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The function of mosquito taste receptors
J. Insect Physiol., 1966, Vol. 12, pp. 1051 to 1060. Pergamon Press Ltd.
THE FUNCTION
OF MOSQUITO
Printed in Great Btitain
TASTE RECEPTORS
H. S. SALAMA” Department
of Entomology,
University
of Alberta,
Edmonton,
Canada
(Received 2 April 1966)
Abstract-Two types of receptors seem to be involved in the feeding of Aedes aegypti. Threshold values of acceptance and rejection for 39 sugars and 25 electrolytes were measured. Most pentoses, hexoses, di- and tri-saccharides are stimulating. Compounds with a-linkage are superior stimuli. Slight molecular differences are reflected in stimulating effectiveness. Metabolic inhibitors at low concentrations do not affect stimulation, whereas CuCl, inhibits acceptance. Competitive inhibition occurs between some sugars. Rejection thresholds for acids and hydroxides are lower than those for neutral salts. The stimulating efficiency of the cations is mostly correlated to their ionic mobilities. Behavioural studies showed the sensitivity of the labral sense organs to blood. The labellar organs are sensitive to sugars and water, and may be capable of detecting unacceptable compounds. The cibarial sensilla alone control ingestion of liquids and are stimulated by blood, sugars, and unacceptable compounds. INTRODUCTION
ALTHOUGHcontact chemoreceptors have been investigated on the, external surface, including the mouth parts and tarsi of a few insects, little is yet known about chemoreception internal to the stylet food canal of sucking insects. The only evidence on this was reported by DAY (1954) wh o suggested that the sense organs in the buccal cavity of Aedes aegypti L. detect some sugars and other components of blood. MILES (1958) also gave evidence for two species of Hemiptera, but the sensilla were not identified. The work done with mosquitoes has been mainly concerned with the sensitivity of contact chemoreceptors on the tarsi, labella, and ligula to sucrose and a few other compounds (FRINGS and HAMRUM, 1950; WALLIS, 1954; HOSOI, 1959; FEIR et al., 1961; OWEN, 1961, 1963). The location and innervation of the labral and cibarial sensilla in a few mosquito species were reported by ROBINSON (1939), FRINGS and HAMRUM(1950), DAY (1954), CHRISTOPHERS(1960), and VON GERNET (unpublished). The present work deals mainly with the possible gustatory function of sensilla located in the cibarial pump of the mosquito A. aegypti L., and their role in the feeding behaviour in relation to the labral and labellar sensilla. The stimulator-y effects of a series of chemical compounds including carbohydrates and electrolytes have been investigated to determine the insect’s ability in discriminating between these compounds when sucked up through the food canal. * Present Egypt,
address:
Department
of Entomology,
U.A.R. 1051
Faculty
of Science,
Cairo
University,
H. S. SALAMA
1052
MATERIALS
AND
METHODS
The experimental mosquitoes used were 2 to 7 days old, from standard cultures fed on 5% sucrose and bred at 27°C and 70 to 80 per cent r.h. For testing the stimulatory effect of different compounds, the insects were anaesthetized with CO, and mounted on thin wax blocks sealed on a carton sheet. The thorax and wings were securely fastened by applying a fine hot needle to the surface of the wax close to the dorsal side of the insect. The individuals were satiated with water and left for 24 hr to recover completely after which they were checked again for water responsiveness. Solutions of the compounds to be tested were then presented in an ascending series from subliminal to supraliminal concentrations, in fine capillary tubes (1.5-2.5 mm i.d.) which fitted over the whole proboscis. The acceptability of a compound can be judged from the flow of the solution down the capillary tube and through the food canal, the pulsation frequency of the oesophageal pump, and the final abdominal distension. The compounds tested were presented as water solutions; for electrolytes a 0.1 M sucrose solution was used as a mediator for positive response. All the experiments were run at 25 to 28”C, with 2 to 3 hr intervals between successive trials. The percentage of insects feeding to repletion on each medium was recorded. Insects which fed were discarded. The most probable value of the logarithmic concentration accepted or rejected by 50 per cent of the insects was determined according to BLISS (1938). The scatter of the thresholds obtained with different individuals was found to be normal wkh respect to the logarithm of concentration. RESULTS
AND
DISCUSSION
Response to carbohydrates The response of various insects to carbohydrates (WEIS, 1930 ; MINNICH, 193 1; VOGEL, 1931; VON FRISCH, 1934; H&LINGER, 1935; HASSETTet al., 1950; DETHIER, 1955) showed a spectrum of activity from complete unresponsiveness to extreme sensitivity. The stimulating effectiveness of 39 sugars and related compounds to A. aegypti was investigated, and the median acceptance thresholds are given in Table 1. Trioses, tetroses, and heptoses are ineffective. All the tested pentoses, hexoses, and di- and tri-saccharides are stimulating with few exceptions. In the polysaccharides, xylan is weakly stimulating. Inositol is the only stimulating polyhydric alcohol. Considering the median acceptance thresholds for sugars tested, the order of stimulating effectiveness is as follows: Monosaccharides : D-fructose > L-fucose > L-sorbose > D-&COW? > D-arabinose > D-xylose > L-arabinose > L-xylose > D-galactose > D-mannose. Disaccharides: Sucrose > maltose = turanose > lactose = melibiose. Trisaccharides: Melezitose > rafKnose. The effectiveness of pentoses and hexoses in the monosaccharides possibly indicates a certain chain length within which the compounds of this group can be stimulating. There is a great difference in the effectiveness of hexoses, such as D-glUCOSeand D-galaCtOSe,D-glUCOSeand D-mannose, despite the slight differences
THE FUNCTION
OF MOSQUITO
TASTE RECEPTORS
1053
Mannose and galactose, however, are equally stimulating, in their configuration. although they differ more in their structural configuration. Compounds with or-linkage are superior stimuli like mannose, turanose, and sucrose. Lactose, melibiose, and cellobiose, which lack the a-linkage, are either weakly or non-stimulating. n-Arabinose and n-xylose are more stimulating than the respective L-forms. U-Methyl glucoside is also a superior stimulus to the /3form. So, the structure of the molecule shows it to be effective in stimulation. DETHIER (1955) showed the significance of the sugar structural configuration in stimulation of the blowfly. VON FRISCH (1934) and WYKES (1952), however, concluded that some biological specificity may be involved in stimulation of the honey bee by sugars. Substitution in the sugar molecule may change its stimulating characteristic as with sucrose octoacetate, diacetone glucose, and gulonic lactone. Glucosamine hydrochloride, however, is approximately as stimulating as glucose. Of the sugar phosphates, n-fructose-1-6-phosphate (magnesium) is effective. DETHIER (1955) obtained similar results for the blowfly, and he assumed that stimulation by sugars does not involve phosphorylytic reactions. Factors afecting stimulation by sugars Competitive inhibition occurs between some sugars in A. aegypti causing an increase in the acceptance threshold of the more stimulating component. For instance, mannose at 0.5 M inhibits glucose, fructose, or sucrose, causing the threshold values to be elevated by factors of 8, 6, and 5, respectively. L-Rhamnose (non-stimulating) also inhibits both fructose and sucrose, but this is probably a rejection effect. The inhibitory effect caused may be due to unequivalent stimuli of both sugars in the mixture, one inducing stimulation and the other interfering with its action, until a certain concentration level is reached, where the more stimulating component overcomes the other one. This may also suggest that there are different sites of action on the sugar receptor as in the blowfly (DETHIER et al., 1956). There is no evidence for synergism or an additive effect. There is no inhibition caused by low concentrations of the metabolic inhibitors cyanide, azide, and fluoride on stimulation with sucrose. This indicates that the mechanism of stimulation does not involve any steps in the glydolytic cycle as in the blowfly (DETHIER, 1955). Heavy metal salts such as CuCl, at’O.005 to O-01 M, on the other hand, inhibit stimulation with sucrose and render the oral receptors insensitive for about 4 hr. This may be related to the reaction of such heavy salts with or within the receptor cells. The housefly and blowfly also, reacted similarly to such salts (DEONIER, 1938; DETHIER, 1955). Response to electrolytes A. aegypti rejects sucrose to which electrolytes have been added. The rejection threshold values illustrated in Table 2 show a higher stimulating effectiveness for either acids or hydroxides than for neutral inorganic salts. There is a response to some chlorides (NaCl and CaCl,) above the ordinary physiological range. Since the mosquito feeds on blood which naturally contains a variety of inorganic salts, the
0.047 0.281 0.497 0.294 0.541
0.020 0.108 0.079 0.916 1.023
0.023 0.058 0.051 1.081 I*005
0.113 0.445
1.224
Hexoses : D-Fructose D-Glucose L-Sorbose D-Calactose D-Mannose
Disaccharides : Sucrose Turanose D-Maltose Melibiose Lactose
Trisaccharides : Melezitose Ralhnose
Polysaccharides : Xylan
Median acceptance threshold (M)
Pentoses : L-Fucose D-Arabinose L-Arabinose D-XylOSe L-Xylose
Compound
1.693 f 0.069 0.968 k 0.050 I.101 f 0.050 0.038 + 0.123 O*OlOf 0.152
If:0.108 + 0.123 f 0.087 f 0.128 f 0.103
0.088 + 0.075
- 0.948 f 0.086 - 0.352 z!z0.077
- I.644 + 0.062 - I.237 + 0.075 - I.294 + 0.069 0.030 f 0.044 0.002 + 0.113
-
- I.331 -0.552 - 0.303 -0.531 - 0.267
Log median acceptance threshold f 2.575 S.E.
+ 0.165 + 0.166 dz0.183 f 0.175 + 0.242
*o*166 + 0.185 + 0.169 + 0.212 + 0.210
+ 0.195 + 0.191 + 0.241 + 0.161 + 0.234
4.933 * 0.220
4.921 + 0.231 5.043 + 0.153
5.632 5.109 5.080 4.574 4.724
4.910 4.716 5.087 4.606 4.525
5.042 5.056 4.972 5.128 4.947
a+ SE.?
+ * + + +
0.650 0.911 1.152 I.850 2.018
!c 0.875 f 1.881 t 1.450 * 1,069 f 1.080
+ I.065 ) 0.722 f 1.340 f 0.543 f I.380
7.492 f 1.069
6.968 c I.330 5.179 + 0.782
6.125 5.703 6.896 IO.769 5.794
5.963 9.922 8.421 4.652 4.131
4.735 3.822 7.221 3.306 5.832
b+S.E.t
I.322 0.538 0.307 0.493 0.276
3
I.639 I.218 1.282 0.006 0.046
0.079
- 0.959 - 0.344
-
- I.708 - 0:997 - 1.091 -0.123 - 0.105
-
TABLE ~-RESPONSE OF A. aegypti FEMALESTO SUGARSAND RELATEDCOMPOUNDS*
100
100
50
100 100 100 100 50
100 7.5 100 50 50
100 80 50 80 50
No. of tests
[
x
v,
glu-
glu-
0.120
I.023
0.240
0.771
O-213
0.555
Median acceptance threshold (M)
f 0.126
kO.088
- 0.922 + 0.283
O*OlOf 0.170
-0.619
-0*113
- 0.671+ 0.108
- 0.256 + 0.088
Log median acceptance threshold + 2.575 S.E.
4.684 + 0.220
4.365 + 0.220
4.804 + 0.204
4.948 + 0.232
5.030 + 0.232
4.915 * 0.194
a+ S.E.t
4.095 z!z0.849
4.972 A I.258
4.193 f 0.929
6.966 + 1.451
5.572 f 1.076
5.944 f 1.950
bkS.E.7
- 0.999
-0.118
- 0.666
-0.120
- 0.666
- 0.270
50
50
50
50
50
75
No. of tests
* The following compounds are non-stimulating at all concentrations : monosaccharides (n-L-glyceraldehyde, D-erythrose, u-ribose, L-rhamnose, u-a-glucoheptose), disaccharides (cellobiose), polysaccharides (glycogen), polyhydric alcohols (sorbitol, glycerol, dulcitol, mannitol), substituted sugars (gulonic lactone, glucose octoacetate, glucose-l-phosphate dipotassium and diacetone glucose). t The fourth, fifth, and sixth columns of this table give the calculated values for a, b and E in the equation Y = a + b(x - E), which is the regression of percentage of insects accepting or rejecting, Y expressed as probites, on log cont., X.
diphosphate (magnesium)
hydrochloride Glucose-6-phosphate (magnesium) a-n-Fructose, l-6
Substituted sugars : Glucosamine
coside
Inositol Glycosides : ol-D-Methyl coside P-n-Methyl
Polyhydric alcohols :
Compound
TABLE 1 (co&.)
8
2 2 a B 2
8
z
8
8 z
$ :! 8
$
4.786 4,978 5.062 4.663
- 0.204 - 0.365 - 0.466 -0.482
0.625 0.432 0.342 0.330
0.199 0.139 0.723
Na,S04 MgSC4 K&SO4 (NH&SC,
KBr K-I Na,HPO*
t See explanation in Table 1.
-0.702 f 0.178 -0.857 + 0.283 - 0.141 + 0.224
f 0.221 zk0.291 f 0.178 k 0.330
4.808 + 0.222 5.014 + 0.196 4.867 f 0.207
-0.182 f 0.193 - 0.402 f 0.252 -0-306 f 0.221
0.658 0.396 0.494
NaCH,COO KCH,COO LiCHaCOO
4.884 f 0.193 5.112 f 0.192 5.335 AI0.208
0.328 0.180 0.149
NaNO, KNO, NH,NO,
3.086 & 0.195 5.351 + 0.205 4.727 + 0.218
f 0.212 * 0.193 + 0.223 -I 0.203
+ 0.196 + 0.196 + 0.210 + 0.201 f 0.187 zk0.193
- 0.484 + 0.173 - 0.744 + 0.283 - 0.827 +_0.283
0.408 I.290 1.105 0.466 0.336 0,307
LiCl NaCl CaCl, MgC1, KC1 NH&l
4.978 4.722 4.760 5.152 4.811 5000
0.084 0.080 0.065
NaOH KOH Ca(CH) 2 - 0.389 + 0.283 0.1121kO.167 0.044 + 0.157 - 0.332 + 0.227 -0.473 +0*155 -0.513 50.134
0.070 0.075 0,044
HCl HN03 H,SCI 4.949 f 0,225 4.829 f 0.186 4.987 f 0.188
a + SE.7
- 1.074 + 0.201 - 1.098 + 0.348 -1~185~0~170
Log median rejection threshold + 2.575 SE.
5.176 + 0.230 5.169 rt:0.201 4.801 + 0.199
(M)
Median rejection threshold
-1*155 +0*188 -1.126+0*185 - 1.357 + 0.234
Compound
I?I0.502 + 0.750 + 0.670 + 0.520 f 0.701 +_0.764
* 0.600 k 0.475 f 0.652 f 0.919 2.874 z!z0.862 2.252 + 0.945 2.631 zk0.633
2.623 I.716 3.057 3.106
3.014 + 0.658 1.975 f 0.501 2.350 + 0.552
3.186 zk0.771 1.739 I 0,655 2.312 +_0.972
1.849 3.155 3.585 2.295 3.193 3.635
2.534 + I.111 1.485 -1:0.682 2.421 ‘I 0.694
3.294 + 1.169 2.873 I!I0.830 2.315 + 0.870
b of:S.E.7
0.286 0.378 0446 0.591 - 0.672 - 0.701 - 0.246
-
- 0.246 - 0.395 -0.362
- 0.520 - 0.680 -0.682
- 0.401 0.024 - 0.024 _ 0.398 - 0.533 - 0.514
- 1.093 - 1.213 - 1.191
- I.102 - 1.067 - 1.443
zt
5.03 7.19 1.39
1.60 2.32 2.92 3.03
1.52 2.53 2.02
3.05 5.56 6.71
2.45 0.78 0.90 2.15 2.98 3.26
11.91 12.50 15.39
14.29 13.33 22.73
Stimulating efficiency (1 /threshold)
TABLE ~--RESPONSESOF A. aegypti FEMALES TO ELECTROLYTES IN 0.1 M SUCROSE
50 50 50
50 50 50 50
50 50 50
60 50 50
50 50 50 50 50 60
50 50 60
50 50 50
No. of tests
E
F
x m
K a\
THEFUNCTION OFMOSQUITO TASTE RECEPTORS
1057
taste of some of them may be causing this stimulating action. The stimulating efficiency of the compounds tested show that the cations in a uniform anion combination stimulate in the order: Na+ < C!a++ < Mg++ < Li+ < K+ < NH,+ < H,O+, which is mostly correlated to their ionic mobilities. In a uniform cation combination, anions stimulate in the order: PO,--- < CHSCOO- < Cl- = SO,-- < Br- d NO,- < I- < OH-, which is less clear and empirical. These results agree with the data obtained for various insects (FRINGS, 1946; FRINGS and O’NEAL, 1946; HODGSON, 1951; DETHIER, 1955). As pointed out by OSTERHOUT et al. (1934) the order of effectiveness of salts is that of partition coefficients. Each electrolyte may have its distinctive taste to the mosquito but rejection cannot be explained upon this basis. It is suggested that the penetration through the receptor cells is according to the order of ionic mobility which must determine the stimulating action. The threshold value of a heavy metal salt like CuCl,, however, is very low (0*005 M), and this may be related to its reaction with or within the receptor cells. In view of this, BEIDLRR(1954) assumed that the magnitude of response in mammalian salt receptors is directly related to the number of ions or molecules reacting with the receptors. These results possibly indicate that rejection in Aedes is not a single modality. A complete understanding of the mechanism of stimulation has to await the application of electrophysiological techniques. In an attempt to determine the effect of increasing sugar concentration on the level of rejection of electrolytes, the threshold values were found to increase with increase of sucrose concentration. The factor of increase in acids and salts ranges from 2.5 to 4 with increase in sucrose concentration from 0.1 to 0.5 M. The inhibition caused by electrolytes in general can be reversed by increasing sucrose concentration or by removing the inhibitor. For instance, when the mosquito is offered a mixture of O-1 M sucrose and 1 M KCl, it rejects it, but when offered 0.1 M sucrose solution after this mixture, it responds positively. The agreement between these results and those of DETHIER(1953, 1955) suggests that there are at least two types of receptors involved in feeding of Aedes as in the blowfly, one for acceptable compounds and the other for unacceptable compounds. If the inhibitory stimulus is strong and the mediator weak, rejection occurs, but when the mediator is strong and the opposing stimulus weak, gorging takes place. Role of chemoreceptors in feeding behaviour The possible function of chemoreceptors on the labrum, labium, and within the cibarium of female A. aegypti and their role in aspiration were investigated by mounting individuals on wax blocks as already described. The mouth parts were unsheathed (MACGREGOR, 1930), with the labium being detached from the fascicle by means of a fine pin between the two. Capillary tubes containing various solutions were then placed over the labium or the fascicle or both. When distilled water or a 0.1 M sucrose solution is presented to the fascicle of thirsty or starved individuals, 17 per cent suck water and 28 per cent suck sucrose to repletion. If the capillary tube is withdrawn from such individuals when a minimal amount has
1058
H. S. SALAMA
been ingested and then restored, aspiration is resumed in most individuals. When the labrum was stimulated with a solution of sucrose and methyl cellulose (viscosity, 100 centipoises), all individuals failed to imbibe the solution through the labral canal. Thus it can be assumed that aspiration in the former case is due either to capillary action or to some external factor influencing the olfactory or thermal receptors when the central excitatory state of the mosquito is high. OWEN (1963), in similar experiments with Culiseta inornata, assumed that aspiration is spontaneous. In a second experiment, the fascicle and labium of each sex were stimulated with sucrose solution, and nearly all individuals gorged within a few seconds. The same was observed when the fascicles of thirsty individuals were stimulated with sucrose and the labium with water. Stimulation of the labrum of the female with human blood or blood corpuscles in suspension lead to gorging, whereas negligible response to presentation of this suspension to the labella and of sucrose to the fascicle was found. Ten per cent of males (3 out of 30) imbibed blood with their fascicles. In another series of experiments, stimulating the fascicle with a mixture of 0.1 M sucrose and 2 M NH&l, resulted in the aspiration of the liquid (20 per cent individuals) through the stylet food canal followed by rejection. The same occurs in 39 per cent of the insects on stimulating the whole proboscis with this mixture. Rejection probably occurs in these after the liquid has reached the cibarium, and this demonstrates the control of cibarial receptors over ingestion. The labella, however, seem to be capable to some extent of detecting NH,CI, judging by the reduction in the percentage of individuals which aspire the liquid through the stylet food canal. When the mosquitoes were offered a mixture of blood and NH&l through chick skin, they pierced, remained in this situation for a while, and then withdrew. This may indicate that NH&l is not detected by the labral sense organs, which come in direct contact with it on piercing, but the stimulation occurs after the blood has reached the cibarium. Control individuals, feeding on blood only, engorged rapidly. So it is probable that the labral sense organs are involved in tasting blood, which is further confirmed by their absence from male mosquitoes. Female mosquitoes with amputated labella showed a reduced sensitivity to sucrose. When the fascicle was amputated the sensitivity to blood was reduced, judging by the reduction in the percentage of individuals responding. This shows that the labral sense organs are probably sensitive to blood and aspiration cannot be due to a capillary action. HOSOI (1954) reported similar observations with C. pipiens, whereas OWEN (1963) indicated that the labrum in C. inornata is insensitive to blood or sugars. This work leads to the belief that the labral sense organs are involved in tasting blood. The fact that the labrum is still slightly sensitive to blood after partial amputation may be referred to the capillary transfer to the cibarium, which leads to cibarial contraction and aspiration. The labellar sense organs, on the other hand, are sensitive to water and sugars and may be capable of detecting unacceptable
THE FUNCTIONOF MOSQUITOTASTERECEPTORS
1059
compounds. The cibarial sensilla alone control ingestion and are stimulated by blood, sugars, and unacceptable compounds. From our work and other available information (FRINGS and HAMRUM, 1950; WALLIS, 1954; OT;VEN,1963), it can be concluded that the chemoreceptors on the tarsi, labella, labrum, and cibarium co-operate to accomplish feeding. In biting, the insect is normally attracted to a host, starts to pierce, sucks, and then the flow of blood will be controlled over the cibarial sensilla. Feeding on sugars and other solutions, the insect would not feed normally at any solution rejected by the tarsi. If the tarsi are incapable of discrimination at this stage, the sense organs on the labella will be, and, if not, the whole control will be passed to the cibarium. It has been observed that the mosquito spreads the labella on stimulation with solutions of some rejected compounds, but the flow through the food canal was controlled, as it reached the cibarium. It is suggested that compounds accepted by the labella will not necessarily be ingested as the cibarial sensilla exercise the final control over ingestion. Acknowledgements-1 would like to thank Professor B. HOCKING for his many valuable suggestions and advice throughout the work and for critically reading the manuscript. I am indebted to Dr. J. SHARPLIN and Dr. C. P. HICKMAN for useful comments on the draft manuscript and to Professor V. G. DETHIER for providing me with his publications. Many chemicals were supplied through the courtesy of Professors R. K. BROWN and J. S. COLTER of the Deparnnents of Chemistry and Biochemistry, University of Alberta. I wish to acknowledge the financial assistance of U.S. Army, Medical Research and Development Command Grant No. 63-G83 in support of this research. REFERENCES BEIDLERL. M. (1954) A theory of taste stimulation. g. gen. Physiol. 38, 133-139. BLISS C. I. (1938) The determination of the dosage mortality curve from small numbers. Quart.r. Pharm. 11, 192-216. CHRISTOPHERSS. R. (1960) Aedes aegypti (L.), the Yellow Fever Mosquito. Its Life History, Bionomics and Structure. Cambridge University Press, London. DAY M. E. (1954) The mechanism of food distribution to midgut or diverticula in the mosquito. Aust.J. biol. Sci. 7, 515-524. DEONIER C. C. (1938) Effects of some common poisons in sucrose solutions on the chemoreceptors of the housefly, lki’usca domestica (L.). J. econ. Ent. 31, 742-745. DETHIER V. G. (1953) Summation and inhibition following contralateral stimulation of the tarsal chemoreceptors of the blowfly. Biol. Bull., Woods Hole 105, 257-268. DETHIER V. G. (1955) The physiology and histology of the contact chemoreceptors of the blowfly. Quart. rev. BioE. 30, 348-371. DETHIER V. G., EVANS D. R., and RHOADE~ M. V. (1956) Some factors controlling the ingestion of carbohydrates by the blowfly. Biol. Bull., Woods Hole 111, 204-222. FEIR D., LENGY J. I., and OWEN W. B. (1961) Contact chemoreceptors in the mosquito, CuZiseta inornata (Williston), sensitivity of the tarsi and labella to sucrose and glucose. J. Insect Physiol. 6, 13-20. FRINGS H. (1946) Gustatory thresholds for sucrose and electrolytes for the cockroach, Periplaneta americana (Linn.). 3. exp. Zool. 102, 23-50. FRINGS H. and HAMRUM C. L. (1950) The contact chemoreceptors of adult yellow fever mosquitoes, Aedes aegypti (L.). J. N.Y. ent. Sot. 58, 133-142. FRINGS H. and O’NEAL. B. R. (1946) The loci and thresholds of contact chemoreceptors in females of the horsefly, Tabanus sulcifrons (Macq.). r. exp. Zool. 103, 61-79. VON FRISCH K. (1934) Uber den Geschmackssirm der Beine. 2. vergl. Physiol. 21, l-156.
1060
H.S.
SALAMA
VON GERNET G. (1965) The labral and cibarial sense organs and their innervation of some mosquitoes (unpublished). H&LINGER F. (1935) Faber den Geschmacksinn von Calliphora erythrocephulu (Meigen) und tiber die Verwertung von Zuckern und Zuckeralkoholen durch diese Fliege. 2. vergl. Physiol. 22, 614-640. HASSETT C. C., DETHIER V. G., and GANS J. (1950) A comparison of nutritive values and taste thresholds of carbohydrates for the blowfly. Biol. Bull., Woods Hole 99, 446-453. HODGSON E. S. (1951) Reaction thresholds of an aquatic beetle, Laccophilus maculosus Germ., to salts and alcohols. Physiol. Ziiol. 24, 131-140. HOSOI T. (1954) Mechanism enabling the mosquito to ingest blood into the stomach and sugary fluids into the oesophageal diverticula. Annot. x001. jap. 27, 82-90. HOSOI T. (1959) Identification of blood components which induce gorging of the mosquito. y. Insect ?hysiol. 3, 191-218. MACGREGOR M. E. (1930) The artificial feeding of mosquitoes by a new method which demonstrates certain functions of the diverticula. Trans. I?. Sot. trop. Med. Hyg. 33, 329-331. MILES P. W. (1958) Contact chemoreceptors in some Heteroptera including chemoreception internal to the stylet food canal. r. Insect Physiol. 2, 338-347. MINNICH D. E. (1931) The sensitivity of the. oral lobes of the proboscis of the blowfly, ,Calliphora vomitoria (Linn.) to various sugars. r. exp. Zool. 60, 121-139. OSTERHOUTW. J. V., KAMERING S.E., and STANLEYW. M. (1934) Kinetics ofpenetrationVII. Molecular versus ionic transport. r. gen. Physiol. 17, 469-480. OWEN W. B. (1961) The contact chemoreceptors of the mosquito, Culisetu inornatu (Williston), and their role in feeding (Abstract). Am. Zool. 1,466. OWEN W. B. (1963) The contact chemoreceptor organs of the mosquito and their function in feeding behaviour. y. Insect Physiol. 9, 73-87. ROBINSON G. G. (1939) The mouth parts and their function in the female mosquito, Anopheles maculipennis. Parasitology 31, 212-242. VOGEL B. (1931) uber die Beziehungen zwischen siissgeschmack und Niihrwert von Zuckern und Zuckeralkoholen bei der Honingbiene. 2. vergl. Physiol. 14, 273-347. WALLIS R. C. (1954) A study of oviposition activity of mosquitoes. Am.J. Hyg. 60,135-168. WEIS L. (1930) Versuche iiber die Geschmacksrezeption durch die Tarsan des Admirals, Pyyameis atalanta (L.). 2. vergl. Physiol. 12, 206-248. WYKES G. R. (1952) The preference of honeybees for solutions of various sugars which occur in nectar. J. exp. Biol. 29, 511-519.
THE FUNCTION
OF MOSQUITO
Printed in Great Btitain
TASTE RECEPTORS
H. S. SALAMA” Department
of Entomology,
University
of Alberta,
Edmonton,
Canada
(Received 2 April 1966)
Abstract-Two types of receptors seem to be involved in the feeding of Aedes aegypti. Threshold values of acceptance and rejection for 39 sugars and 25 electrolytes were measured. Most pentoses, hexoses, di- and tri-saccharides are stimulating. Compounds with a-linkage are superior stimuli. Slight molecular differences are reflected in stimulating effectiveness. Metabolic inhibitors at low concentrations do not affect stimulation, whereas CuCl, inhibits acceptance. Competitive inhibition occurs between some sugars. Rejection thresholds for acids and hydroxides are lower than those for neutral salts. The stimulating efficiency of the cations is mostly correlated to their ionic mobilities. Behavioural studies showed the sensitivity of the labral sense organs to blood. The labellar organs are sensitive to sugars and water, and may be capable of detecting unacceptable compounds. The cibarial sensilla alone control ingestion of liquids and are stimulated by blood, sugars, and unacceptable compounds. INTRODUCTION
ALTHOUGHcontact chemoreceptors have been investigated on the, external surface, including the mouth parts and tarsi of a few insects, little is yet known about chemoreception internal to the stylet food canal of sucking insects. The only evidence on this was reported by DAY (1954) wh o suggested that the sense organs in the buccal cavity of Aedes aegypti L. detect some sugars and other components of blood. MILES (1958) also gave evidence for two species of Hemiptera, but the sensilla were not identified. The work done with mosquitoes has been mainly concerned with the sensitivity of contact chemoreceptors on the tarsi, labella, and ligula to sucrose and a few other compounds (FRINGS and HAMRUM, 1950; WALLIS, 1954; HOSOI, 1959; FEIR et al., 1961; OWEN, 1961, 1963). The location and innervation of the labral and cibarial sensilla in a few mosquito species were reported by ROBINSON (1939), FRINGS and HAMRUM(1950), DAY (1954), CHRISTOPHERS(1960), and VON GERNET (unpublished). The present work deals mainly with the possible gustatory function of sensilla located in the cibarial pump of the mosquito A. aegypti L., and their role in the feeding behaviour in relation to the labral and labellar sensilla. The stimulator-y effects of a series of chemical compounds including carbohydrates and electrolytes have been investigated to determine the insect’s ability in discriminating between these compounds when sucked up through the food canal. * Present Egypt,
address:
Department
of Entomology,
U.A.R. 1051
Faculty
of Science,
Cairo
University,
H. S. SALAMA
1052
MATERIALS
AND
METHODS
The experimental mosquitoes used were 2 to 7 days old, from standard cultures fed on 5% sucrose and bred at 27°C and 70 to 80 per cent r.h. For testing the stimulatory effect of different compounds, the insects were anaesthetized with CO, and mounted on thin wax blocks sealed on a carton sheet. The thorax and wings were securely fastened by applying a fine hot needle to the surface of the wax close to the dorsal side of the insect. The individuals were satiated with water and left for 24 hr to recover completely after which they were checked again for water responsiveness. Solutions of the compounds to be tested were then presented in an ascending series from subliminal to supraliminal concentrations, in fine capillary tubes (1.5-2.5 mm i.d.) which fitted over the whole proboscis. The acceptability of a compound can be judged from the flow of the solution down the capillary tube and through the food canal, the pulsation frequency of the oesophageal pump, and the final abdominal distension. The compounds tested were presented as water solutions; for electrolytes a 0.1 M sucrose solution was used as a mediator for positive response. All the experiments were run at 25 to 28”C, with 2 to 3 hr intervals between successive trials. The percentage of insects feeding to repletion on each medium was recorded. Insects which fed were discarded. The most probable value of the logarithmic concentration accepted or rejected by 50 per cent of the insects was determined according to BLISS (1938). The scatter of the thresholds obtained with different individuals was found to be normal wkh respect to the logarithm of concentration. RESULTS
AND
DISCUSSION
Response to carbohydrates The response of various insects to carbohydrates (WEIS, 1930 ; MINNICH, 193 1; VOGEL, 1931; VON FRISCH, 1934; H&LINGER, 1935; HASSETTet al., 1950; DETHIER, 1955) showed a spectrum of activity from complete unresponsiveness to extreme sensitivity. The stimulating effectiveness of 39 sugars and related compounds to A. aegypti was investigated, and the median acceptance thresholds are given in Table 1. Trioses, tetroses, and heptoses are ineffective. All the tested pentoses, hexoses, and di- and tri-saccharides are stimulating with few exceptions. In the polysaccharides, xylan is weakly stimulating. Inositol is the only stimulating polyhydric alcohol. Considering the median acceptance thresholds for sugars tested, the order of stimulating effectiveness is as follows: Monosaccharides : D-fructose > L-fucose > L-sorbose > D-&COW? > D-arabinose > D-xylose > L-arabinose > L-xylose > D-galactose > D-mannose. Disaccharides: Sucrose > maltose = turanose > lactose = melibiose. Trisaccharides: Melezitose > rafKnose. The effectiveness of pentoses and hexoses in the monosaccharides possibly indicates a certain chain length within which the compounds of this group can be stimulating. There is a great difference in the effectiveness of hexoses, such as D-glUCOSeand D-galaCtOSe,D-glUCOSeand D-mannose, despite the slight differences
THE FUNCTION
OF MOSQUITO
TASTE RECEPTORS
1053
Mannose and galactose, however, are equally stimulating, in their configuration. although they differ more in their structural configuration. Compounds with or-linkage are superior stimuli like mannose, turanose, and sucrose. Lactose, melibiose, and cellobiose, which lack the a-linkage, are either weakly or non-stimulating. n-Arabinose and n-xylose are more stimulating than the respective L-forms. U-Methyl glucoside is also a superior stimulus to the /3form. So, the structure of the molecule shows it to be effective in stimulation. DETHIER (1955) showed the significance of the sugar structural configuration in stimulation of the blowfly. VON FRISCH (1934) and WYKES (1952), however, concluded that some biological specificity may be involved in stimulation of the honey bee by sugars. Substitution in the sugar molecule may change its stimulating characteristic as with sucrose octoacetate, diacetone glucose, and gulonic lactone. Glucosamine hydrochloride, however, is approximately as stimulating as glucose. Of the sugar phosphates, n-fructose-1-6-phosphate (magnesium) is effective. DETHIER (1955) obtained similar results for the blowfly, and he assumed that stimulation by sugars does not involve phosphorylytic reactions. Factors afecting stimulation by sugars Competitive inhibition occurs between some sugars in A. aegypti causing an increase in the acceptance threshold of the more stimulating component. For instance, mannose at 0.5 M inhibits glucose, fructose, or sucrose, causing the threshold values to be elevated by factors of 8, 6, and 5, respectively. L-Rhamnose (non-stimulating) also inhibits both fructose and sucrose, but this is probably a rejection effect. The inhibitory effect caused may be due to unequivalent stimuli of both sugars in the mixture, one inducing stimulation and the other interfering with its action, until a certain concentration level is reached, where the more stimulating component overcomes the other one. This may also suggest that there are different sites of action on the sugar receptor as in the blowfly (DETHIER et al., 1956). There is no evidence for synergism or an additive effect. There is no inhibition caused by low concentrations of the metabolic inhibitors cyanide, azide, and fluoride on stimulation with sucrose. This indicates that the mechanism of stimulation does not involve any steps in the glydolytic cycle as in the blowfly (DETHIER, 1955). Heavy metal salts such as CuCl, at’O.005 to O-01 M, on the other hand, inhibit stimulation with sucrose and render the oral receptors insensitive for about 4 hr. This may be related to the reaction of such heavy salts with or within the receptor cells. The housefly and blowfly also, reacted similarly to such salts (DEONIER, 1938; DETHIER, 1955). Response to electrolytes A. aegypti rejects sucrose to which electrolytes have been added. The rejection threshold values illustrated in Table 2 show a higher stimulating effectiveness for either acids or hydroxides than for neutral inorganic salts. There is a response to some chlorides (NaCl and CaCl,) above the ordinary physiological range. Since the mosquito feeds on blood which naturally contains a variety of inorganic salts, the
0.047 0.281 0.497 0.294 0.541
0.020 0.108 0.079 0.916 1.023
0.023 0.058 0.051 1.081 I*005
0.113 0.445
1.224
Hexoses : D-Fructose D-Glucose L-Sorbose D-Calactose D-Mannose
Disaccharides : Sucrose Turanose D-Maltose Melibiose Lactose
Trisaccharides : Melezitose Ralhnose
Polysaccharides : Xylan
Median acceptance threshold (M)
Pentoses : L-Fucose D-Arabinose L-Arabinose D-XylOSe L-Xylose
Compound
1.693 f 0.069 0.968 k 0.050 I.101 f 0.050 0.038 + 0.123 O*OlOf 0.152
If:0.108 + 0.123 f 0.087 f 0.128 f 0.103
0.088 + 0.075
- 0.948 f 0.086 - 0.352 z!z0.077
- I.644 + 0.062 - I.237 + 0.075 - I.294 + 0.069 0.030 f 0.044 0.002 + 0.113
-
- I.331 -0.552 - 0.303 -0.531 - 0.267
Log median acceptance threshold f 2.575 S.E.
+ 0.165 + 0.166 dz0.183 f 0.175 + 0.242
*o*166 + 0.185 + 0.169 + 0.212 + 0.210
+ 0.195 + 0.191 + 0.241 + 0.161 + 0.234
4.933 * 0.220
4.921 + 0.231 5.043 + 0.153
5.632 5.109 5.080 4.574 4.724
4.910 4.716 5.087 4.606 4.525
5.042 5.056 4.972 5.128 4.947
a+ SE.?
+ * + + +
0.650 0.911 1.152 I.850 2.018
!c 0.875 f 1.881 t 1.450 * 1,069 f 1.080
+ I.065 ) 0.722 f 1.340 f 0.543 f I.380
7.492 f 1.069
6.968 c I.330 5.179 + 0.782
6.125 5.703 6.896 IO.769 5.794
5.963 9.922 8.421 4.652 4.131
4.735 3.822 7.221 3.306 5.832
b+S.E.t
I.322 0.538 0.307 0.493 0.276
3
I.639 I.218 1.282 0.006 0.046
0.079
- 0.959 - 0.344
-
- I.708 - 0:997 - 1.091 -0.123 - 0.105
-
TABLE ~-RESPONSE OF A. aegypti FEMALESTO SUGARSAND RELATEDCOMPOUNDS*
100
100
50
100 100 100 100 50
100 7.5 100 50 50
100 80 50 80 50
No. of tests
[
x
v,
glu-
glu-
0.120
I.023
0.240
0.771
O-213
0.555
Median acceptance threshold (M)
f 0.126
kO.088
- 0.922 + 0.283
O*OlOf 0.170
-0.619
-0*113
- 0.671+ 0.108
- 0.256 + 0.088
Log median acceptance threshold + 2.575 S.E.
4.684 + 0.220
4.365 + 0.220
4.804 + 0.204
4.948 + 0.232
5.030 + 0.232
4.915 * 0.194
a+ S.E.t
4.095 z!z0.849
4.972 A I.258
4.193 f 0.929
6.966 + 1.451
5.572 f 1.076
5.944 f 1.950
bkS.E.7
- 0.999
-0.118
- 0.666
-0.120
- 0.666
- 0.270
50
50
50
50
50
75
No. of tests
* The following compounds are non-stimulating at all concentrations : monosaccharides (n-L-glyceraldehyde, D-erythrose, u-ribose, L-rhamnose, u-a-glucoheptose), disaccharides (cellobiose), polysaccharides (glycogen), polyhydric alcohols (sorbitol, glycerol, dulcitol, mannitol), substituted sugars (gulonic lactone, glucose octoacetate, glucose-l-phosphate dipotassium and diacetone glucose). t The fourth, fifth, and sixth columns of this table give the calculated values for a, b and E in the equation Y = a + b(x - E), which is the regression of percentage of insects accepting or rejecting, Y expressed as probites, on log cont., X.
diphosphate (magnesium)
hydrochloride Glucose-6-phosphate (magnesium) a-n-Fructose, l-6
Substituted sugars : Glucosamine
coside
Inositol Glycosides : ol-D-Methyl coside P-n-Methyl
Polyhydric alcohols :
Compound
TABLE 1 (co&.)
8
2 2 a B 2
8
z
8
8 z
$ :! 8
$
4.786 4,978 5.062 4.663
- 0.204 - 0.365 - 0.466 -0.482
0.625 0.432 0.342 0.330
0.199 0.139 0.723
Na,S04 MgSC4 K&SO4 (NH&SC,
KBr K-I Na,HPO*
t See explanation in Table 1.
-0.702 f 0.178 -0.857 + 0.283 - 0.141 + 0.224
f 0.221 zk0.291 f 0.178 k 0.330
4.808 + 0.222 5.014 + 0.196 4.867 f 0.207
-0.182 f 0.193 - 0.402 f 0.252 -0-306 f 0.221
0.658 0.396 0.494
NaCH,COO KCH,COO LiCHaCOO
4.884 f 0.193 5.112 f 0.192 5.335 AI0.208
0.328 0.180 0.149
NaNO, KNO, NH,NO,
3.086 & 0.195 5.351 + 0.205 4.727 + 0.218
f 0.212 * 0.193 + 0.223 -I 0.203
+ 0.196 + 0.196 + 0.210 + 0.201 f 0.187 zk0.193
- 0.484 + 0.173 - 0.744 + 0.283 - 0.827 +_0.283
0.408 I.290 1.105 0.466 0.336 0,307
LiCl NaCl CaCl, MgC1, KC1 NH&l
4.978 4.722 4.760 5.152 4.811 5000
0.084 0.080 0.065
NaOH KOH Ca(CH) 2 - 0.389 + 0.283 0.1121kO.167 0.044 + 0.157 - 0.332 + 0.227 -0.473 +0*155 -0.513 50.134
0.070 0.075 0,044
HCl HN03 H,SCI 4.949 f 0,225 4.829 f 0.186 4.987 f 0.188
a + SE.7
- 1.074 + 0.201 - 1.098 + 0.348 -1~185~0~170
Log median rejection threshold + 2.575 SE.
5.176 + 0.230 5.169 rt:0.201 4.801 + 0.199
(M)
Median rejection threshold
-1*155 +0*188 -1.126+0*185 - 1.357 + 0.234
Compound
I?I0.502 + 0.750 + 0.670 + 0.520 f 0.701 +_0.764
* 0.600 k 0.475 f 0.652 f 0.919 2.874 z!z0.862 2.252 + 0.945 2.631 zk0.633
2.623 I.716 3.057 3.106
3.014 + 0.658 1.975 f 0.501 2.350 + 0.552
3.186 zk0.771 1.739 I 0,655 2.312 +_0.972
1.849 3.155 3.585 2.295 3.193 3.635
2.534 + I.111 1.485 -1:0.682 2.421 ‘I 0.694
3.294 + 1.169 2.873 I!I0.830 2.315 + 0.870
b of:S.E.7
0.286 0.378 0446 0.591 - 0.672 - 0.701 - 0.246
-
- 0.246 - 0.395 -0.362
- 0.520 - 0.680 -0.682
- 0.401 0.024 - 0.024 _ 0.398 - 0.533 - 0.514
- 1.093 - 1.213 - 1.191
- I.102 - 1.067 - 1.443
zt
5.03 7.19 1.39
1.60 2.32 2.92 3.03
1.52 2.53 2.02
3.05 5.56 6.71
2.45 0.78 0.90 2.15 2.98 3.26
11.91 12.50 15.39
14.29 13.33 22.73
Stimulating efficiency (1 /threshold)
TABLE ~--RESPONSESOF A. aegypti FEMALES TO ELECTROLYTES IN 0.1 M SUCROSE
50 50 50
50 50 50 50
50 50 50
60 50 50
50 50 50 50 50 60
50 50 60
50 50 50
No. of tests
E
F
x m
K a\
THEFUNCTION OFMOSQUITO TASTE RECEPTORS
1057
taste of some of them may be causing this stimulating action. The stimulating efficiency of the compounds tested show that the cations in a uniform anion combination stimulate in the order: Na+ < C!a++ < Mg++ < Li+ < K+ < NH,+ < H,O+, which is mostly correlated to their ionic mobilities. In a uniform cation combination, anions stimulate in the order: PO,--- < CHSCOO- < Cl- = SO,-- < Br- d NO,- < I- < OH-, which is less clear and empirical. These results agree with the data obtained for various insects (FRINGS, 1946; FRINGS and O’NEAL, 1946; HODGSON, 1951; DETHIER, 1955). As pointed out by OSTERHOUT et al. (1934) the order of effectiveness of salts is that of partition coefficients. Each electrolyte may have its distinctive taste to the mosquito but rejection cannot be explained upon this basis. It is suggested that the penetration through the receptor cells is according to the order of ionic mobility which must determine the stimulating action. The threshold value of a heavy metal salt like CuCl,, however, is very low (0*005 M), and this may be related to its reaction with or within the receptor cells. In view of this, BEIDLRR(1954) assumed that the magnitude of response in mammalian salt receptors is directly related to the number of ions or molecules reacting with the receptors. These results possibly indicate that rejection in Aedes is not a single modality. A complete understanding of the mechanism of stimulation has to await the application of electrophysiological techniques. In an attempt to determine the effect of increasing sugar concentration on the level of rejection of electrolytes, the threshold values were found to increase with increase of sucrose concentration. The factor of increase in acids and salts ranges from 2.5 to 4 with increase in sucrose concentration from 0.1 to 0.5 M. The inhibition caused by electrolytes in general can be reversed by increasing sucrose concentration or by removing the inhibitor. For instance, when the mosquito is offered a mixture of O-1 M sucrose and 1 M KCl, it rejects it, but when offered 0.1 M sucrose solution after this mixture, it responds positively. The agreement between these results and those of DETHIER(1953, 1955) suggests that there are at least two types of receptors involved in feeding of Aedes as in the blowfly, one for acceptable compounds and the other for unacceptable compounds. If the inhibitory stimulus is strong and the mediator weak, rejection occurs, but when the mediator is strong and the opposing stimulus weak, gorging takes place. Role of chemoreceptors in feeding behaviour The possible function of chemoreceptors on the labrum, labium, and within the cibarium of female A. aegypti and their role in aspiration were investigated by mounting individuals on wax blocks as already described. The mouth parts were unsheathed (MACGREGOR, 1930), with the labium being detached from the fascicle by means of a fine pin between the two. Capillary tubes containing various solutions were then placed over the labium or the fascicle or both. When distilled water or a 0.1 M sucrose solution is presented to the fascicle of thirsty or starved individuals, 17 per cent suck water and 28 per cent suck sucrose to repletion. If the capillary tube is withdrawn from such individuals when a minimal amount has
1058
H. S. SALAMA
been ingested and then restored, aspiration is resumed in most individuals. When the labrum was stimulated with a solution of sucrose and methyl cellulose (viscosity, 100 centipoises), all individuals failed to imbibe the solution through the labral canal. Thus it can be assumed that aspiration in the former case is due either to capillary action or to some external factor influencing the olfactory or thermal receptors when the central excitatory state of the mosquito is high. OWEN (1963), in similar experiments with Culiseta inornata, assumed that aspiration is spontaneous. In a second experiment, the fascicle and labium of each sex were stimulated with sucrose solution, and nearly all individuals gorged within a few seconds. The same was observed when the fascicles of thirsty individuals were stimulated with sucrose and the labium with water. Stimulation of the labrum of the female with human blood or blood corpuscles in suspension lead to gorging, whereas negligible response to presentation of this suspension to the labella and of sucrose to the fascicle was found. Ten per cent of males (3 out of 30) imbibed blood with their fascicles. In another series of experiments, stimulating the fascicle with a mixture of 0.1 M sucrose and 2 M NH&l, resulted in the aspiration of the liquid (20 per cent individuals) through the stylet food canal followed by rejection. The same occurs in 39 per cent of the insects on stimulating the whole proboscis with this mixture. Rejection probably occurs in these after the liquid has reached the cibarium, and this demonstrates the control of cibarial receptors over ingestion. The labella, however, seem to be capable to some extent of detecting NH,CI, judging by the reduction in the percentage of individuals which aspire the liquid through the stylet food canal. When the mosquitoes were offered a mixture of blood and NH&l through chick skin, they pierced, remained in this situation for a while, and then withdrew. This may indicate that NH&l is not detected by the labral sense organs, which come in direct contact with it on piercing, but the stimulation occurs after the blood has reached the cibarium. Control individuals, feeding on blood only, engorged rapidly. So it is probable that the labral sense organs are involved in tasting blood, which is further confirmed by their absence from male mosquitoes. Female mosquitoes with amputated labella showed a reduced sensitivity to sucrose. When the fascicle was amputated the sensitivity to blood was reduced, judging by the reduction in the percentage of individuals responding. This shows that the labral sense organs are probably sensitive to blood and aspiration cannot be due to a capillary action. HOSOI (1954) reported similar observations with C. pipiens, whereas OWEN (1963) indicated that the labrum in C. inornata is insensitive to blood or sugars. This work leads to the belief that the labral sense organs are involved in tasting blood. The fact that the labrum is still slightly sensitive to blood after partial amputation may be referred to the capillary transfer to the cibarium, which leads to cibarial contraction and aspiration. The labellar sense organs, on the other hand, are sensitive to water and sugars and may be capable of detecting unacceptable
THE FUNCTIONOF MOSQUITOTASTERECEPTORS
1059
compounds. The cibarial sensilla alone control ingestion and are stimulated by blood, sugars, and unacceptable compounds. From our work and other available information (FRINGS and HAMRUM, 1950; WALLIS, 1954; OT;VEN,1963), it can be concluded that the chemoreceptors on the tarsi, labella, labrum, and cibarium co-operate to accomplish feeding. In biting, the insect is normally attracted to a host, starts to pierce, sucks, and then the flow of blood will be controlled over the cibarial sensilla. Feeding on sugars and other solutions, the insect would not feed normally at any solution rejected by the tarsi. If the tarsi are incapable of discrimination at this stage, the sense organs on the labella will be, and, if not, the whole control will be passed to the cibarium. It has been observed that the mosquito spreads the labella on stimulation with solutions of some rejected compounds, but the flow through the food canal was controlled, as it reached the cibarium. It is suggested that compounds accepted by the labella will not necessarily be ingested as the cibarial sensilla exercise the final control over ingestion. Acknowledgements-1 would like to thank Professor B. HOCKING for his many valuable suggestions and advice throughout the work and for critically reading the manuscript. I am indebted to Dr. J. SHARPLIN and Dr. C. P. HICKMAN for useful comments on the draft manuscript and to Professor V. G. DETHIER for providing me with his publications. Many chemicals were supplied through the courtesy of Professors R. K. BROWN and J. S. COLTER of the Deparnnents of Chemistry and Biochemistry, University of Alberta. I wish to acknowledge the financial assistance of U.S. Army, Medical Research and Development Command Grant No. 63-G83 in support of this research. REFERENCES BEIDLERL. M. (1954) A theory of taste stimulation. g. gen. Physiol. 38, 133-139. BLISS C. I. (1938) The determination of the dosage mortality curve from small numbers. Quart.r. Pharm. 11, 192-216. CHRISTOPHERSS. R. (1960) Aedes aegypti (L.), the Yellow Fever Mosquito. Its Life History, Bionomics and Structure. Cambridge University Press, London. DAY M. E. (1954) The mechanism of food distribution to midgut or diverticula in the mosquito. Aust.J. biol. Sci. 7, 515-524. DEONIER C. C. (1938) Effects of some common poisons in sucrose solutions on the chemoreceptors of the housefly, lki’usca domestica (L.). J. econ. Ent. 31, 742-745. DETHIER V. G. (1953) Summation and inhibition following contralateral stimulation of the tarsal chemoreceptors of the blowfly. Biol. Bull., Woods Hole 105, 257-268. DETHIER V. G. (1955) The physiology and histology of the contact chemoreceptors of the blowfly. Quart. rev. BioE. 30, 348-371. DETHIER V. G., EVANS D. R., and RHOADE~ M. V. (1956) Some factors controlling the ingestion of carbohydrates by the blowfly. Biol. Bull., Woods Hole 111, 204-222. FEIR D., LENGY J. I., and OWEN W. B. (1961) Contact chemoreceptors in the mosquito, CuZiseta inornata (Williston), sensitivity of the tarsi and labella to sucrose and glucose. J. Insect Physiol. 6, 13-20. FRINGS H. (1946) Gustatory thresholds for sucrose and electrolytes for the cockroach, Periplaneta americana (Linn.). 3. exp. Zool. 102, 23-50. FRINGS H. and HAMRUM C. L. (1950) The contact chemoreceptors of adult yellow fever mosquitoes, Aedes aegypti (L.). J. N.Y. ent. Sot. 58, 133-142. FRINGS H. and O’NEAL. B. R. (1946) The loci and thresholds of contact chemoreceptors in females of the horsefly, Tabanus sulcifrons (Macq.). r. exp. Zool. 103, 61-79. VON FRISCH K. (1934) Uber den Geschmackssirm der Beine. 2. vergl. Physiol. 21, l-156.
1060
H.S.
SALAMA
VON GERNET G. (1965) The labral and cibarial sense organs and their innervation of some mosquitoes (unpublished). H&LINGER F. (1935) Faber den Geschmacksinn von Calliphora erythrocephulu (Meigen) und tiber die Verwertung von Zuckern und Zuckeralkoholen durch diese Fliege. 2. vergl. Physiol. 22, 614-640. HASSETT C. C., DETHIER V. G., and GANS J. (1950) A comparison of nutritive values and taste thresholds of carbohydrates for the blowfly. Biol. Bull., Woods Hole 99, 446-453. HODGSON E. S. (1951) Reaction thresholds of an aquatic beetle, Laccophilus maculosus Germ., to salts and alcohols. Physiol. Ziiol. 24, 131-140. HOSOI T. (1954) Mechanism enabling the mosquito to ingest blood into the stomach and sugary fluids into the oesophageal diverticula. Annot. x001. jap. 27, 82-90. HOSOI T. (1959) Identification of blood components which induce gorging of the mosquito. y. Insect ?hysiol. 3, 191-218. MACGREGOR M. E. (1930) The artificial feeding of mosquitoes by a new method which demonstrates certain functions of the diverticula. Trans. I?. Sot. trop. Med. Hyg. 33, 329-331. MILES P. W. (1958) Contact chemoreceptors in some Heteroptera including chemoreception internal to the stylet food canal. r. Insect Physiol. 2, 338-347. MINNICH D. E. (1931) The sensitivity of the. oral lobes of the proboscis of the blowfly, ,Calliphora vomitoria (Linn.) to various sugars. r. exp. Zool. 60, 121-139. OSTERHOUTW. J. V., KAMERING S.E., and STANLEYW. M. (1934) Kinetics ofpenetrationVII. Molecular versus ionic transport. r. gen. Physiol. 17, 469-480. OWEN W. B. (1961) The contact chemoreceptors of the mosquito, Culisetu inornatu (Williston), and their role in feeding (Abstract). Am. Zool. 1,466. OWEN W. B. (1963) The contact chemoreceptor organs of the mosquito and their function in feeding behaviour. y. Insect Physiol. 9, 73-87. ROBINSON G. G. (1939) The mouth parts and their function in the female mosquito, Anopheles maculipennis. Parasitology 31, 212-242. VOGEL B. (1931) uber die Beziehungen zwischen siissgeschmack und Niihrwert von Zuckern und Zuckeralkoholen bei der Honingbiene. 2. vergl. Physiol. 14, 273-347. WALLIS R. C. (1954) A study of oviposition activity of mosquitoes. Am.J. Hyg. 60,135-168. WEIS L. (1930) Versuche iiber die Geschmacksrezeption durch die Tarsan des Admirals, Pyyameis atalanta (L.). 2. vergl. Physiol. 12, 206-248. WYKES G. R. (1952) The preference of honeybees for solutions of various sugars which occur in nectar. J. exp. Biol. 29, 511-519.