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Variation in chemosensitivity and the control of dietary selection behaviour in the locust
Appetite, 1991, 17, 141-154
Variation Selection
in Chemosensitivity and the Control Behaviour in the Locust
S. J. SIMPSON
of Dietary
and S. JAMES
Department of Zoology and University Museum, University of Oxford
M. S. J. SIMMONDS Jodrell Laboratory, Royal Botanic Gardens, Kew
W. M. BLANEY Department of Biology, Birkbeck College, University of London
Investigations into the hehavioural and underlying physiological mechanisms of dietary selection are presented for the locust, Locusta migratoria. Locusts were fed for 4,8 or 12 h on one of four chemically defined artificial diets: diet PC, which was nutritionally complete; diet P, containing no digestible carbohydrate; diet C, containing no protein; and diet 0, which lacked both protein and digestible carbohydrate. Following this pretreatment, the locusts were provided with both the P and the C diet in a choice test. Detailed analyses of selection hehaviour indicated that diets lacking a nutrient for which the insect was deficient were either rejected before a meal was initiated, or, if feeding commenced, eaten in meals of only short duration, while those containing the appropriate nutrients were accepted more readily and eaten in longer meals. Electrophysiological studies showed that this hehaviour was paralled by nutrient-specific changes in gustatory responsiveness. Locusts pretreated for 4 h on C diet had increased gustatory responsiveness to stimulation with an amino acid mix, but not to sucrose, while insects fed on P diet showed increased responsiveness to stimulation with sucrose, but not to the amino acid mix. This result is consistent with earlier experiments in which levels of blood nutrients were shown to modulate taste responsiveness in the locust.
INTRODUCTION
Herbivorous insects do not live in a nutritionally homogeneous environment: their food is variable in both the quantity and quality of nutrients it contains (Slansky & Rodriguez, 1987). Like vertebrates (reviewed by Le Magnen, 1985), insects are able to compensate for such variability by altering their feeding behaviour. They can do this in two ways-by altering the amount of food eaten and by selecting between available foods (Simpson & Simpson, 1990; Waldbauer & Friedman, 1991). During recent years both types of compensatory response have been found to occur for protein and carbohydrate in the locust. Such experiments have involved the use of defined artificial diets presented in several types of assay. In some experiments,
Address correspondence and reprint requests to: Dr S. J. Simpson, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, U.K. 019556663/91/050141+ 14 $03.00/O
0 1991 Academic Press Limited
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insects were given a single diet in which concentrations of both protein and carbohydrate were varied (Simpson & Abisgold, 1985; Raubenheimer & Simpson, 1990). In others, locusts were given a choice of two diets, one of which had no protein but contained one of a graded series of carbohydrate levels, and the other contained no digestible carbohydrate but had one of a series of protein levels (Simpson et al., 1988; Chyb & Simpson, 1990). The third type of assay has involved manipulating the nutritional state of insects by feeding them for a defined period on one of several artificial diets, and then providing them with the opportunity to make good any deficiency incurred by allowing them to select between diets (Simpson et al., 1988, 1990). This last assay style is used in the present study. It has been criticized for not providing conclusive evidence for dietary selection (Booth, 1985), but the combination of all three approaches, along with direct manipulation of blood nutrient titres by injection (Abisgold 8z Simpson, 1988; C. L. Simpson et al., 1990b), has already shown beyond doubt that protein and carbohydrate intake are regulated in locusts and that nutrient-specific feedbacks are involved. Compensatory selection by locusts is exhibited for protein after only a single meal taken during ad libitum feeding on a diet lacking protein (Simpson et al., 1990a). The response to a deficiency of carbohydrate takes somewhat longer to become apparent but is clearly seen after feeding for 4 h on a diet lacking the nutrient. Compensatory selection for both protein and carbohydrate continues for several hours and many meals after pretreatment on deficient diets for 4,8 or 12 h (Simpson et al., 1988). Such persistence strongly suggests that the response is not a simple matter of sensory adaptation or central habituation to the taste of the pretreatment diets (Blaney & Duckett, 1975; Clifton et al., 1987; Hetherington et al., 1989). However, the detailed behavioural mechanisms involved and their underlying physiological control are still to be discovered. The mechanisms of dietary selection behaviour in vertebrates are also not fully explained, and have been the cause of considerable controversy (see, for example, Booth, 1987; Fernstrom, 1987, and the rest of that issue of Appetite). Dietary selection involves behavioural decisions both before and after a food is contacted. Some of these decisions involve associatively or non-associatively learned responses, while others are based on innate preferences or direct physiological feedbacks (Booth, 1985; Baker et al., 1987; Simpson & Simpson, 1990). Simpson & White (1990) showed that locusts learn to associate the odour of a food with its protein content. Nymphs are attracted to such odours when they are deprived of protein, but not when they are deficient in carbohydrate. Once food is contacted the chemosensory system plays a central role in the initiation and maintenance of feeding, with nutrients such as sugars and amino acids acting as important phagostimulants (Bernays & Simpson, 1982). It has been shown recently that the nutritional state of a locust influences the responsiveness of taste receptors on the mouthparts. Abisgold & Simpson (1987, 1988) and C. L. Simpson et al. (1990b) demonstrated, using nutrient injections, that amino acids in the blood provide a nutritional feedback controlling the duration of intermeal intervals in the locust, and that part of this effect is apparently mediated by specific changes in the responsiveness of mouthpart sensilla to stimulation by amino acids. In those studies, locusts were fed an artificial diet with the protein component diluted to varying degrees. Compensation involved eating more of the diluted diets. It was hypothesized that such a mechanism could also control dietary selection behaviour for protein and perhaps, via blood sugar levels, even for carbohydrate
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(Abisgold & Simpson, 1988). A mechanism whereby the nutritional needs of the insect are reflected in the responsiveness of its chemoreceptors and, as a result, cause compensatory selective feeding, would be of singular elegance. The aims of the present study were, firstly, to discover whether the probability of initiating and maintaining ingestion on a diet were influenced by the specific nutritional state of the insect, and, secondly, to investigate the role, if any, of variability in chemoreceptor responsiveness in the control of such behaviour. Detailed behavioural observations were carried out during the same series of experiments described by Simpson et al. (1988), although that paper only gives data for the total amounts of diet eaten during the pretreatment and choice periods. An analysis of detailed behavioural mechanisms is presented in the present publication.
MATERIALSAND METHODS
Experimental insects Locusts were reared at the Department of Zoology, Oxford, using seedling wheat and bran as the standard diet. On the day of ecdysis to the fifth stadium, equal numbers of male and female nymphs were placed individually in 17 x 12 x 6 cm plastic containers with an expanded aluminium perch, a supply of free water and a dish of PC diet (see below). They were kept at 30 + 1°C under an L:D 12 h: 12 h light regime until the morning of the fourth day after ecdysis.
Behavioural observations On the morning of the fourth day the array of insects was observed, starting 2 h after the lights came on, until 12 male and 12 female insects had taken a meal of the PC diet during the course of ad libitum feeding. As each insect finished feeding the PC diet was removed and replaced by two Petri dishes, placed diametrically opposite each other in the container, both containing weighed amounts of one of the following pretreatment diets: PC (the same as the rearing diet), P, C or 0 (see below for details of diet composition). Locusts were then left for 4, 8 or 12 h (+ 15 min), at the end of which the diets were removed, dried and weighed so that the amounts eaten could be calculated. These data have been published previously (Simpson et al., 1988). Immediately after the pretreatment period the locusts were transferred individually into containers with two Petri dishes, one containing a known weight of P diet and the other a known weight of C diet. The position of these was alternated between insects to control for any standard positional bias. After 20sec settling time the behaviour of the locusts was recorded at IO-set intervals until both diets had been contacted and, if feeding was initiated, until the first meal had been completed on each diet (a maximum of 77 min). The diets were then reweighed, any spilt diet having being replaced in the appropriate dish, to establish the amount eaten. Amounts eaten have been published previously (Simpson et al., 1988). The following categories of behaviour were recorded: ingesting; chewing (masticating during a pause in ingesting); contacting the food with at least one fore or mid tarsus but not ingesting; resting without ingesting or chewing; and locomoting. The
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definitions for the termination of meals were based on log-survivorship feeding and non-feeding episodes (Simpson, 1982, 1990).
analyses of
Recording gustatory responsiveness
Different insects from those used for behavioural observations were used to record gustatory responsiveness. Locusts were set up as described above and on the fourth day, starting 2 h after lights-on and at 30-min intervals thereafter, one locust at a time had the PC diet in its dish replaced by P, C or more PC diet. The 0 diet was not tested. Each insect was then left for 4 h, after which it was prepared for electrophysiology. It took about 10-l 5 min to set up each insect, and another 15 min to complete the recordings. The gustatory receptors of locusts are borne in hollow cuticular pegs (the gustatory sensilla) with a terminal pore and are located predominantly on the tarsi (feet) and the external and internal mouthparts. The mouthpart palps, of which there are four (the paired maxillary and labial palps), are particularly important structures in food selection. On their distal tip they each have a flexible dome, upon which are found approximately 350 taste hairs (Blaney et al., 1971). Tip-recording (Hodgson et al., 1955) was used to investigate the responsiveness of taste sensilla on the domes of the maxillary palps. In brief, an indifferent electrode is placed in the haemolymph and a micropipette, containing a recording electrode and filled with a stimulant mixed in a dilute electrolyte, is placed over a single taste hair. Extracellular recordings are obtained from whichever of the 6-10 taste neurones within the hair are responding. Three of the 350 or so gustatory sensilla on one palp were each stimulated once with seven solutions. The three hairs were chosen for their accessibility to stimulation (and so tended to lie centrally on the dome of the palp) and were followed individually throughout the experiment. It is possible to record from a larger sample than this but it is difficult to track individual sensilla when more than three are stimulated by a given solution. In previous studies we have found that it is statistically more powerful to compare the responses within a sensillum, and so record from fewer sensilla on each insect, than it is to record from lo-15 sensilla per insect, but only be able to compare population responses. Work by Blaney (1974, 1975,198l) suggests that there is no obvious specialization among sensilla, although variation within and between sensilla is high. It could be argued that three sensilla is insufficient to provide a representative sample. If that were true, then variation between individual insects ought to obscure any effects of experimental treatment. Results show that the experimental effects in the present experiment are clearly apparent, despite the small intra-insect sample. All seven stimulating solutions were mixed in 0.05 M KC1 (the electrolyte) and were as follows: 0.01, 0.05 and 0.1 M sucrose; 0.01, 0.05 and 0.1 M amino acids; and 0.05~ KC1 alone. The amino acid solutions contained a 41:46:33:47:52:46:37:28 ratio of L 1eu:glu: ser:met :phe: 1ys:val:ala. These amino acids have been demonstrated to be involved in the control of protein compensation in the locust (C. L. Simpson et al., 1990b) and the ratio tested is that present in hydrolysates of the artificial diet (Abisgold & Simpson, 1987). A series of three concentrations were tested for both sucrose and amino acids to provide an indication of the dose-responsiveness of any modulation due to nutritional state. Both amino acid and sugar series were
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tested in ascending concentration, but the order in which amino acids, sucrose or KC1 were tested was randomized, as was the order in which the three hairs were stimulated with a given solution.
Artljicial diets Full details of the dry, granular artificial diets used are given elsewhere (Simpson & Abisgold, 1985; Simpson et al., 1988). The PC diet contained 20% protein (a 3: 1: 1 mix of casein, peptone and egg albumen) by dry weight and 10% digestible carbohydrate (a 1: 1 mix of sucrose and white dextrin), in addition to salts, essential lipids, vitamins and cellulose. The P and C diets were the same as PC, except that the former lacked digestible carbohydrate and the latter lacked protein, these being replaced with indigestible cellulose. The 0 diet had both protein and digestible carbohydrate substituted by cellulose.
RESULTS
Selection behaviour Figure 1 summarizes the response of locusts to the P and C diets upon first contacting the diets during the choice period, following pretreatment on P, C, PC and 0 diets. In Simpson et al. (1988) it was shown that the strength of selection exhibited (measured as the amounts of P and C diet eaten during the first hour after being provided with a choice of P and C diets) was not statistically significantly influenced by the duration of pretreatment, being maximal after 4 h. This was verified for the more detailed behavioural data [Zway ANOVA, with pretreatment duration (4, 8 or 12 h) and pretreatment diet (PC, P, C and 0) as main effects: F-value for pretreatment duration= 1.07 (N.S.), when duration of first feeding period was the dependent variable; df 2, 1131. As a result, data for 4, 8 and 12-h conditioned locusts have been pooled, giving a replicate number per pretreatment diet of 30 insects. It is clear from Figure 1 and Table 1 that the nature of the pretreatment diet influenced the response of the locusts to the diets in the choice test. Insects pretreated on the nutritionally complete PC diet had a high and statistically similar probability of rejecting both the P and the C diets on first contacting them during the choice test (rejection here being defined as a feeding episode of less than 1 min in duration, or a contact without any feeding). If feeding commenced, meals tended to be short, especially on the C diet. Locusts conditioned on the 0 diet, which lacked both protein and digestible carbohydrate, almost invariably commenced ingestion upon contacting either the P or the C diet, and meals were of a considerably longer duration than for PC-conditioned insects. There was no significant difference in the response of Oconditioned locusts to the P and the C diets. The C-conditioned insects (which had been deprived of protein during pretreatment), however, did show a marked difference in their response to the choice diets. They had a very much higher probability of rejecting the C than the P diet and, on the few occasions when feeding continued for longer than 1 min, meals were very much shorter than on P diet. The reverse of this response was exhibited by the locusts deprived of carbohydrate
S.
J. SIMPSON ETAL.
C-Conditioned
locusts
P-Conditioned
locusts
‘%., ‘.., I
O-Conditioned
2
3
4
5
6
7
PC-conditioned
locusts
8
91OlI
locusts
20-
I I
2
3
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7
8
9 IO II I2 I3 I4 15 Duration of first feeding
I23456789 episodes t (min)
FIGURE1. Responses exhibited by locusts pretreated on PC, P, C or 0 diets when they first contacted the P and C diets during the choice test. Graphs show the distribution of feeding times plotted as a linear survivorship function. If a locust either did not commence ingestion upon first contacting a diet, or, if it initiated feeding but only continued for 60 set or less, then the feeding bout has been included as being less than 1 min in duration. The contact or feeding episode was deemed ended if there was a following gap lasting at least 4min with no further ingestion of that diet. [This bout criterion was based on log-survivorship analyses from earlier work (Simpson & Abisgold, 1985).]n= 30 insects per pretreatment diet. *, PcO.05; **, P< 0.01; Kolmogorov-Smirnov 2-Sample Test.
TABLE 1. F-ratios from the ANOVA testing the effects of pretreatment diet and stimulating solution on the responsiveness of gustatory sensilla on the maxillary palps of locust nymphs. Data are based on the mean response produced by three sensilla per insect F-ratios: Source
df
Pretr. Resid. Total
2 24 26
F-ratio: KC1 0.2
Source
df
Sucrose
Amino acids
Pretr. Resid. Concn PXC Resid. Total
2 24 2 4 48 80
7.16**
3.43*
7.65*** 1.11
3.99* 0.68
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CHEMOSENSITIVITY IN LOCUSTS
(P-conditioned), such insects being more likely to reject and less likely to feed for long on the P than the C diet during the choice test. Gustatory responsiveness Figures 2 and 3 show the mean (f S.E.) response of gustatory sensilla to stimulation by sucrose and amino acids, following pretreatment on the PC, P and C diets for 4 h. There was a particularly marked effect of pretreatment on chemoresponsiveness (Table 1). Sensilla of locusts conditioned on the diet lacking digestible carbohydrate (the P diet) exhibited a considerably higher mean rate of firing to stimulation with sucrose than did sensilla of PC- and C-conditioned insects, the latter two not differing from each other. On the other hand, sensilla of locusts pretreated on the diet lacking protein (the C diet) fired at a higher mean rate to stimulation with amino acids than did those of PC- and P-conditioned insects, which also did not differ from each other. These effects were most noticeable at the lowest concentrations tested (0.01 M). Mean firing rates declined with increasing concentrations of sucrose
P cond it boned C conditioned PC conditioned
Sucrose concentration Amino
I 0.01
a q
(M)
acids P conditioned C condttioned PC condltioned
0
n
I
q
I
0.05
Amino actd concentrotlon
n
A
0.10
(M1
FIGURE 2. The responses of maxillary palp chemosensilla of locusts to stimulation with various concentrations of sucrose in 0.05 M KC1 and a mix of amino acids in 0.05 M KCI. The responses to 0.05 M KC1 alone are given to the left of the x-axis. Insects were fed for 4 h prior to the experiment with one of three artificial diets: PC, containing both protein and digestible carbohydrate; P, containing protein but no digestible carbohydrate and C, containing carbohydrate but no protein. Responses are meansfS.E. for total number of spikes elicited during the first second of stimulation. Means are based on responses from three sensilla on each of nine insects (i.e. 27 stimulations).
S. J. SIMPSON H-AL.
148 C-Conditioned, 0.05
M
O-05
M Sucrose
insect
24,
sensillum
Amino acids in 0.05
in 0.05
2
P-Conditioned, O-05
M KCI
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insect
25,
sensillum
M Amino acids in 0.05
M Sucrose in 0.05
M
2
KCI
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J 0.05
C-ConditIoned,
insect 27, sensillum
0.01 M Amino acids in 0.05
M KCI
3
M Sucrose in 0.05
KCI
P-Conditioned,
insect
14, sensillum
0.01 M Amino acids in 0.05
0.01 0.01
M
M Sucrose
in 0.05
M
M
3
KCI
KCI
M KCI J
O-05
MKCI 0-05
M
KCI
FIGURE3. Sample traces (1 set duration in each case) for sensilla stimulated with an amino acid mix in 0.05~ KC1, sucrose in 0.05 M KCl, and 0.05 M KCI alone from two insects conditioned for the previous 4 h on the carbohydrate-deficient P diet and two insects conditioned on the protein-deficient C diet. Note that several neurones respond to each of the stimulants.
and amino acids. This was especially the case for sucrose, where the mean response of PC- and C-conditioned insects was inhibited relative to stimulation with 0.05 M KC1 alone. Sample traces in Figure 3 show a sensillum from each of two C-conditioned locusts where complete inhibition of firing to sucrose occurred. There was no difference in the mean response of PC-, P- and C-conditioned insects to stimulation of their chemosensilla with only 0.05 M KCl.
DISCUSSION We have demonstrated that specific changes in gustatory responsiveness occur with previous nutritional experience in fifth-instar Locusta, and that these changes correlate with the probability that the insect will initiate and maintain feeding on a diet containing nutrients which were lacking in the previous food. Gustatory responsiveness to sucrose and an amino acid mix are modulated independently: locusts fed on a diet lacking protein show increased responsiveness to stimulation with amino acids but not to sucrose, while insects fed a diet lacking digestible carbohydrate show the opposite effect.
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Abisgold & Simpson (1988) and Simpson et al. (1991) have shown that levels of free amino acids in the blood of locust nymphs determine chemosensitivity to stimulation with amino acids. It is likely that the same mechanism is involved in the responses to amino acids found in the present study; those locusts pretreated with the C diet presumably having lower blood levels of free amino acids than nymphs pretreated with the PC or P diets. No work has yet been done to investigate the role of blood sugars on feeding behaviour or gustatory responsiveness in locusts. It would be most parsimonious to hypothesize a similar mechanism to that for amino acids, and future work is planned to measure blood sugars and manipulate levels by injection, as done by Abisgold & Simpson (1987) for amino acids. Very recent data from caterpillars indicates a role for blood sugar levels in the control of sugar selection (Friedman et al., unpublished data). Reports of dietary quality affecting chemosensitivity in other insects are scarce, but there are some suggestive data for some caterpillars. The responsiveness of maxillary receptors in Manduca sexta, Spodoptera exempta and Spodoptera littoralis are influenced by the nature of the rearing diet. In particular, specific changes have been reported for receptors responding to allelochemicals, with a reduction in responsiveness occurring when diets contain the compounds (Schoonhoven, 1969, 1976; Schoonhoven et al., 1987; StHdler & Hanson, 1976). Recently, C. L. Simpson et al. (1990a) found in adult Locusta that the relative responsiveness of chemosensilla on the maxillary palps to amino acids and sucrose changed during the somatic growth phase. Such variation was consistent with relative changes that occurred in the pattern of protein and carbohydrate ingestion exhibited by locusts able to select their intake of the two macronutrients (Chyb & Simpson, 1990). Once again, a mechanism involving titres of amino acids and sugars in the haemolymph could explain the results, except that the variation in blood levels would be due to changes in rates of uptake and synthesis by the fat body, rather than to dietary pretreatment as in the present study. The few reports of changes in gustatory responsiveness with development or reproductive state in other insects have been reviewed by Blaney et al. (1986). Apart from the modulation of responsiveness with nutritional state, the firing rates of sensilla to the various stimulating solutions tested in the present study are very much as expected from previous work on locusts (Blaney, 1974, 1975; Winstanley & Blaney, 1978; Abisgold & Simpson, 1988). Most insects show a positive relationship between firing rate of gustatory receptors responding to phagostimulants and the intensity of feeding behaviour (Blom, 1978; Wieczorek, 1981; Stadler, 1984). The results of pretreatment in the present experiments support this. In addition to the effect of pretreatment, however, there was evidence, particularly for sucrose, of a decrease in firing rate with increasing concentration of stimulant. Behavioural tests on locusts fed dry pith discs have shown that the dose-response curve to sucrose is bell shaped, with high concentrations being rejected (Cook, 1977). The same bell-shaped response has been reported for gustatory receptors in some insects (Wieczorek & Koppl, 1982). Although it is very difficult to equate concentrations in a stimulating solution with those on a dry surface, it is possible that the concentrations used to stimulate sensilla in the present study lie on the falling part of the behavioural dose-response curve. Another possible explanation is that we have not distinguished the responses of individual neurones within a sensillum in this study. It is evident from Figure 3 that in each case a number of neurones contribute to the responses of a sensillum to salt,
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amino acids and sugars. This was reported earlier by Blaney (1974, 1975, 1981) and, recently, White & Chapman (1990) have found similar responses to sugars and salts from tarsal chemosensilla of another grasshopper, Schistocerca americana. No one has yet succeeded in separating the responses of the six or ten individual neurones within the maxillary palp chemosensilla of locusts (Blaney et al., 1971). Recent advances in the computer-aided analysis of spike trains (Frazier & Hanson, 1986; Mitchell et al., 1990) may yet help to identify the activity of individual taste neurones. Perhaps then more typical dose-response effects will emerge. The maxillary palps are known to be important in the selection of food by locusts [Blaney & Chapman, 1970; Mordue (Luntz), 19791, and experiments by Blaney & Duckett (1975) and Mordue (Luntz) (1979) indicate that they influence meal duration, particularly on relatively unpalatable foods. However, amino acids and sugars are found in highest concentrations within plant tissues rather than on their surfaces and it seems that palp chemosensilla do not usually contact the internal contents of leaves during feeding. Perhaps the chemosensilla within the pre-oral cavity play an important role. We do not know whether such chemoreceptors are modulated in a similar way to those on the maxilla. Despite sugars and amino acids occurring predominantly within plant tissues, they do occur on leaf surfaces at very low concentrations and, at least in the case of ovipositing European corn borers, appear to influence behaviour at these low levels (Derridj et al., 1986; Derridj et al., 1989). The concentration of sugars and free amino acids on the surface of the artificial diets used in the present experiments will be the same as that occurring throughout. Analysis of the PC diet indicates that the concentration of free amino acids is 1.1 x 1O- 5 mole/g while sucrose concentration is 1.5 x 10-4mole/g. In the case of the diets there is also a strong correlation between the free amino acid profile and the ratio of amino acids present in the protein. Whether the same is true for plants is not clear, nor is it known whether insects can taste proteins (the present consensus is that they cannot). During food selection behaviour and throughout the meal, locusts drum their palps on the surface of the food at a rate of about lo- 15 Hz (Blaney & Chapman, 1970). Perhaps such high frequency sampling behaviour serves to concentrate in the viscous fluid surrounding the sensory dendrites stimulants occurring at very low density on the surface of a plant. This could provide another function for such behaviours, in addition to those already suggested (see Blaney & Duckett, 1975; Stldler, 1986; Woodhead & Chapman, 1986). We still do not understand the way in which blood levels of nutrients influence gustatory responsiveness. A number of possibilities have been discussed by Abisgold & Simpson (1988) and Blaney et aE. (1986). Since then, C. L. Simpson (Note 1) has shown that the effect in the locust is not mediated by centrifugal neural feedback from the central nervous system. Centrifugal control of taste sensitivity has been suggested in some vertebrates (Brush & Halpern, 1970; Roper, 1989). Other possibilities include hormonal feedbacks or direct interactions between nutrients in the blood and acceptor sites on the peripheral taste neurones-perhaps even the same sites that bind with stimulants entering the pore at the tip of each sensillum, so that the gustatory ne-.trones of a nutrient-satiated animal are adapted by nutrients from the blood and hence are relatively unresponsive to further, external stimulation. Thus they would act as difference detectors. Such adaptation of taste receptors “from the inside” is an exciting possibility which we are currently investigating. A similar type of phenomenon might possibly be involved in selective feeding for salt in sodium-deprived rats. In this case, variation in levels of sodium in the saliva, which correlates with degree of
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salt deprivation, might account for the changed responsiveness of taste afferents to stimulation of the tongue with NaCl (Contreras & Frank, 1979; Rowland & Fregly, 1988). A mechanism in which acceptor sites are directly affected would confer the necessary specificity in modulation, even if the same receptor cells contain different acceptors for salts, sugars and amino acids, as seems to be the case for Locusta (Blaney, 1974, 1975, 1981; White & Chapman, 1990). The possibility that the composition of the receptor lymph reflects that of the blood is known to exist through selective absorption by the accessory cells which act as a barrier between the two (Phillips & Vande Berg, 1976). The hormonally induced increase in electrical resistance across the palp tips found after feeding in Locustu by Bernays et al. (1972) could not provide the necessary specificity to account for present results, nor those of Abisgold & Simpson (1988), since the release of the factor is apparently caused by distension of the crop, something which is independent of the specific nutritional composition of the food. The data in the present paper suggest that modulation of chemosensory inputs could play an important role in selective feeding behaviour in insects. That peripheral integration is involved is, perhaps, not very surprising, given the need for economy within the central nervous system of a small animal. After all, peripheral integration is well documented in insect motor systems and contrasts with the way in which vertebrates are organized (Hoyle, 1982; Shepherd, 1988). There is no evidence that modulation of taste receptor responsiveness is involved in compensatory feeding for protein or carbohydrate in vertebrates. Giza and Scott (1983, 1987) found that neurones in the nucleus of the solitary tract exhibited reduced responsiveness to stimulation of the tongue of a rat with sugars, but not NaCl or HCl, after intravenous insulin or glucose injection. Modulation at the level of the sense cells was not considered as a possible explanation of the results. Rats appear to differ from monkeys. Feeding monkeys to satiety on a glucose solution has no effect on the gustatory responses of neurones in the nucleus of the solitary tract, although neurones in brain regions further along the integrative pathway, notably the orbitofrontal cortex and lateral hypothalamus, are specifically modulated (Yaxley et al., 1985; Rolls et al., 1988; Rolls, 1989). It has been proposed that such neural changes in the monkey provide the mechanism underlying sensoryspecific satiety, whereby the acceptability of a particular food declines immediately after it has been eaten to repletion, while the acceptability of other foods remains relatively unchanged (Rolls et al., 1986). Although it exists and is important, peripheral integration in insect chemosensory systems does not imply any lesser a role for the central nervous system: primacy of behavioural control still resides in the C.N.S. ACKNOWLEDGEMENT Many thanks to Dr Reg Chapman for his constructive criticism of the manuscript and for his thoughts and ideas.
REFERENCENOTE Simpson, C. L. (1990) Dietary compensation in Locusta migratoria: behauiour. D.Phil. thesis, Oxford University.
aspects of physiology
and
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Abisgold, J. D. & Simpson, S. J. (1987) The physiology of compensation by locusts for changes in dietary protein. Journal of Experimental Biology, 129, 329-346. Abisgold, J. D. & Simpson, S. J. (1988) The effect of dietary protein levels and haemolymph composition on the sensitivity of the maxillary palp chemoreceptors of locusts. Journal of Experimental Biology, 135, 21 S-229.
Baker, B. J., Booth, D. A., Duggan, J. P. & Gibson, E. L. (1987) Protein appetite demonstrated: learned specificity of protein-cue preference to protein need in adult rats. Nutrition Research, 7, 481-486.
Bernays, E. A. & Simpson, S. J. (1982) Control of food intake. Advances in Insect Physiology, 16, 59-118. Bernays, E. A., Blaney, W. M. & Chapman, R. F. (1972) Changes in chemoreceptor sensilla on the maxillary palps of Locusta migratoria in relation to feeding. Journal of Experimental Biology, 57, 745-753.
Blaney, W. M. (1974) Electrophysiological responses of the terminal sensilla on the maxillary palps of Locusta migratoria (L.) to some electrolytes and non-electrolytes. Journal of Experimental Biology, 60, 275-293.
Blaney, W. M. (1975) Behavioural and electrophysiological studies of taste discrimination by the maxillary palps of larvae of Locusta migratoria. Journal of Experimental Biology, 62, 555-569.
Blaney, W. M. (1981) Chemoreception
and food selection in locusts. Trends in Neuroscience, 4,
35-38.
Blaney, W. M. & Chapman, R. F. (1970) The functions of the maxillary palps of Acrididae (Orthoptera). Entomologia experimentalis et Applicata, 13, 363-376. Blaney, W. M. & Duckett, A. M. (1975) The significance of palpation of the maxillary palps of Locusta migratoria (L.): an electrophysiological and behavioural study. Journal of Experimental Biology, 63, 701-712.
Blaney, W. M., Chapman, R. F. & Cook, A. G. (1971) The structure of the terminal sensilla of the maxillary palps of Locusta migratoria (L.) and changes associated with moulting. Zeitschrift fur Zellforschung und mikroskopische Anatomie, 121, 48-61.
Blaney, W. M., Schoonhoven, L. M. & Simmonds, M. S. J. (1986) Sensitivity variations in insect chemoreceptors; a review. Experientia, 42, 13- 19. Blom, F. (1978) Sensory input behavioural output relationships in the feeding activity of some lepidopterous larvae. Entomologia experimentalis et Applicata, 24, 258-263. Booth, D. A. (1985) Food-conditioned eating preferences and aversions with interoreceptive elements: conditioned appetites and aversions. Annals of the New York Academy of Sciences, 443, 22-41.
Booth, D. A. (1987) Central dietary ‘feedback onto nutrient selection’: not even a scientific hypothesis. Appetite, 8, 195-201. Brush, A. D. & Halpern, B. P. (1970) Centrifugal control of gustatory responses. Physiology and Behavior, 5, 743-746.
Chapman, R. F. (1988) Sensory aspects of host-plant recognition by Acridoidea: Questions associated with the multiplicity of receptors and variability of response. Journal of Insect Physiology, 34, 167- 174.
Chyb, S. & Simpson, S. J. (1990) Dietary selection in adult Locusta migratoria L. Entomologia experimentalis et Applicata, 56, 47-60.
Clifton, P. G., Burton, M. J. & Sharp, C. (1987) Rapid loss of stimulus-specific satiety after consumption of a second food. Appetite, 9, 149- 156. Cook, A. G. (1977) Nutrient chemicals as phagostimulants for Locusta migratoria. Ecological Entomology, 2, 113-121. Contreras, R. J. & Frank, M. (1979) Sodium deprivation alters neural responses to gustatory stimuli. Journal of General Physiology, 73, 569-594. Derridj, S., Fiala, E. & Jolivet, E. (1987) Low molecular carbohydrates of Zea mays L. leaves and the egg laying of Ostrinia nubialis Hbn. (Lep. Pyralidae). In V. Labeyrie, G. Fabres & D. Lachaise (Eds.), Insects-Plants. Proceedings of the 6th International Symposium on Insect-Plant Relationships, Pp. 295-299. Dordrecht: Dr W. Junk Publishers.
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Derridj, S.,Gregoire, V., Boutin, J P. & Fiala, V. (1989) Plant growth stages in the interspecific oviposition preference of the European corn borer and relations with chemicals present on the leaf surfaces. Entomologia experimentalis et Applicata, 53, 267-276. Fernstrom, J. D. (1987) Food-induced changes in brain serotonin synthesis: Is there a relationship to appetite for specific nutrients? Appetite, 7, 163-182. Frazier, J. L. & Hanson, F. E. (1986) Electrophysiological recordings and analysis of insect chemosensory responses. In J. R. Miller & T. A. Miller (Eds.), Insect-plant interactions, Pp. 285-330. New York: Springer Verlag. Giza, B. K. & Scott, T. R. (1983) Blood glucose selectively affects taste-evoked activity in rat nucleus tractus solitarious. Physiology and Behavior, 31, 643-650. Giza, B. K. & Scott, T. R. (1987) Intravenous insulin infusions in rats decrease gustatory evoked responses to sugars. American Journal of Physiology, 252, R994-1002. Hetherington, M., Rolls, B. J. & Burley, V. J. (1989) The time course of sensory-specific satiety. Appetite, 12, 57-68.
Hodgson, E. S., Lettvin, J. Y. & Roeder, K. D. (1966) Physiology of a primary chemoreceptor unit. Science, 122, 417-418. Hoyle, G. (1982) Muscles and their nervous control. New York: Wiley. Le Magnen, J. (1985) Hunger. Cambridge: Cambridge University Press. Mitchell, B. K., Smith, J. J. B., Roselth, B. M. & Whitehead, A. (1990) SAPZD Tools, Version 3.5. Manual available from Prof. Mitchell, Department of Entomology, University of Alberta, Edmonton, Canada. Mordue (Luntz), A. J. (1979) The role of the maxillary and labial palps in the feeding behaviour of Schistocerca gregaria. Entomologia experimentalis et Applicata, 25, 279-288. Phillips, C. E. & Vande Berg, J. S. (1976) Directional flow of sensillum liquor in blowfly (Phormia regina) chemoreceptors. Journal of Insect Physiology, 22, 425-429. Raubenheimer, D. & Simpson, S. J. (1990) The effects of simultaneous variation in protein, digestible carbohydrate and tannic acid on the feeding behaviour of larval Locusta migratoria (L.) and Schistocerca gregaria (Forskal). I. Short-term studies. Physiological Entomology, 15, 219-233.
Rolls, E. T., Scott, T. R., Sienkiewicz, Z. J. & Yaxley, S. (1988) The responsiveness of neurones in the frontal opercular gustatory cortex of the macaque monkey is independent of hunger. Journal of Physiology, London, 397, 1-12. Rolls, E. T. (1989) Information processing in the taste system in primates. Journal qf Experimental Biology, 146, 141-164.
Rolls, E. T., Murzi, E., Yaxley, S., Thorpe, S. J. and Simpson, S. J. (1986) Sensory-specific satiety: food-specific reduction in responsiveness of ventral forebrain neurones after feeding in the monkey. Brain Research, 368, 79-86. Roper, S. D. (1989) The cell biology of taste receptors. Annual Reoiew of Neroscience, 12, 329-353.
Rowland, N. E. & Fregly, M. J. (1988) Sodium appetite: species and strain differences and role of renin-angiotensin aldosterone system. Appetite, 11, 143-178. Schoonhoven, L. M. (1969) Sensitivity changes in some insect chemoreceptors and their effect on food selection behaviour. Proceedings. Koniklijke Nederlandse akademie van Wetenschappen (C), 72, 491-498. Schoonhoven, L. M. (1976) On the variability of chemosensory information. Symposia Biologica Hungarica, 16, 261-266.
Schoonhoven, L. M., Blaney, W. M. & Simmonds, M. S. J. (1987) Inconstancies of chemoreceptor sensitivities. In V. Labeyrie, G. Fabres & D. Lachaise (Eds.), ZnsectsPlants: Proceedings of the 6th International Symposium on Insect-Plant Relationships,
pp. 141-145. Shepherd, G. M. (1988) Neurobiology (2nd edition). Oxford: Oxford University Press. Simpson, C. L., Chyb, S. & Simpson, S. J. (1990a) Changes in chemoreceptor sensitivity in relation to dietary selection by adult Locusta migratoria L. Entomologia experimentalis et Applicata, 56, 259-268.
Simpson, C. L., Simpson, S. J. & Abisgold, J. D. (1990b) An amino acid feedback and the control of locust feeding. Symposia Biologica Hungarica, 39, 39-46. Simpson, S. J. (1982) Patterns in feeding: a behavioural analysis using Locusta migratoria nymphs. Physiological Entomology, 7, 325-336.
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Simpson, S. J. (1990) The pattern of feeding. In R. F. Chapman & A. Joern (Eds.), Biology of grasshoppers, Pp. 73-103. New York: John Wiley & Sons. Simpson, S. J. & Abisgold, J. D. (1985) Compensation by locusts for changes in dietary nutrients: behavioural mechanisms. Physiological Entomology, IO, 443-452. Simpson, S. J., Simmonds, M. S. J. & Blaney, W. M. (1988) A comparison of dietary selection behaviour in larval Locusta migratoria and Spodoptera littoralis. Physiological Entomology, 13, 225-238.
Simpson, S. J. & Simpson, C. L. (1990) The mechanisms of compensation by phytophagous insects. In E. A. Bernays (Ed.), Insect-plant interactions Vol. ZZ,Pp. 11l-160. Florida, Boca Raton: CRC Press. Simpson, S. J. & White, P. R. (1990) Associative learning and locust feeding: evidence for a “learned hunger” for protein. Animal Behaviour, 40, 506-513. Simpson, S. J., Simmonds, M. S. J., Blaney, W. M. & Jones, J. P. (1990) Compensatory dietary selection occurs in larval Locusta migratoria but not Spodoptera littoralis after a single deficient meal during ad libitum feeding. Physiological Entomology, 15, 235-242. Slansky, F. Jr. & Rodriguez, J. G. (Eds.) (1987) Nutritional ecology of insects, mites, spiders and related invertebrates New York: John Wiley. StHdler, E. (1984) Contact chemoreception. In W. J. Bell & R. T. Carde (Eds.), Chemical ecology of insects, Pp. 3-35. London: Chapman & Hall. Stldler, E. (1986). Oviposition and feeding stimuli in leaf surface waxes. In B. E. Juniper & T. R. E. Southwood (Eds.), Insects and plant surfaces, Pp. 105-121, London: Edward Arnold. Stlidler, E. & Hanson, F. E. (1986) Influence of induction of host preference on chemoreception of Manduca sexta: behavioral and electrophysiological studies. Symposia Biologica Ht ‘tgarica, 16, 267-273.
Waldbauer, G. P. & Friedman,
S. (1991) Self-selection of optimal diets by insects. Annual
Review of Entomology, 36, 43-63.
White, P. R. & Chapman, R. F. (1990) Tarsal chemoreception in the polyphagous grasshopper Schistocerca americana: behavioural assays, sensilla distribution and electrophysiology. Physiological Entomology, 15, 105-121. Wieczorek, H. (1981) Sugar receptors in the labellar taste hairs of the fly. Advances in physiological sciences, 23, 481-493.
Wieczorek, H. & Koppl, R. (1982) Reaction spectra of sugar receptors in different taste hairs of the fly. Journal of Comparative Physiology, A, 149, 207-213. Winstanley, C. & Blaney, W. M. (1978) Chemosensory mechanisms in locusts in relation to feeding. Entomologia experimentalis et Applicata, 24, 550-558. Woodhead, S. & Chapman, R. F. (1986) Insect behaviour and the chemistry of plant surface waxes. In B. E. Juniper & T. R. E. Southwood (Eds.), Insects and the plant surface, Pp. 123-135. London: Edward Arnold. Yaxley, S., Rolls, E. T., Sienkiewicz, Z. J. & Scott, T. R. (1985) Satiety does not affect gustatory activity in the nucleus of the solitary tract of the alert monkey. Brain Research, 347,85-93.
Variation Selection
in Chemosensitivity and the Control Behaviour in the Locust
S. J. SIMPSON
of Dietary
and S. JAMES
Department of Zoology and University Museum, University of Oxford
M. S. J. SIMMONDS Jodrell Laboratory, Royal Botanic Gardens, Kew
W. M. BLANEY Department of Biology, Birkbeck College, University of London
Investigations into the hehavioural and underlying physiological mechanisms of dietary selection are presented for the locust, Locusta migratoria. Locusts were fed for 4,8 or 12 h on one of four chemically defined artificial diets: diet PC, which was nutritionally complete; diet P, containing no digestible carbohydrate; diet C, containing no protein; and diet 0, which lacked both protein and digestible carbohydrate. Following this pretreatment, the locusts were provided with both the P and the C diet in a choice test. Detailed analyses of selection hehaviour indicated that diets lacking a nutrient for which the insect was deficient were either rejected before a meal was initiated, or, if feeding commenced, eaten in meals of only short duration, while those containing the appropriate nutrients were accepted more readily and eaten in longer meals. Electrophysiological studies showed that this hehaviour was paralled by nutrient-specific changes in gustatory responsiveness. Locusts pretreated for 4 h on C diet had increased gustatory responsiveness to stimulation with an amino acid mix, but not to sucrose, while insects fed on P diet showed increased responsiveness to stimulation with sucrose, but not to the amino acid mix. This result is consistent with earlier experiments in which levels of blood nutrients were shown to modulate taste responsiveness in the locust.
INTRODUCTION
Herbivorous insects do not live in a nutritionally homogeneous environment: their food is variable in both the quantity and quality of nutrients it contains (Slansky & Rodriguez, 1987). Like vertebrates (reviewed by Le Magnen, 1985), insects are able to compensate for such variability by altering their feeding behaviour. They can do this in two ways-by altering the amount of food eaten and by selecting between available foods (Simpson & Simpson, 1990; Waldbauer & Friedman, 1991). During recent years both types of compensatory response have been found to occur for protein and carbohydrate in the locust. Such experiments have involved the use of defined artificial diets presented in several types of assay. In some experiments,
Address correspondence and reprint requests to: Dr S. J. Simpson, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, U.K. 019556663/91/050141+ 14 $03.00/O
0 1991 Academic Press Limited
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insects were given a single diet in which concentrations of both protein and carbohydrate were varied (Simpson & Abisgold, 1985; Raubenheimer & Simpson, 1990). In others, locusts were given a choice of two diets, one of which had no protein but contained one of a graded series of carbohydrate levels, and the other contained no digestible carbohydrate but had one of a series of protein levels (Simpson et al., 1988; Chyb & Simpson, 1990). The third type of assay has involved manipulating the nutritional state of insects by feeding them for a defined period on one of several artificial diets, and then providing them with the opportunity to make good any deficiency incurred by allowing them to select between diets (Simpson et al., 1988, 1990). This last assay style is used in the present study. It has been criticized for not providing conclusive evidence for dietary selection (Booth, 1985), but the combination of all three approaches, along with direct manipulation of blood nutrient titres by injection (Abisgold 8z Simpson, 1988; C. L. Simpson et al., 1990b), has already shown beyond doubt that protein and carbohydrate intake are regulated in locusts and that nutrient-specific feedbacks are involved. Compensatory selection by locusts is exhibited for protein after only a single meal taken during ad libitum feeding on a diet lacking protein (Simpson et al., 1990a). The response to a deficiency of carbohydrate takes somewhat longer to become apparent but is clearly seen after feeding for 4 h on a diet lacking the nutrient. Compensatory selection for both protein and carbohydrate continues for several hours and many meals after pretreatment on deficient diets for 4,8 or 12 h (Simpson et al., 1988). Such persistence strongly suggests that the response is not a simple matter of sensory adaptation or central habituation to the taste of the pretreatment diets (Blaney & Duckett, 1975; Clifton et al., 1987; Hetherington et al., 1989). However, the detailed behavioural mechanisms involved and their underlying physiological control are still to be discovered. The mechanisms of dietary selection behaviour in vertebrates are also not fully explained, and have been the cause of considerable controversy (see, for example, Booth, 1987; Fernstrom, 1987, and the rest of that issue of Appetite). Dietary selection involves behavioural decisions both before and after a food is contacted. Some of these decisions involve associatively or non-associatively learned responses, while others are based on innate preferences or direct physiological feedbacks (Booth, 1985; Baker et al., 1987; Simpson & Simpson, 1990). Simpson & White (1990) showed that locusts learn to associate the odour of a food with its protein content. Nymphs are attracted to such odours when they are deprived of protein, but not when they are deficient in carbohydrate. Once food is contacted the chemosensory system plays a central role in the initiation and maintenance of feeding, with nutrients such as sugars and amino acids acting as important phagostimulants (Bernays & Simpson, 1982). It has been shown recently that the nutritional state of a locust influences the responsiveness of taste receptors on the mouthparts. Abisgold & Simpson (1987, 1988) and C. L. Simpson et al. (1990b) demonstrated, using nutrient injections, that amino acids in the blood provide a nutritional feedback controlling the duration of intermeal intervals in the locust, and that part of this effect is apparently mediated by specific changes in the responsiveness of mouthpart sensilla to stimulation by amino acids. In those studies, locusts were fed an artificial diet with the protein component diluted to varying degrees. Compensation involved eating more of the diluted diets. It was hypothesized that such a mechanism could also control dietary selection behaviour for protein and perhaps, via blood sugar levels, even for carbohydrate
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(Abisgold & Simpson, 1988). A mechanism whereby the nutritional needs of the insect are reflected in the responsiveness of its chemoreceptors and, as a result, cause compensatory selective feeding, would be of singular elegance. The aims of the present study were, firstly, to discover whether the probability of initiating and maintaining ingestion on a diet were influenced by the specific nutritional state of the insect, and, secondly, to investigate the role, if any, of variability in chemoreceptor responsiveness in the control of such behaviour. Detailed behavioural observations were carried out during the same series of experiments described by Simpson et al. (1988), although that paper only gives data for the total amounts of diet eaten during the pretreatment and choice periods. An analysis of detailed behavioural mechanisms is presented in the present publication.
MATERIALSAND METHODS
Experimental insects Locusts were reared at the Department of Zoology, Oxford, using seedling wheat and bran as the standard diet. On the day of ecdysis to the fifth stadium, equal numbers of male and female nymphs were placed individually in 17 x 12 x 6 cm plastic containers with an expanded aluminium perch, a supply of free water and a dish of PC diet (see below). They were kept at 30 + 1°C under an L:D 12 h: 12 h light regime until the morning of the fourth day after ecdysis.
Behavioural observations On the morning of the fourth day the array of insects was observed, starting 2 h after the lights came on, until 12 male and 12 female insects had taken a meal of the PC diet during the course of ad libitum feeding. As each insect finished feeding the PC diet was removed and replaced by two Petri dishes, placed diametrically opposite each other in the container, both containing weighed amounts of one of the following pretreatment diets: PC (the same as the rearing diet), P, C or 0 (see below for details of diet composition). Locusts were then left for 4, 8 or 12 h (+ 15 min), at the end of which the diets were removed, dried and weighed so that the amounts eaten could be calculated. These data have been published previously (Simpson et al., 1988). Immediately after the pretreatment period the locusts were transferred individually into containers with two Petri dishes, one containing a known weight of P diet and the other a known weight of C diet. The position of these was alternated between insects to control for any standard positional bias. After 20sec settling time the behaviour of the locusts was recorded at IO-set intervals until both diets had been contacted and, if feeding was initiated, until the first meal had been completed on each diet (a maximum of 77 min). The diets were then reweighed, any spilt diet having being replaced in the appropriate dish, to establish the amount eaten. Amounts eaten have been published previously (Simpson et al., 1988). The following categories of behaviour were recorded: ingesting; chewing (masticating during a pause in ingesting); contacting the food with at least one fore or mid tarsus but not ingesting; resting without ingesting or chewing; and locomoting. The
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definitions for the termination of meals were based on log-survivorship feeding and non-feeding episodes (Simpson, 1982, 1990).
analyses of
Recording gustatory responsiveness
Different insects from those used for behavioural observations were used to record gustatory responsiveness. Locusts were set up as described above and on the fourth day, starting 2 h after lights-on and at 30-min intervals thereafter, one locust at a time had the PC diet in its dish replaced by P, C or more PC diet. The 0 diet was not tested. Each insect was then left for 4 h, after which it was prepared for electrophysiology. It took about 10-l 5 min to set up each insect, and another 15 min to complete the recordings. The gustatory receptors of locusts are borne in hollow cuticular pegs (the gustatory sensilla) with a terminal pore and are located predominantly on the tarsi (feet) and the external and internal mouthparts. The mouthpart palps, of which there are four (the paired maxillary and labial palps), are particularly important structures in food selection. On their distal tip they each have a flexible dome, upon which are found approximately 350 taste hairs (Blaney et al., 1971). Tip-recording (Hodgson et al., 1955) was used to investigate the responsiveness of taste sensilla on the domes of the maxillary palps. In brief, an indifferent electrode is placed in the haemolymph and a micropipette, containing a recording electrode and filled with a stimulant mixed in a dilute electrolyte, is placed over a single taste hair. Extracellular recordings are obtained from whichever of the 6-10 taste neurones within the hair are responding. Three of the 350 or so gustatory sensilla on one palp were each stimulated once with seven solutions. The three hairs were chosen for their accessibility to stimulation (and so tended to lie centrally on the dome of the palp) and were followed individually throughout the experiment. It is possible to record from a larger sample than this but it is difficult to track individual sensilla when more than three are stimulated by a given solution. In previous studies we have found that it is statistically more powerful to compare the responses within a sensillum, and so record from fewer sensilla on each insect, than it is to record from lo-15 sensilla per insect, but only be able to compare population responses. Work by Blaney (1974, 1975,198l) suggests that there is no obvious specialization among sensilla, although variation within and between sensilla is high. It could be argued that three sensilla is insufficient to provide a representative sample. If that were true, then variation between individual insects ought to obscure any effects of experimental treatment. Results show that the experimental effects in the present experiment are clearly apparent, despite the small intra-insect sample. All seven stimulating solutions were mixed in 0.05 M KC1 (the electrolyte) and were as follows: 0.01, 0.05 and 0.1 M sucrose; 0.01, 0.05 and 0.1 M amino acids; and 0.05~ KC1 alone. The amino acid solutions contained a 41:46:33:47:52:46:37:28 ratio of L 1eu:glu: ser:met :phe: 1ys:val:ala. These amino acids have been demonstrated to be involved in the control of protein compensation in the locust (C. L. Simpson et al., 1990b) and the ratio tested is that present in hydrolysates of the artificial diet (Abisgold & Simpson, 1987). A series of three concentrations were tested for both sucrose and amino acids to provide an indication of the dose-responsiveness of any modulation due to nutritional state. Both amino acid and sugar series were
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tested in ascending concentration, but the order in which amino acids, sucrose or KC1 were tested was randomized, as was the order in which the three hairs were stimulated with a given solution.
Artljicial diets Full details of the dry, granular artificial diets used are given elsewhere (Simpson & Abisgold, 1985; Simpson et al., 1988). The PC diet contained 20% protein (a 3: 1: 1 mix of casein, peptone and egg albumen) by dry weight and 10% digestible carbohydrate (a 1: 1 mix of sucrose and white dextrin), in addition to salts, essential lipids, vitamins and cellulose. The P and C diets were the same as PC, except that the former lacked digestible carbohydrate and the latter lacked protein, these being replaced with indigestible cellulose. The 0 diet had both protein and digestible carbohydrate substituted by cellulose.
RESULTS
Selection behaviour Figure 1 summarizes the response of locusts to the P and C diets upon first contacting the diets during the choice period, following pretreatment on P, C, PC and 0 diets. In Simpson et al. (1988) it was shown that the strength of selection exhibited (measured as the amounts of P and C diet eaten during the first hour after being provided with a choice of P and C diets) was not statistically significantly influenced by the duration of pretreatment, being maximal after 4 h. This was verified for the more detailed behavioural data [Zway ANOVA, with pretreatment duration (4, 8 or 12 h) and pretreatment diet (PC, P, C and 0) as main effects: F-value for pretreatment duration= 1.07 (N.S.), when duration of first feeding period was the dependent variable; df 2, 1131. As a result, data for 4, 8 and 12-h conditioned locusts have been pooled, giving a replicate number per pretreatment diet of 30 insects. It is clear from Figure 1 and Table 1 that the nature of the pretreatment diet influenced the response of the locusts to the diets in the choice test. Insects pretreated on the nutritionally complete PC diet had a high and statistically similar probability of rejecting both the P and the C diets on first contacting them during the choice test (rejection here being defined as a feeding episode of less than 1 min in duration, or a contact without any feeding). If feeding commenced, meals tended to be short, especially on the C diet. Locusts conditioned on the 0 diet, which lacked both protein and digestible carbohydrate, almost invariably commenced ingestion upon contacting either the P or the C diet, and meals were of a considerably longer duration than for PC-conditioned insects. There was no significant difference in the response of Oconditioned locusts to the P and the C diets. The C-conditioned insects (which had been deprived of protein during pretreatment), however, did show a marked difference in their response to the choice diets. They had a very much higher probability of rejecting the C than the P diet and, on the few occasions when feeding continued for longer than 1 min, meals were very much shorter than on P diet. The reverse of this response was exhibited by the locusts deprived of carbohydrate
S.
J. SIMPSON ETAL.
C-Conditioned
locusts
P-Conditioned
locusts
‘%., ‘.., I
O-Conditioned
2
3
4
5
6
7
PC-conditioned
locusts
8
91OlI
locusts
20-
I I
2
3
4
5
6
7
8
9 IO II I2 I3 I4 15 Duration of first feeding
I23456789 episodes t (min)
FIGURE1. Responses exhibited by locusts pretreated on PC, P, C or 0 diets when they first contacted the P and C diets during the choice test. Graphs show the distribution of feeding times plotted as a linear survivorship function. If a locust either did not commence ingestion upon first contacting a diet, or, if it initiated feeding but only continued for 60 set or less, then the feeding bout has been included as being less than 1 min in duration. The contact or feeding episode was deemed ended if there was a following gap lasting at least 4min with no further ingestion of that diet. [This bout criterion was based on log-survivorship analyses from earlier work (Simpson & Abisgold, 1985).]n= 30 insects per pretreatment diet. *, PcO.05; **, P< 0.01; Kolmogorov-Smirnov 2-Sample Test.
TABLE 1. F-ratios from the ANOVA testing the effects of pretreatment diet and stimulating solution on the responsiveness of gustatory sensilla on the maxillary palps of locust nymphs. Data are based on the mean response produced by three sensilla per insect F-ratios: Source
df
Pretr. Resid. Total
2 24 26
F-ratio: KC1 0.2
Source
df
Sucrose
Amino acids
Pretr. Resid. Concn PXC Resid. Total
2 24 2 4 48 80
7.16**
3.43*
7.65*** 1.11
3.99* 0.68
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CHEMOSENSITIVITY IN LOCUSTS
(P-conditioned), such insects being more likely to reject and less likely to feed for long on the P than the C diet during the choice test. Gustatory responsiveness Figures 2 and 3 show the mean (f S.E.) response of gustatory sensilla to stimulation by sucrose and amino acids, following pretreatment on the PC, P and C diets for 4 h. There was a particularly marked effect of pretreatment on chemoresponsiveness (Table 1). Sensilla of locusts conditioned on the diet lacking digestible carbohydrate (the P diet) exhibited a considerably higher mean rate of firing to stimulation with sucrose than did sensilla of PC- and C-conditioned insects, the latter two not differing from each other. On the other hand, sensilla of locusts pretreated on the diet lacking protein (the C diet) fired at a higher mean rate to stimulation with amino acids than did those of PC- and P-conditioned insects, which also did not differ from each other. These effects were most noticeable at the lowest concentrations tested (0.01 M). Mean firing rates declined with increasing concentrations of sucrose
P cond it boned C conditioned PC conditioned
Sucrose concentration Amino
I 0.01
a q
(M)
acids P conditioned C condttioned PC condltioned
0
n
I
q
I
0.05
Amino actd concentrotlon
n
A
0.10
(M1
FIGURE 2. The responses of maxillary palp chemosensilla of locusts to stimulation with various concentrations of sucrose in 0.05 M KC1 and a mix of amino acids in 0.05 M KCI. The responses to 0.05 M KC1 alone are given to the left of the x-axis. Insects were fed for 4 h prior to the experiment with one of three artificial diets: PC, containing both protein and digestible carbohydrate; P, containing protein but no digestible carbohydrate and C, containing carbohydrate but no protein. Responses are meansfS.E. for total number of spikes elicited during the first second of stimulation. Means are based on responses from three sensilla on each of nine insects (i.e. 27 stimulations).
S. J. SIMPSON H-AL.
148 C-Conditioned, 0.05
M
O-05
M Sucrose
insect
24,
sensillum
Amino acids in 0.05
in 0.05
2
P-Conditioned, O-05
M KCI
0.05
M
insect
25,
sensillum
M Amino acids in 0.05
M Sucrose in 0.05
M
2
KCI
M
J 0.05
C-ConditIoned,
insect 27, sensillum
0.01 M Amino acids in 0.05
M KCI
3
M Sucrose in 0.05
KCI
P-Conditioned,
insect
14, sensillum
0.01 M Amino acids in 0.05
0.01 0.01
M
M Sucrose
in 0.05
M
M
3
KCI
KCI
M KCI J
O-05
MKCI 0-05
M
KCI
FIGURE3. Sample traces (1 set duration in each case) for sensilla stimulated with an amino acid mix in 0.05~ KC1, sucrose in 0.05 M KCl, and 0.05 M KCI alone from two insects conditioned for the previous 4 h on the carbohydrate-deficient P diet and two insects conditioned on the protein-deficient C diet. Note that several neurones respond to each of the stimulants.
and amino acids. This was especially the case for sucrose, where the mean response of PC- and C-conditioned insects was inhibited relative to stimulation with 0.05 M KC1 alone. Sample traces in Figure 3 show a sensillum from each of two C-conditioned locusts where complete inhibition of firing to sucrose occurred. There was no difference in the mean response of PC-, P- and C-conditioned insects to stimulation of their chemosensilla with only 0.05 M KCl.
DISCUSSION We have demonstrated that specific changes in gustatory responsiveness occur with previous nutritional experience in fifth-instar Locusta, and that these changes correlate with the probability that the insect will initiate and maintain feeding on a diet containing nutrients which were lacking in the previous food. Gustatory responsiveness to sucrose and an amino acid mix are modulated independently: locusts fed on a diet lacking protein show increased responsiveness to stimulation with amino acids but not to sucrose, while insects fed a diet lacking digestible carbohydrate show the opposite effect.
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Abisgold & Simpson (1988) and Simpson et al. (1991) have shown that levels of free amino acids in the blood of locust nymphs determine chemosensitivity to stimulation with amino acids. It is likely that the same mechanism is involved in the responses to amino acids found in the present study; those locusts pretreated with the C diet presumably having lower blood levels of free amino acids than nymphs pretreated with the PC or P diets. No work has yet been done to investigate the role of blood sugars on feeding behaviour or gustatory responsiveness in locusts. It would be most parsimonious to hypothesize a similar mechanism to that for amino acids, and future work is planned to measure blood sugars and manipulate levels by injection, as done by Abisgold & Simpson (1987) for amino acids. Very recent data from caterpillars indicates a role for blood sugar levels in the control of sugar selection (Friedman et al., unpublished data). Reports of dietary quality affecting chemosensitivity in other insects are scarce, but there are some suggestive data for some caterpillars. The responsiveness of maxillary receptors in Manduca sexta, Spodoptera exempta and Spodoptera littoralis are influenced by the nature of the rearing diet. In particular, specific changes have been reported for receptors responding to allelochemicals, with a reduction in responsiveness occurring when diets contain the compounds (Schoonhoven, 1969, 1976; Schoonhoven et al., 1987; StHdler & Hanson, 1976). Recently, C. L. Simpson et al. (1990a) found in adult Locusta that the relative responsiveness of chemosensilla on the maxillary palps to amino acids and sucrose changed during the somatic growth phase. Such variation was consistent with relative changes that occurred in the pattern of protein and carbohydrate ingestion exhibited by locusts able to select their intake of the two macronutrients (Chyb & Simpson, 1990). Once again, a mechanism involving titres of amino acids and sugars in the haemolymph could explain the results, except that the variation in blood levels would be due to changes in rates of uptake and synthesis by the fat body, rather than to dietary pretreatment as in the present study. The few reports of changes in gustatory responsiveness with development or reproductive state in other insects have been reviewed by Blaney et al. (1986). Apart from the modulation of responsiveness with nutritional state, the firing rates of sensilla to the various stimulating solutions tested in the present study are very much as expected from previous work on locusts (Blaney, 1974, 1975; Winstanley & Blaney, 1978; Abisgold & Simpson, 1988). Most insects show a positive relationship between firing rate of gustatory receptors responding to phagostimulants and the intensity of feeding behaviour (Blom, 1978; Wieczorek, 1981; Stadler, 1984). The results of pretreatment in the present experiments support this. In addition to the effect of pretreatment, however, there was evidence, particularly for sucrose, of a decrease in firing rate with increasing concentration of stimulant. Behavioural tests on locusts fed dry pith discs have shown that the dose-response curve to sucrose is bell shaped, with high concentrations being rejected (Cook, 1977). The same bell-shaped response has been reported for gustatory receptors in some insects (Wieczorek & Koppl, 1982). Although it is very difficult to equate concentrations in a stimulating solution with those on a dry surface, it is possible that the concentrations used to stimulate sensilla in the present study lie on the falling part of the behavioural dose-response curve. Another possible explanation is that we have not distinguished the responses of individual neurones within a sensillum in this study. It is evident from Figure 3 that in each case a number of neurones contribute to the responses of a sensillum to salt,
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amino acids and sugars. This was reported earlier by Blaney (1974, 1975, 1981) and, recently, White & Chapman (1990) have found similar responses to sugars and salts from tarsal chemosensilla of another grasshopper, Schistocerca americana. No one has yet succeeded in separating the responses of the six or ten individual neurones within the maxillary palp chemosensilla of locusts (Blaney et al., 1971). Recent advances in the computer-aided analysis of spike trains (Frazier & Hanson, 1986; Mitchell et al., 1990) may yet help to identify the activity of individual taste neurones. Perhaps then more typical dose-response effects will emerge. The maxillary palps are known to be important in the selection of food by locusts [Blaney & Chapman, 1970; Mordue (Luntz), 19791, and experiments by Blaney & Duckett (1975) and Mordue (Luntz) (1979) indicate that they influence meal duration, particularly on relatively unpalatable foods. However, amino acids and sugars are found in highest concentrations within plant tissues rather than on their surfaces and it seems that palp chemosensilla do not usually contact the internal contents of leaves during feeding. Perhaps the chemosensilla within the pre-oral cavity play an important role. We do not know whether such chemoreceptors are modulated in a similar way to those on the maxilla. Despite sugars and amino acids occurring predominantly within plant tissues, they do occur on leaf surfaces at very low concentrations and, at least in the case of ovipositing European corn borers, appear to influence behaviour at these low levels (Derridj et al., 1986; Derridj et al., 1989). The concentration of sugars and free amino acids on the surface of the artificial diets used in the present experiments will be the same as that occurring throughout. Analysis of the PC diet indicates that the concentration of free amino acids is 1.1 x 1O- 5 mole/g while sucrose concentration is 1.5 x 10-4mole/g. In the case of the diets there is also a strong correlation between the free amino acid profile and the ratio of amino acids present in the protein. Whether the same is true for plants is not clear, nor is it known whether insects can taste proteins (the present consensus is that they cannot). During food selection behaviour and throughout the meal, locusts drum their palps on the surface of the food at a rate of about lo- 15 Hz (Blaney & Chapman, 1970). Perhaps such high frequency sampling behaviour serves to concentrate in the viscous fluid surrounding the sensory dendrites stimulants occurring at very low density on the surface of a plant. This could provide another function for such behaviours, in addition to those already suggested (see Blaney & Duckett, 1975; Stldler, 1986; Woodhead & Chapman, 1986). We still do not understand the way in which blood levels of nutrients influence gustatory responsiveness. A number of possibilities have been discussed by Abisgold & Simpson (1988) and Blaney et aE. (1986). Since then, C. L. Simpson (Note 1) has shown that the effect in the locust is not mediated by centrifugal neural feedback from the central nervous system. Centrifugal control of taste sensitivity has been suggested in some vertebrates (Brush & Halpern, 1970; Roper, 1989). Other possibilities include hormonal feedbacks or direct interactions between nutrients in the blood and acceptor sites on the peripheral taste neurones-perhaps even the same sites that bind with stimulants entering the pore at the tip of each sensillum, so that the gustatory ne-.trones of a nutrient-satiated animal are adapted by nutrients from the blood and hence are relatively unresponsive to further, external stimulation. Thus they would act as difference detectors. Such adaptation of taste receptors “from the inside” is an exciting possibility which we are currently investigating. A similar type of phenomenon might possibly be involved in selective feeding for salt in sodium-deprived rats. In this case, variation in levels of sodium in the saliva, which correlates with degree of
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salt deprivation, might account for the changed responsiveness of taste afferents to stimulation of the tongue with NaCl (Contreras & Frank, 1979; Rowland & Fregly, 1988). A mechanism in which acceptor sites are directly affected would confer the necessary specificity in modulation, even if the same receptor cells contain different acceptors for salts, sugars and amino acids, as seems to be the case for Locusta (Blaney, 1974, 1975, 1981; White & Chapman, 1990). The possibility that the composition of the receptor lymph reflects that of the blood is known to exist through selective absorption by the accessory cells which act as a barrier between the two (Phillips & Vande Berg, 1976). The hormonally induced increase in electrical resistance across the palp tips found after feeding in Locustu by Bernays et al. (1972) could not provide the necessary specificity to account for present results, nor those of Abisgold & Simpson (1988), since the release of the factor is apparently caused by distension of the crop, something which is independent of the specific nutritional composition of the food. The data in the present paper suggest that modulation of chemosensory inputs could play an important role in selective feeding behaviour in insects. That peripheral integration is involved is, perhaps, not very surprising, given the need for economy within the central nervous system of a small animal. After all, peripheral integration is well documented in insect motor systems and contrasts with the way in which vertebrates are organized (Hoyle, 1982; Shepherd, 1988). There is no evidence that modulation of taste receptor responsiveness is involved in compensatory feeding for protein or carbohydrate in vertebrates. Giza and Scott (1983, 1987) found that neurones in the nucleus of the solitary tract exhibited reduced responsiveness to stimulation of the tongue of a rat with sugars, but not NaCl or HCl, after intravenous insulin or glucose injection. Modulation at the level of the sense cells was not considered as a possible explanation of the results. Rats appear to differ from monkeys. Feeding monkeys to satiety on a glucose solution has no effect on the gustatory responses of neurones in the nucleus of the solitary tract, although neurones in brain regions further along the integrative pathway, notably the orbitofrontal cortex and lateral hypothalamus, are specifically modulated (Yaxley et al., 1985; Rolls et al., 1988; Rolls, 1989). It has been proposed that such neural changes in the monkey provide the mechanism underlying sensoryspecific satiety, whereby the acceptability of a particular food declines immediately after it has been eaten to repletion, while the acceptability of other foods remains relatively unchanged (Rolls et al., 1986). Although it exists and is important, peripheral integration in insect chemosensory systems does not imply any lesser a role for the central nervous system: primacy of behavioural control still resides in the C.N.S. ACKNOWLEDGEMENT Many thanks to Dr Reg Chapman for his constructive criticism of the manuscript and for his thoughts and ideas.
REFERENCENOTE Simpson, C. L. (1990) Dietary compensation in Locusta migratoria: behauiour. D.Phil. thesis, Oxford University.
aspects of physiology
and
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Abisgold, J. D. & Simpson, S. J. (1987) The physiology of compensation by locusts for changes in dietary protein. Journal of Experimental Biology, 129, 329-346. Abisgold, J. D. & Simpson, S. J. (1988) The effect of dietary protein levels and haemolymph composition on the sensitivity of the maxillary palp chemoreceptors of locusts. Journal of Experimental Biology, 135, 21 S-229.
Baker, B. J., Booth, D. A., Duggan, J. P. & Gibson, E. L. (1987) Protein appetite demonstrated: learned specificity of protein-cue preference to protein need in adult rats. Nutrition Research, 7, 481-486.
Bernays, E. A. & Simpson, S. J. (1982) Control of food intake. Advances in Insect Physiology, 16, 59-118. Bernays, E. A., Blaney, W. M. & Chapman, R. F. (1972) Changes in chemoreceptor sensilla on the maxillary palps of Locusta migratoria in relation to feeding. Journal of Experimental Biology, 57, 745-753.
Blaney, W. M. (1974) Electrophysiological responses of the terminal sensilla on the maxillary palps of Locusta migratoria (L.) to some electrolytes and non-electrolytes. Journal of Experimental Biology, 60, 275-293.
Blaney, W. M. (1975) Behavioural and electrophysiological studies of taste discrimination by the maxillary palps of larvae of Locusta migratoria. Journal of Experimental Biology, 62, 555-569.
Blaney, W. M. (1981) Chemoreception
and food selection in locusts. Trends in Neuroscience, 4,
35-38.
Blaney, W. M. & Chapman, R. F. (1970) The functions of the maxillary palps of Acrididae (Orthoptera). Entomologia experimentalis et Applicata, 13, 363-376. Blaney, W. M. & Duckett, A. M. (1975) The significance of palpation of the maxillary palps of Locusta migratoria (L.): an electrophysiological and behavioural study. Journal of Experimental Biology, 63, 701-712.
Blaney, W. M., Chapman, R. F. & Cook, A. G. (1971) The structure of the terminal sensilla of the maxillary palps of Locusta migratoria (L.) and changes associated with moulting. Zeitschrift fur Zellforschung und mikroskopische Anatomie, 121, 48-61.
Blaney, W. M., Schoonhoven, L. M. & Simmonds, M. S. J. (1986) Sensitivity variations in insect chemoreceptors; a review. Experientia, 42, 13- 19. Blom, F. (1978) Sensory input behavioural output relationships in the feeding activity of some lepidopterous larvae. Entomologia experimentalis et Applicata, 24, 258-263. Booth, D. A. (1985) Food-conditioned eating preferences and aversions with interoreceptive elements: conditioned appetites and aversions. Annals of the New York Academy of Sciences, 443, 22-41.
Booth, D. A. (1987) Central dietary ‘feedback onto nutrient selection’: not even a scientific hypothesis. Appetite, 8, 195-201. Brush, A. D. & Halpern, B. P. (1970) Centrifugal control of gustatory responses. Physiology and Behavior, 5, 743-746.
Chapman, R. F. (1988) Sensory aspects of host-plant recognition by Acridoidea: Questions associated with the multiplicity of receptors and variability of response. Journal of Insect Physiology, 34, 167- 174.
Chyb, S. & Simpson, S. J. (1990) Dietary selection in adult Locusta migratoria L. Entomologia experimentalis et Applicata, 56, 47-60.
Clifton, P. G., Burton, M. J. & Sharp, C. (1987) Rapid loss of stimulus-specific satiety after consumption of a second food. Appetite, 9, 149- 156. Cook, A. G. (1977) Nutrient chemicals as phagostimulants for Locusta migratoria. Ecological Entomology, 2, 113-121. Contreras, R. J. & Frank, M. (1979) Sodium deprivation alters neural responses to gustatory stimuli. Journal of General Physiology, 73, 569-594. Derridj, S., Fiala, E. & Jolivet, E. (1987) Low molecular carbohydrates of Zea mays L. leaves and the egg laying of Ostrinia nubialis Hbn. (Lep. Pyralidae). In V. Labeyrie, G. Fabres & D. Lachaise (Eds.), Insects-Plants. Proceedings of the 6th International Symposium on Insect-Plant Relationships, Pp. 295-299. Dordrecht: Dr W. Junk Publishers.
CHEMOSENSITIVITY
IN LOCUSTS
153
Derridj, S.,Gregoire, V., Boutin, J P. & Fiala, V. (1989) Plant growth stages in the interspecific oviposition preference of the European corn borer and relations with chemicals present on the leaf surfaces. Entomologia experimentalis et Applicata, 53, 267-276. Fernstrom, J. D. (1987) Food-induced changes in brain serotonin synthesis: Is there a relationship to appetite for specific nutrients? Appetite, 7, 163-182. Frazier, J. L. & Hanson, F. E. (1986) Electrophysiological recordings and analysis of insect chemosensory responses. In J. R. Miller & T. A. Miller (Eds.), Insect-plant interactions, Pp. 285-330. New York: Springer Verlag. Giza, B. K. & Scott, T. R. (1983) Blood glucose selectively affects taste-evoked activity in rat nucleus tractus solitarious. Physiology and Behavior, 31, 643-650. Giza, B. K. & Scott, T. R. (1987) Intravenous insulin infusions in rats decrease gustatory evoked responses to sugars. American Journal of Physiology, 252, R994-1002. Hetherington, M., Rolls, B. J. & Burley, V. J. (1989) The time course of sensory-specific satiety. Appetite, 12, 57-68.
Hodgson, E. S., Lettvin, J. Y. & Roeder, K. D. (1966) Physiology of a primary chemoreceptor unit. Science, 122, 417-418. Hoyle, G. (1982) Muscles and their nervous control. New York: Wiley. Le Magnen, J. (1985) Hunger. Cambridge: Cambridge University Press. Mitchell, B. K., Smith, J. J. B., Roselth, B. M. & Whitehead, A. (1990) SAPZD Tools, Version 3.5. Manual available from Prof. Mitchell, Department of Entomology, University of Alberta, Edmonton, Canada. Mordue (Luntz), A. J. (1979) The role of the maxillary and labial palps in the feeding behaviour of Schistocerca gregaria. Entomologia experimentalis et Applicata, 25, 279-288. Phillips, C. E. & Vande Berg, J. S. (1976) Directional flow of sensillum liquor in blowfly (Phormia regina) chemoreceptors. Journal of Insect Physiology, 22, 425-429. Raubenheimer, D. & Simpson, S. J. (1990) The effects of simultaneous variation in protein, digestible carbohydrate and tannic acid on the feeding behaviour of larval Locusta migratoria (L.) and Schistocerca gregaria (Forskal). I. Short-term studies. Physiological Entomology, 15, 219-233.
Rolls, E. T., Scott, T. R., Sienkiewicz, Z. J. & Yaxley, S. (1988) The responsiveness of neurones in the frontal opercular gustatory cortex of the macaque monkey is independent of hunger. Journal of Physiology, London, 397, 1-12. Rolls, E. T. (1989) Information processing in the taste system in primates. Journal qf Experimental Biology, 146, 141-164.
Rolls, E. T., Murzi, E., Yaxley, S., Thorpe, S. J. and Simpson, S. J. (1986) Sensory-specific satiety: food-specific reduction in responsiveness of ventral forebrain neurones after feeding in the monkey. Brain Research, 368, 79-86. Roper, S. D. (1989) The cell biology of taste receptors. Annual Reoiew of Neroscience, 12, 329-353.
Rowland, N. E. & Fregly, M. J. (1988) Sodium appetite: species and strain differences and role of renin-angiotensin aldosterone system. Appetite, 11, 143-178. Schoonhoven, L. M. (1969) Sensitivity changes in some insect chemoreceptors and their effect on food selection behaviour. Proceedings. Koniklijke Nederlandse akademie van Wetenschappen (C), 72, 491-498. Schoonhoven, L. M. (1976) On the variability of chemosensory information. Symposia Biologica Hungarica, 16, 261-266.
Schoonhoven, L. M., Blaney, W. M. & Simmonds, M. S. J. (1987) Inconstancies of chemoreceptor sensitivities. In V. Labeyrie, G. Fabres & D. Lachaise (Eds.), ZnsectsPlants: Proceedings of the 6th International Symposium on Insect-Plant Relationships,
pp. 141-145. Shepherd, G. M. (1988) Neurobiology (2nd edition). Oxford: Oxford University Press. Simpson, C. L., Chyb, S. & Simpson, S. J. (1990a) Changes in chemoreceptor sensitivity in relation to dietary selection by adult Locusta migratoria L. Entomologia experimentalis et Applicata, 56, 259-268.
Simpson, C. L., Simpson, S. J. & Abisgold, J. D. (1990b) An amino acid feedback and the control of locust feeding. Symposia Biologica Hungarica, 39, 39-46. Simpson, S. J. (1982) Patterns in feeding: a behavioural analysis using Locusta migratoria nymphs. Physiological Entomology, 7, 325-336.
154
S. J. SIMPSON ET AL.
Simpson, S. J. (1990) The pattern of feeding. In R. F. Chapman & A. Joern (Eds.), Biology of grasshoppers, Pp. 73-103. New York: John Wiley & Sons. Simpson, S. J. & Abisgold, J. D. (1985) Compensation by locusts for changes in dietary nutrients: behavioural mechanisms. Physiological Entomology, IO, 443-452. Simpson, S. J., Simmonds, M. S. J. & Blaney, W. M. (1988) A comparison of dietary selection behaviour in larval Locusta migratoria and Spodoptera littoralis. Physiological Entomology, 13, 225-238.
Simpson, S. J. & Simpson, C. L. (1990) The mechanisms of compensation by phytophagous insects. In E. A. Bernays (Ed.), Insect-plant interactions Vol. ZZ,Pp. 11l-160. Florida, Boca Raton: CRC Press. Simpson, S. J. & White, P. R. (1990) Associative learning and locust feeding: evidence for a “learned hunger” for protein. Animal Behaviour, 40, 506-513. Simpson, S. J., Simmonds, M. S. J., Blaney, W. M. & Jones, J. P. (1990) Compensatory dietary selection occurs in larval Locusta migratoria but not Spodoptera littoralis after a single deficient meal during ad libitum feeding. Physiological Entomology, 15, 235-242. Slansky, F. Jr. & Rodriguez, J. G. (Eds.) (1987) Nutritional ecology of insects, mites, spiders and related invertebrates New York: John Wiley. StHdler, E. (1984) Contact chemoreception. In W. J. Bell & R. T. Carde (Eds.), Chemical ecology of insects, Pp. 3-35. London: Chapman & Hall. Stldler, E. (1986). Oviposition and feeding stimuli in leaf surface waxes. In B. E. Juniper & T. R. E. Southwood (Eds.), Insects and plant surfaces, Pp. 105-121, London: Edward Arnold. Stlidler, E. & Hanson, F. E. (1986) Influence of induction of host preference on chemoreception of Manduca sexta: behavioral and electrophysiological studies. Symposia Biologica Ht ‘tgarica, 16, 267-273.
Waldbauer, G. P. & Friedman,
S. (1991) Self-selection of optimal diets by insects. Annual
Review of Entomology, 36, 43-63.
White, P. R. & Chapman, R. F. (1990) Tarsal chemoreception in the polyphagous grasshopper Schistocerca americana: behavioural assays, sensilla distribution and electrophysiology. Physiological Entomology, 15, 105-121. Wieczorek, H. (1981) Sugar receptors in the labellar taste hairs of the fly. Advances in physiological sciences, 23, 481-493.
Wieczorek, H. & Koppl, R. (1982) Reaction spectra of sugar receptors in different taste hairs of the fly. Journal of Comparative Physiology, A, 149, 207-213. Winstanley, C. & Blaney, W. M. (1978) Chemosensory mechanisms in locusts in relation to feeding. Entomologia experimentalis et Applicata, 24, 550-558. Woodhead, S. & Chapman, R. F. (1986) Insect behaviour and the chemistry of plant surface waxes. In B. E. Juniper & T. R. E. Southwood (Eds.), Insects and the plant surface, Pp. 123-135. London: Edward Arnold. Yaxley, S., Rolls, E. T., Sienkiewicz, Z. J. & Scott, T. R. (1985) Satiety does not affect gustatory activity in the nucleus of the solitary tract of the alert monkey. Brain Research, 347,85-93.