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The control of food flow in a society of the ant Myrmica rubra L
Anim . Behav., 1977,25, 1047-1055
THE CONTROL OF FOOD FLOW IN A SOCIETY OF THE ANT MYRMICA RUBRA L . BY
M . V. BRIAN & A. ABBOTT
Institute of Terrestrial Ecology, Furzebrook Research Station, Wareham, Dorset
Abstract. The control of prey, thin syrup and water flow, through a society of Myrmica was studied . Larval intake increases if they are deprived of prey, but not if they are deprived of water or sugar . Deprivation causes them to take prey juices from workers and they get more if the workers themselves have also been deprived ; this is because such workers over-collect and readily pass on their surplus . Even well-fed larvae will take prey juices from these surfeited workers ; they will also take sugary fluids but not water. The head of a larva elicits some food collection by workers even if it is immobile, but the real cause of food flow towards larvae is their ability to absorb and assimilate the prey juices which they can obtain from workers . Starved nurse workers can obtain prey and water from foragers but a reverse flow does not occur ; only thin syrup is exchanged freely between workers . Myrmica rubra L. (=M. laevinodis Nyl.) workers identify larvae by means of their surface chemistry, hairiness and rotundity ; their movement and possession of a head are not used (Brian 1975a) . Workers that tend larvae (which are supine) collect more food than workers that have none (Brian 1973) . This paper reports an investigation of the causal chain relating the presence of larvae to the food collected by foragers . The passage of food through ant societies was discussed by Sudd (1967) . Sugars pass quickly from worker to worker ; proteins, either crude or prepared and concentrated, go to the larvae and the queens . Larvae are known to stimulate prey collection even when the adults are mainly nectarivorus (Haskins & Haskins 1950 ; Vowles 1955) . LeMasne (1953) and Vowles (1955) suggested that it was the movement of larvae that stimulated workers to collect prey, and Schneirla (cited Vowles 1955) produced evidence that this was so when he stopped Eciton workers foraging by killing their larvae . Such larvae, though indubitably still, were unfortunately unable to eat and an important part of the stimulus may have been absent. LeMasne (1953) moreover noticed that workers in the course of licking and generally manipulating larvae gave food to those that extended their heads in line with their body axis . In this position the mouth lies in front at the apex of the cone formed by the thorax and head . Quiescent larvae often responded to the proximity of workers by adopting this pose and workers then found their mouths and passed them regurgitated food juices . Not all larvae were receptive ; quite clearly they have an internal state which is refractory to feeding. Sudd (1967)
pointed out that it is not necessarily the movement which releases regurgitation by the workers, it might simply be that the mouth is made more accessible to them . Before feeding a larva workers usually place their fore-tarsi on opposite sides of the head and their mid- and hind-tarsi on the ventral thoracic segments (the scratches caused by their claws can easily be shown with silver nitrate). There is thus a characteristic feeding stance which is only possible if the larva extends its head whilst lying on its back . After the experimental methods have been described three series of trials will be detailed . The first concerns the passage of different types of food from workers to larvae in the presence or absence of queens . The second analyses the stimuli used by larvae to obtain prey juices from workers . The third concerns whether the nurses collect food directly from the source or use the foragers as agents. General Methods Myrmica rubra were cultured in the laboratory
as described in Brian (1973, 1975a), using subsamples of large colonies in spring condition . Three foods were offered : crushed Drosophila flies (prey), 5 % sucrose in water (thin syrup), and de-ionized water (water) ; the sugar solution must not exceed 10 %, otherwise workers are liable to regurgitate it on to the nest floor (Brian 1973) . Each of the ant categories (thirdstage larvae, workers or queens) were pretreated for a week at 20 C and 100 % relative humidity by depriving them entirely of one of these three foods (never more than one at a time) . Water tests needed only 3 days pretreatment. Afterwards all possible combinations 1047
1 048
ANIMAL BEHAVIOUR, 25,
of pretreated and untreated ant categories were set in a factorial design with both successive and simultaneous replication. This did not always necessitate separate nest containers, for larvae of different pre-treatments can be distinctly marked by cuticle punctures that leave black scars, and foragers can be distinguished from nurses after removing an epinotal spine . Cohabitation of differently treated ant classes reduces error variance . Foragers were separated from other workers whilst collecting food in an open illuminated arena. The weaker attachment of foragers to brood was also used : a slight disturbance brings them out of the brood chamber . Food uptake during an experiment was assessed after 24 h, using the degree of penetration of vital dyes and the consumption of flies and syrup. The best dyes are neutral red and brilliant cresyl blue ; crystals can be dissolved in thin syrup or sprinkled on fresh fly corpses . Both these vital dyes are quite harmless in the strengths needed to give strong coloration ; they stay in the urate cells of the fat body where they can be seen through the cuticle in larvae, but workers have to be dissected. To avoid any possibility of their confusing treatment influences they were used alternately . Three degrees of penetration were distinguished : the dye was entirely absent, it was only in the crop, or it was present in the crop, the mid-gut and throughout the body cavity. Whereas the second degree shows that the food was received and perhaps
4
passed on, the third degree shows that it was absorbed. In an experiment on the larval stimulation of workers some larvae were beheaded by ligature with steel wire ; others were deprived of their cerebral ganglion (both as in Brian 1974a). In such cases, since the larvae cannot eat uptake was measured as numbers or weight consumed . Thus for prey, the number of Drosophila flies eaten each day out of the surplus of fresh ones given was counted . For syrup, the weight of 5 sucrose solution drunk was obtained from the loss in weight of the syrup tube less an allowance for evaporation ; this was assessed on each syrup tube individually (as in Brian 1973) . Flow Between Larvae and Adults The first aim was to find what effect starvation had on food uptake and distribution in a typical composite group of Myrmica. Foods were tested in the order: prey, syrup, water, with intervals of 2 weeks for recovery on full diet . Two pre-treatment levels (fed and unfed) of the three ant categories (larva, workers and queen) were set in all 23 = 8 combinations and replicated twice . When possible, 2 queens, 10 workers and 15 larvae were examined for dye but deaths sometimes prevented the full number being examined . Results were tested for homogeneity using the usual x2 method for each food and for pooled data (Table I). There can be no doubt that workers and queens both react strongly to water
Table I. The Number of Individuals (Workers, Queens or Larvae) That Took Food (Prey, Syrup or Water) After a Pretreatment Either With or Without That Food. The Total Numbers Used are Also Given . X2 Tests for Homogeneity are Given and One for Interaction . Workers (W) Food type Prey
Fed Total Fed Total
Unfed
Fed
Unfed
Fed
Unfed
50 80
68 80
2 16
5 16
32 69
22 68
10 . 5** 79 80
X2 (1 df)
Water
Fed Total Fed Total x2 (3 df)
NS
78 80
8 12
NS
8 80
68 75
137 240
11 12
2 15
214 235 106. 7***
Level of significance : *P<0. 05 ; **P<0 .01 ; ***P<0.001 .
NS
29 40
31 39 6 112
6.68**
NS
1 107
69 220
52 215 6.84
46 80
30 75 4. 78*
NS
23 38 10 . 6*
(Q x W)
NS
7 10
12 43
Interaction
NS
NS
96 .2***
x2 (1 df)
Sum
Larvae (L)
Fed
x2 (1 df)
Syrup
Queens (Q)
(Ns)
BRIAN & ABBOTT : CONTROL OF FOOD FLOW IN MYRMICA RUBRA
1049
Table IL The nmaeber of Lrvae That Took Either Prey, Syrup or Water After Either They or Their Atta chat Workers had boa Fed or Not Fed with It Befombsnd . X2 Homogeneity Tests are Given and SkwNka ee Indicated as In Table I Worker pre-treatment Food type Prey
Fed Total
Fed
Unfed
Fed
Un-fed
18 58
36 52
18 54
36 56
14.6***
x2
Syrup
Fed Total
16 42
Fed Total
9 . 32** 24 44
21 43
2 . 29 NS
x2
Water
Larval pre-treatment
x2
loss and increase their replacement rates . For workers this is also true of prey, but not of thin syrup which they drink abundantly, whether deprived or not . Queens show signs of increased uptake of prey, syrup and water after deprivation, but the figures taken together only just give statistical significance (P<0.05) . Uptake by larvae was not affected at all by pre-treatment, indeed there are signs that, the more they have, the more they can take. Their growth at this stage is complicated by a tendency to become dormant if unfed and by the presence of queens which stimulate worker attention to them . One interaction is statistically significant (P<0 .05) : the reaction of workers to a water deficit is stronger if their queens are also deprived . The figures are : with saturated queens the number of workers absorbing water rises from 8/40 (saturated) to 30/35 (unsaturated) whereas if the queens are unsaturated the corresponding worker frequencies are 0/40 and 38/40. The nutritive state of queens thus has a substantial effect on worker behaviour. The next experiment was designed simply to clarify the reaction of larvae to starvation ; no queens were present . Four combinations of workers, fed or unfed and larvae fed or unfed were used. This time only the larvae were investigated for dye penetration and the results of several replicates were pooled (Table II) . They show that there is a strong prey effect but no syrup or water effects . Thus there is no doubt in this experiment, unlike that above, that starving larvae of prey increases their uptake when prey are later made available. Since workers supply larvae with less food when they are queenless, it is likely that they respond more to starved ones . Neither syrup nor water produce dif-
0. 187 NS 7 116
4 116 0 .861 NS
19 43
5 116
6 116 0.096 Ns
ferences but it is evident that sugar makes water more acceptable to larvae : less than 5 % took plain water, compared with 50 % that took 5 sucrose solution . Notice the very significant increase that unfed workers cause to larval prey consumption twice as many larvae received food . This presumably means that these workers over-collect and share their surplus with larvae . There are also signs of this being the case for syrup and water though the figures are not statistically significant ;. again it is clear that larvae drink very little plain water . It can be asked whether the extra larval intake induced by unfed workers goes to unfed larvae or to well-fed ones . The relevant figures are as follows : if both workers and larvae are fed, 4/38 larvae receive dye ; if workers are fed but larvae not, 14/30 ; if larvae are fed but workers not 14/26 ; if neither are fed 22/26 . This shows first that starved larvae can get dyed food from well-fed workers that would collect little for themselves ; and second, that well-fed larvae will accept surplus collected by unfed workers . If both workers and larvae are deprived before the experiment, food reaches the majority of individuals . Starvation of either larvae or workers increases larval uptake . In the third experiment mixtures of equal numbers of sugar-deprived and prey-deprived larvae (distinguishable by a cuticular mark) were reared by groups of workers that had experienced various pre-treatments . One group were all starved of sugar, another were all starved of prey and a third was made up of half one and half the other sort . After being set up they were each offered red-dyed flies and bluedyed sugar solution . During this period the number of flies eaten and the weight of sugar
1 050
ANIMAL BEHAVIOUR, 25, 4 Table III. Workers were Fed on Either Prey or Syrup for a Week and Then Set in Three Groups of which One was Composite. In Each Group Half the Larvae had Been Fed en Prey and Half en -Syrup and Such Type was Marked Distinctively . During the Experiment, the Amount of Food Collected was Measured and the Number of Larvae Dyed Red (From Prey) or Blue (From Syrup) was Recorded . There were 20 Larvae In Each of the Three Cultures Worker pretreatment
Syrup taken (mg)
Flies eaten (no.)
Larval pretreatment
All had syrup
10 . 5
10
Syrup Flies
0 0
10 8
Half had syrup, half had prey
16 . 7
7
Syrup Flies
0 0
9 9
All had prey
28. 3
1
Syrup Flies
0 4
9 4
solution drunk was measured ; finally the larvae were examined for dye . The results (Table III) show a close association between the sugar drunk and the proportion of sugar-starved workers in the group ; similar results were obtained for prey-starved workers . As expected from earlier results, workers well-fed on sugar still took quite a lot of their syrup but workers well-fed on prey took only one fly. It is striking that nearly all the larvae were given prey juices, though slightly fewer prey-fed larvae took this (21) than prey-starved ones (28) . This difference was due mainly to the fact that the workers were so busy drinking syrup after being deprived of it, that they not only failed to induce all the preyfed larvae to take more prey, but gave them syrup instead. Presumably their crops were so full of thin syrup that they had no room for prey juices . Thus these results demonstrate that sugarstarved workers give priority to supplying prey to prey-starved larvae even when they have a source of sugar available . Also, that the ingestion of sugar solution by workers can, in extreme cases, temporarily interfere with the distribution of prey juices but only to larvae that are already well supplied with these . Workers deprived of prey or water compensate by taking more . This appears to be true for sugar too, though workers take it readily even if it is constantly available . Larvae increase their prey intake if temporarily deprived, but not their sugar or water intake . Workers overcompensate after prey deprivation and pass the surplus on to larvae even to those already well-fed with prey . Larvae will take some sugary water but not plain water . The Nature of the Larval Stimulus to Workers One may first ask whether or not workers vary their prey collection in relation to the mass, or
No . out of 10 larvae receiving : Syrup Flies
surface area of the larval population . This would be consistent with the fact that Myrmica workers have a feeding bias towards big larvae (Brian & Hibble 1963) and that food collection is related to larval number (Brian 1973) . To test this hypothesis seven cultures of 20 workers with either 0, 2, 4, 6, 8, 10 or 12 large normal larvae were set up. After assessing fly consumption for 5 days, intake (Y) regressed positively on larval number (X) and gave the equation Y = 0 . 62X-J-0. 59
(P<0 .05)
The larvae were then decapitated and returned to the culture. No relationship between intake and larval number could be detected during the next 5 days . As a variant of this experiment, nine cultures were set with either 0, 5 or 20 decapitated larvae in three replications . Fly consumption was again measured and analysis of the results showed that the effects of treatment differences were insignificant. These two experiments show that a variable population of headless larvae does not raise prey consumption above the levels associated with workers alone. Yet it is well attested that big normal larvae attract more food than small ones and that many normal larvae attract more than few. One must presume therefore that size as an individual attribute, stimulates the workers indirectly. As third-stage larvae all have heads the same size (though the body size varies by a factor of 8) it is likely that their ability to take and store more food is the essential factor governing prey collection by workers . Larvae which are normal in every way except that they cannot swallow can be obtained by decerebration ; such larvae have heads which they can move, but they cannot eat . An experi-
BRIAN & ABBOTT : CONTROL OF FOOD FLOW IN MYRMICA RUBRA
ment was set with the following three treatments normal larvae, decerebrate larvae, and no larvae . There were five replications and the consumption of flies and 5 % sucrose solution was measured in the usual way. Results (Table IV) give highly significant treatment differences (P<0 . 01). Most of these are due to the differences between cultures with normal larvae and those with no larvae : the relative consumption is six times for flies but only 1 .8 times for fluid. However, workers collected more flies for decerebrate larvae than for themselves alone : averaging 4 . 2 more (a 49 % increase) and more than 2 SE of difference. Though this is not strictly an orthogonal comparison it is a sign that suggests that the head may be a shape stimulus to workers for prey collection. There is, by contrast no evidence at all that the head increases fluid collection . This analysis can be taken a little further, for it has been shown that workers cannot distinguish pharate pupae from third-stage larvae when they are collecting them from a light arena and taking them into the nest (Brian 1975b). Workers recognize larvae by means of a widely spread surface chemical reinforced by body rotundity and hairiness ; the head plays no part in this . Pharate pupae are pupae in the third-stage larval skin but they are immobile and cannot urinate, salivate or swallow food ; yet they have the old larval head (containing pupal antennae) projecting fixedly forwards. They are thus, at least externally, larvae with correctly shaped but inTable IV. The Flies and Fluid Collected by Workers with Either Normal Larvae, Larvae Without Brains or No Larvae : One Experiment with Five Replicates
Material collected Flies (no .)
Normal larvae
Larvae without brains
32 30 40 34 33 169 33 . 8
18 7 17 12 10 64 12 . 8
No larvae
6 12 7 7 11 Sum 43 Mean 8.6 sE Differences between means = 2 .03 flies
5 % Sugar (mg)
sE
682 1045 1102 894 1100 4823 965
457 551 558 555 498 2619 524
569 622 536 613 525 2865 573
Sum Mean Difference between means = 52 . 2 mg fluid
1 05 1
active heads and if they can stimulate as much prey collection as decerebrate third-stage larvae the effective clue must be the head shape not the head movement. To test these various possibilities in one experiment the following graded series from normal larvae through decapitate pharate pupae to no brood at all was tested for stimulatory power : (1) normal third-stage larvae, (2) as 1 but unable to swallow (decerebrate larvae), (3) as 1 but without a head (decapitate larvae), (4) as 1 but immobile and with a protruding head (normal pharate pupae), (5) as 4 but without head (decapitate pharate pupae), (6) no brood at all . Ten large larvae of each grade were placed with 30 workers for 3 days . Food was assessed in terms of the flies eaten, and the 5 % sugar drunk (after correction for evaporation) ; dyes were naturally useless with larvae that could not eat . Larvae were starved beforehand to maximize their potential uptake, though those in all treatments except the first continued to be short of food, for obvious reasons . Four experiments of this design including eleven replicates are summarized in Tables V and VI. The usual analysis of variance shows that the treatments differ significantly (P<0 . 001) . Workers by themselves ate, as expected, a considerable number of flies ; but those with normal larvae took 4 . 8 times as many, and there is no doubt that this difference accounts for most of the treatment variation . The only other two means that stand out are those for decerebrate larvae and pharate pupae ; both `larvae' with heads. If one pools them and then compares them with the pooled `headless' and 'nolarvae' treatments one gets means of 11 .30 and 8.23 flies respectively . This margin of 3 .07 flies (a 37 % increase) is more than twice the standard error of differences and, even allowing for the non-orthogonality of the comparison, can be taken to point once more to the role of the larval head as a shape stimulus . Nevertheless, the main factor is undoubtedly the ability of larvae to ingest prey juices . These results show that deprived larvae must obtain extra prey by taking what workers offer them . Even satiated workers respond to larval heads by collecting food . The data for syrup (Table VI) also show that the major part of the highly significant treatment variance arose from differences between no
1052
ANIMAL BEHAVIOUR, 25, 4 Table V. The Number of Flies Collected by Workers with Either Normal Larvae, Larvae Without Brains, Larvae With the Head Li A d Off, Pharate Pupae, Picrate Pupae with the Head Ligat ed Off or No Brood at Data from Four Experiments Indadtg 11 Replicates are Listed
Trial no.
Larvae without heads
Pharate pupae
Pharate pupae without heads
No . brood
1 2 3
43 45 53
2 6 2
12 9 10
12 8 6
1 4 8
5 2 3
4 5
13 11
3 1
2 5
2 2
2 1
3 1
6 7 8
61 67 49
29 26 20
9 24 16
18 20 8
13 15 7
13 17 13
9 10 11
51 40 37
17 10 6
10 3 1
9 27 14
9 4 9
22 11 8
470 42 . 7
122 11 . 1
101 9 .2
126 11 . 5
73 6.6
Sum Mean sE
Normal larvae
Larvae without brains
98 8.9
Difference between means (s2) = 1 . 42 flies .
Table VI . The Weight (g) of 5 °% Sucrose Solution Collected By the Same Workers in the Same Cir. cumstances as the Previous Table but After 2 Weeks Rest
No larvae
0 . 80 0 . 75 0. 71
1 . 02 0 . 90 1 . 07
1 . 00 0. 87 0.74
0. 65 0.31 0 . 75
0 .77 0. 90
0 . 79 0. 87
0 . 59 0. 76
0 . 66 0. 77
0.58 0.71
1 . 82 1-67 1 . 65
0. 83 1 . 03 1 . 14
0. 76 0.91 0.76
0.83 1 . 14 0.91
0 . 92 0 . 74 0 . 77
0 . 74 0 . 87 0 . 71
1 .71 1 .60 1 . 50
0 .88 0 .78 0 .79
0 . 87 0 . 84 0. 76
1 . 03 0 . 97 0 . 85
0. 85 1 . 03 0.85
1 .00 0.91 0. 86
16 . 10 1 . 46
9 .47 0. 86
8 . 81 0. 80
10 . 06 0. 91
9 . 19 0.84
8.04 0.73
Normal larvae
Larvae without heads
1 2 3
1 . 39 1 . 24 1 . 60
0 .77 0 .78 0. 79
4 5
0 . 81 1 . 11
6 7 8 9 10 11
Trial no .
Sum Mean SE
Pharate pupae
Pharate pupae without heads
Larvae without brains
Difference between means (sd) = 0 .031 g.
larvae and normal larvae : in fact workers with normal larvae took twice as much as workers alone. Larvae with heads come next in the array of means (average 0 .89 g) with 13 % more food taken than the rest combined (averaged 0 . 79 g) . The magnitude of this effect is more than 3 sE of difference .
Thus all these results are in accord and taken together indicate that a head even though no more than an immovable knob on one end of a larvae stimulates food collection by workers . Nevertheless, larval ability to ingest and assimilate food is the really important factor governing food flow from workers to larvae .
BRIAN & ABBOTT : CONTROL OF FOOD FLOW IN MYRMICA RUBRA
Food Communication Between Nurses and Foragers Nurses are strongly attracted by brood and struggle to get near it . They feed larvae more than the foragers though the latter do feed in some circumstances (Brian 1974b) . Nurses do not normally leave the nest at all (they are not equipped physically for exposure to sunlight and enemies) and they can therefore be expected to communicate food needs to foragers. A laboratory experiment to test this will now be described . Foragers and nurses were separated as previously described and marked distinctively by cutting off one epinotal spine . They were then fed or not fed, and set for experiment in all the four possible combinations, with 25 foragers and 25 nurses in each culture . All of the three foods were tested in turn. Starved growing larvae were present in all cases and dyed food of the type under test was provided . The dye was traced next day by dissection of the workers and three degrees of penetration as already described
1 05 3
were identified ; those without any trace (encoded 0), those with the dye as far as their crops but no further (encoded 1) and those with the dye perfusing the whole body (encoded 2) . The results are set out in Table VII . This table provides (in column 7 the totals) data for a comparison of nurses and foragers as far as the uptake of the three foods is concerned . Of prey, the nurses appear to take more than the foragers, but this difference is not significant (x2 = 1 . 36, 2 df) ; for water there is no clear difference (x2 = 0.729, 2 df) ; of syrup, which is the only food that all workers took, there is a slightly greater penetration in foragers than nurses and this is significant (x 2 = 4 . 06 P < 0 . 05) . This indication that nurses absorb less sugar than foragers is no doubt related to their different energy requirements ; they might also be expected to take more protein than foragers for they lay eggs as well as feed larvae . Examination of the manner of prey distribution (Table VII) shows that starvation of the nurses reduced the number that took none (from
Table VII. Foragers or Nurses were Either Fed or Unfed and then Put Together in the Four possible , 25 of Each Class. Next Day the Dye from Fresh Food had Either Not Entered (Encoded 0), Reached the Crop (Encoded 1), or Been Fully Absorbed (Encoded 2) . Statistical Significance of x2 Tests for Homogeneity Is Indicated as in Other Tables Nurses
Class of workers
Fed foragers
Unfed foragers
Degree of penetration of dye
Fed
Unfed
~0 1 2
4 21 0
10
1 2
x2 for fed versus unfed
Fed
Unfed
Total
16 4
4 7 14
2 7 16
15 51 34
30 .9**, nurses 1 . 62, foragers 1 .21, interaction
9 15 1
4 16 5
5 14 6
1 9 15
19 54 27
12. 10**, nurses 10-82**, foragers 1 . 65, interaction
(0 { 1 l2
0 7 18
0 9 16
0 10 15
0 10 15
0 36 64
0.694, nurses 0 . 174, foragers 0. 174, interaction
1 01 2
0 7 18
0 4 21
0 5 20
0 7 18
0 23 77
0. 056, nurses 0 .056, foragers 1 . 57, interaction
Nurses
~0 1 L2
4 14 7
9 11 5
0 14 11
1 10 14
14 49 37
14. 9***, nurses 3.60, foragers 1 . 84, interaction
Foragers
(0 { 1 ~, 2
7 13 5
3 13 9
0 19 6
2 10 13
12 55 33
6.25*, nurses 5. 47, foragers 4.75, interaction
Prey Nurses
Foragers
5
Syrup Nurses
Foragers
1
Water
1054
ANIMAL BEHAVIOUR, 25, 4
9 to 6) and increased the number that fully absorbed it (from 4 to 30) . The number of nurses with dye confined to their crops fell (from 37 to 14). Well fed nurses evidently either refrain from taking more, or if they do, hold it in their crops . These differences are statistically very significant (P<0 . 001), and accord well with expectation . Foragers show a similar trend ; starvation reduced the number that took no prey (from 14 to 5), increased the number that absorbed it (from 7 to 20) and decreased the number with prey juices in their crops only (from 29 to 25) . These differences between foragers, fed or not fed beforehand, even though they are slightly smaller than the same differences for nurses are nevertheless statistically significant (P<0.01) . Both these results, though predictable, were worth experimental exploration ; they show that well fed workers either hold food in their crop or do not take any ; whereas starved ones absorb it . Of much more interest, in the present context, is the effect of nurses on foragers and vice versa. First, one may look for an effect of forager pretreatment on nurses . (Compare columns 3 and 5 with columns 4 and 6 under `prey' in the nurse rows of Table VII .) The data show that depriving foragers of food, reduces the number of nurses that do not feed (by only one), decreases the number with full crops (by five), and increases the number that fully absorb food (by six) . This might be taken to suggest that foragers whilst satisfying their own prey needs, influence nurses to do the same but the x2 test is not significant . When communication in the opposite direction is considered, the changes are larger : starved nurses reduce the non-feeding foragers from 13 to 6 and the ones with full crop from 31 to 23 whilst those fully-absorbing food increase from 6 to 21 . These changes, in contrast to the above are highly significant statistically (P<0.001) . This result thus provides the evidence needed to show that nurses can stimulate prey collection by foragers, even in small laboratory cultures where it would be very easy for them to collect prey themselves . A point of further interest is that they cause foragers to absorb more juices rather than to carry more in their crops . Presumably nurses by taking prey juices from foragers effectively deprive them of their own food supply with the result that they collect and absorb more . However, crop filling frequently occurs independently of assimilation ; the crop in fact has long been known as a reservoir of socially available food . The interaction
between the effects of pre-treatment (fed or unfed) and the classes of worker so treated (nurses or foragers) can be tested by comparing the sum of the vectors for dye penetration in those cases where both classes are treated the same (whether fed or unfed) with the sum where both classes are treated differently (one starved and one not) . The vectors are : 6, 28, 16 and 9, 23, 18 respectively (where the figures represent 0, 1 and 2 degrees of dye entry) and the difference is not statistically significant . Syrup distribution (Table VII) provides no statistically significant effects at all . No degree of saturation with syrup could stop workers feeding when the opportunity presented itself . The only difference between the classes of workers was the slightly greater penetration in foragers than in nurses, already mentioned . Water (Table VII) has a strong influence on nurses : deprivation caused non-drinkers to fall from 13 to 1, and absorbers to rise from 12 to 25 (though crop carriers fell by only 1) . Foragers were also affected by starvation in the usual way but the actual changes are just too small to give a significant x 2 test . Foragers did not affect nurses, but they were affected by nurse pretreatment . The vector of foragers with fed nurses (10, 26, 14) compared with that of foragers with unfed nurses (2, 29, 19) is just statistically significant (P<0.05) . There is thus a weak communication of water needs as of prey needs from nurses to foragers but not in the opposite direction . In this water experiment, as for both the other two foods, interaction is not statistically significant. This experiment shows that depriving both nurses and foragers of either prey or water (but not sugar) increases their subsequent uptake . It also shows that though nurses can induce foragers to take more prey, or more water, the influence is not reciprocal ; in fact the society would gain nothing if foragers could extract prey juices, or water, from nurses . The causal sequence from larvae in the nest via nurses to foragers in the field thus leads to the collection of prey and water and its transport back to the larvae . Thin syrup is collected and distributed widely and foragers absorb more than either nurses or larvae ; they no doubt use it as a source of energy . Discussion This experimental analysis of the flow of food from outside the nest to the larvae inside has confirmed the existence of a `gradient of hunger' (Sudd 1967) which though generally accepted has
BRIAN & ABBOTT : CONTROL OF FOOD FLOW IN MYRMICA RUBRA
not previously been demonstrated . Larval acceptance of food has been shown to be influenced both by prior starvation and by the attention of nurses with food . Unless the larvae can swallow the food and dispose of it, very little is collected and there is no doubt that this is the main factor increasing flow of food inwards . However, evidence now exists that the larval head stimulates food collection slightly and, it can be presumed, guides its administration . Although movement is not necessary for the initial stimulation, extension of the head into line with the body brings the mouth to a forward projecting position where it can receive food more easily . Thus in ants other than Myrmica it is likely that the head itself and the act of swallowing are the generative stimuli not the movement. In this then it differs from wasps, whose larvae make similar movements which have been shown to attract food-laden workers . Vespa orientalis workers rasp the sides of the cell with their mandibles and the sound so produced if recorded and played back attracts workers (Ishay & Schwartz 1973) . Larvae of Vespula spp. also move, often in unison, with the same result (Akre et al . 1976) . Wasp larvae are thus able to project their needs in a way that Myrmica larvae appear unable to do. That prior prey-starvation of workers leads to an improved feeding of larvae, even well-fed ones, has been shown to be true for the ant fridomyrmex humilis by Markin (1970) . Presumably transfer in such cases depends on nurses manipulating larvae in a way similar to that described by LeMasne (1953) so that superficially refractory ones are aroused to take food. It is important that the food collected in excess as a result perhaps of a sudden glut can be absorbed by the society even if it is not immediately used as ants have very limited storage facilities outside their own bodies . Vespula spp . when glutted offer pieces of food to larvae and those which do not eat it quickly have it removed and offered to other larvae . Foragers on occasion also bring in more prey than nurses at the nest entrance can deal with and it is dropped into the nest cavity . They do not deliver it to larvae themselves in later stages of nest development (Akre et al. 1976) . Queens influence the passage of food in the Myrmica society in various ways . This work has shown that they can increase the uptake by workers if they themselves are deprived of food
1055
beforehand, but earlier work has shown that they have more subtle, passive ways . Dead ones in fact as long as they are mature queens, not virgins, can cause an increase in the feeding rate of small larvae by spring-type workers though not even live queens can do this with young autumn-type workers (Brian 1975b) . In spring too, queens are able to reduce the solid protein (not the liquid) given to those big female larvae that indicate an ability to grow into sexuals (Brian 1973) . A similar situation occurs in Formica polyctena for Lange (1958) has shown that dead queens can increase the amount of food given to workers in their vicinity as compared with the amount given to other workers . Thus queens can regulate food distribution without taking any part in it themselves . REFERENCES Akre, R. D ., Garnett, W. B ., MacDonald, J. F ., Greene, A. & Landolt, P. 1976 . Behaviour and colony development of Vespula pensylvanica and V. atropilosa (Hymenoptera : Vespidae) . J. Kansas Entomol. Soc., 49 (1), 63-84. Brian, M . V. 1973. Feeding and growth in the ant Myrmica . J. Anim. Ecol., 42, 37-53. Brian, M. V . 1974a. Caste differentiation in Myrmica rubra: the role of hormones. J. Insect. Physiol., 20, 1351-1365. Brian, M . 1974b . Brood rearing behaviour in small cultures of the ant Myrmica rubra L. Anim. Behav., 22, 879-889 . Brian, M . V. 1975a. Larval recognition by workers of the ant Myrmica. Anim. Behav., 23, 745-756. Brian, M . V. 1975b. Caste determination through a queen influence on diapause in larvae of the ant Myrmica rubra. Ent. Exp . & Appl. 18, 429-442 . Brian, M . V . & Hibble, J. 1963 . Larval size and influence of the queen on growth in Myrmica . Insectes soc., 10 (1),71-81 . Haskins, C. P. & Haskins E. F . 1950. Notes on the biology of social behaviour of the archaic ponerine ants of the genera Myrmecia and Promyrmecia. Ann. ent. Soc. Am ., 43, 461-491 . Ishay, J. & Schwartz, A . 1973. Acoustical communication between the members of the oriental hornet (Vespa orientalis) colony . J. accoust. Soc. Am., 53 (2), 640-649. Lange, R . 1958 . Der Einfluss der Konigin auf die Futterverteilung im Ameisenstaat . Natures ., 8, 196. LeMasne, G. 1953 . Observations sur les relations entre le couvain et les adultes chez les fourmis . Annis . Scil nat. Zool., 15, 1-56. Markin, G. P. 1970. Food distribution within laboratory colonies of the Argentine ant Irldomyrmex humilis (Mayr) . Insectes soc ., 17 (2), 127-158. Sudd, J . H . 1967. An Introduction to the Behaviour of Ants . London : Edward Arnold . Vowles, D . M. 1955 . The foraging of ants. Br. J. Anim . Behav ., 3, 1-13 . (Received 29 November 1976 ; revised 11 February 1977 ; MS. number : 1588)
THE CONTROL OF FOOD FLOW IN A SOCIETY OF THE ANT MYRMICA RUBRA L . BY
M . V. BRIAN & A. ABBOTT
Institute of Terrestrial Ecology, Furzebrook Research Station, Wareham, Dorset
Abstract. The control of prey, thin syrup and water flow, through a society of Myrmica was studied . Larval intake increases if they are deprived of prey, but not if they are deprived of water or sugar . Deprivation causes them to take prey juices from workers and they get more if the workers themselves have also been deprived ; this is because such workers over-collect and readily pass on their surplus . Even well-fed larvae will take prey juices from these surfeited workers ; they will also take sugary fluids but not water. The head of a larva elicits some food collection by workers even if it is immobile, but the real cause of food flow towards larvae is their ability to absorb and assimilate the prey juices which they can obtain from workers . Starved nurse workers can obtain prey and water from foragers but a reverse flow does not occur ; only thin syrup is exchanged freely between workers . Myrmica rubra L. (=M. laevinodis Nyl.) workers identify larvae by means of their surface chemistry, hairiness and rotundity ; their movement and possession of a head are not used (Brian 1975a) . Workers that tend larvae (which are supine) collect more food than workers that have none (Brian 1973) . This paper reports an investigation of the causal chain relating the presence of larvae to the food collected by foragers . The passage of food through ant societies was discussed by Sudd (1967) . Sugars pass quickly from worker to worker ; proteins, either crude or prepared and concentrated, go to the larvae and the queens . Larvae are known to stimulate prey collection even when the adults are mainly nectarivorus (Haskins & Haskins 1950 ; Vowles 1955) . LeMasne (1953) and Vowles (1955) suggested that it was the movement of larvae that stimulated workers to collect prey, and Schneirla (cited Vowles 1955) produced evidence that this was so when he stopped Eciton workers foraging by killing their larvae . Such larvae, though indubitably still, were unfortunately unable to eat and an important part of the stimulus may have been absent. LeMasne (1953) moreover noticed that workers in the course of licking and generally manipulating larvae gave food to those that extended their heads in line with their body axis . In this position the mouth lies in front at the apex of the cone formed by the thorax and head . Quiescent larvae often responded to the proximity of workers by adopting this pose and workers then found their mouths and passed them regurgitated food juices . Not all larvae were receptive ; quite clearly they have an internal state which is refractory to feeding. Sudd (1967)
pointed out that it is not necessarily the movement which releases regurgitation by the workers, it might simply be that the mouth is made more accessible to them . Before feeding a larva workers usually place their fore-tarsi on opposite sides of the head and their mid- and hind-tarsi on the ventral thoracic segments (the scratches caused by their claws can easily be shown with silver nitrate). There is thus a characteristic feeding stance which is only possible if the larva extends its head whilst lying on its back . After the experimental methods have been described three series of trials will be detailed . The first concerns the passage of different types of food from workers to larvae in the presence or absence of queens . The second analyses the stimuli used by larvae to obtain prey juices from workers . The third concerns whether the nurses collect food directly from the source or use the foragers as agents. General Methods Myrmica rubra were cultured in the laboratory
as described in Brian (1973, 1975a), using subsamples of large colonies in spring condition . Three foods were offered : crushed Drosophila flies (prey), 5 % sucrose in water (thin syrup), and de-ionized water (water) ; the sugar solution must not exceed 10 %, otherwise workers are liable to regurgitate it on to the nest floor (Brian 1973) . Each of the ant categories (thirdstage larvae, workers or queens) were pretreated for a week at 20 C and 100 % relative humidity by depriving them entirely of one of these three foods (never more than one at a time) . Water tests needed only 3 days pretreatment. Afterwards all possible combinations 1047
1 048
ANIMAL BEHAVIOUR, 25,
of pretreated and untreated ant categories were set in a factorial design with both successive and simultaneous replication. This did not always necessitate separate nest containers, for larvae of different pre-treatments can be distinctly marked by cuticle punctures that leave black scars, and foragers can be distinguished from nurses after removing an epinotal spine . Cohabitation of differently treated ant classes reduces error variance . Foragers were separated from other workers whilst collecting food in an open illuminated arena. The weaker attachment of foragers to brood was also used : a slight disturbance brings them out of the brood chamber . Food uptake during an experiment was assessed after 24 h, using the degree of penetration of vital dyes and the consumption of flies and syrup. The best dyes are neutral red and brilliant cresyl blue ; crystals can be dissolved in thin syrup or sprinkled on fresh fly corpses . Both these vital dyes are quite harmless in the strengths needed to give strong coloration ; they stay in the urate cells of the fat body where they can be seen through the cuticle in larvae, but workers have to be dissected. To avoid any possibility of their confusing treatment influences they were used alternately . Three degrees of penetration were distinguished : the dye was entirely absent, it was only in the crop, or it was present in the crop, the mid-gut and throughout the body cavity. Whereas the second degree shows that the food was received and perhaps
4
passed on, the third degree shows that it was absorbed. In an experiment on the larval stimulation of workers some larvae were beheaded by ligature with steel wire ; others were deprived of their cerebral ganglion (both as in Brian 1974a). In such cases, since the larvae cannot eat uptake was measured as numbers or weight consumed . Thus for prey, the number of Drosophila flies eaten each day out of the surplus of fresh ones given was counted . For syrup, the weight of 5 sucrose solution drunk was obtained from the loss in weight of the syrup tube less an allowance for evaporation ; this was assessed on each syrup tube individually (as in Brian 1973) . Flow Between Larvae and Adults The first aim was to find what effect starvation had on food uptake and distribution in a typical composite group of Myrmica. Foods were tested in the order: prey, syrup, water, with intervals of 2 weeks for recovery on full diet . Two pre-treatment levels (fed and unfed) of the three ant categories (larva, workers and queen) were set in all 23 = 8 combinations and replicated twice . When possible, 2 queens, 10 workers and 15 larvae were examined for dye but deaths sometimes prevented the full number being examined . Results were tested for homogeneity using the usual x2 method for each food and for pooled data (Table I). There can be no doubt that workers and queens both react strongly to water
Table I. The Number of Individuals (Workers, Queens or Larvae) That Took Food (Prey, Syrup or Water) After a Pretreatment Either With or Without That Food. The Total Numbers Used are Also Given . X2 Tests for Homogeneity are Given and One for Interaction . Workers (W) Food type Prey
Fed Total Fed Total
Unfed
Fed
Unfed
Fed
Unfed
50 80
68 80
2 16
5 16
32 69
22 68
10 . 5** 79 80
X2 (1 df)
Water
Fed Total Fed Total x2 (3 df)
NS
78 80
8 12
NS
8 80
68 75
137 240
11 12
2 15
214 235 106. 7***
Level of significance : *P<0. 05 ; **P<0 .01 ; ***P<0.001 .
NS
29 40
31 39 6 112
6.68**
NS
1 107
69 220
52 215 6.84
46 80
30 75 4. 78*
NS
23 38 10 . 6*
(Q x W)
NS
7 10
12 43
Interaction
NS
NS
96 .2***
x2 (1 df)
Sum
Larvae (L)
Fed
x2 (1 df)
Syrup
Queens (Q)
(Ns)
BRIAN & ABBOTT : CONTROL OF FOOD FLOW IN MYRMICA RUBRA
1049
Table IL The nmaeber of Lrvae That Took Either Prey, Syrup or Water After Either They or Their Atta chat Workers had boa Fed or Not Fed with It Befombsnd . X2 Homogeneity Tests are Given and SkwNka ee Indicated as In Table I Worker pre-treatment Food type Prey
Fed Total
Fed
Unfed
Fed
Un-fed
18 58
36 52
18 54
36 56
14.6***
x2
Syrup
Fed Total
16 42
Fed Total
9 . 32** 24 44
21 43
2 . 29 NS
x2
Water
Larval pre-treatment
x2
loss and increase their replacement rates . For workers this is also true of prey, but not of thin syrup which they drink abundantly, whether deprived or not . Queens show signs of increased uptake of prey, syrup and water after deprivation, but the figures taken together only just give statistical significance (P<0.05) . Uptake by larvae was not affected at all by pre-treatment, indeed there are signs that, the more they have, the more they can take. Their growth at this stage is complicated by a tendency to become dormant if unfed and by the presence of queens which stimulate worker attention to them . One interaction is statistically significant (P<0 .05) : the reaction of workers to a water deficit is stronger if their queens are also deprived . The figures are : with saturated queens the number of workers absorbing water rises from 8/40 (saturated) to 30/35 (unsaturated) whereas if the queens are unsaturated the corresponding worker frequencies are 0/40 and 38/40. The nutritive state of queens thus has a substantial effect on worker behaviour. The next experiment was designed simply to clarify the reaction of larvae to starvation ; no queens were present . Four combinations of workers, fed or unfed and larvae fed or unfed were used. This time only the larvae were investigated for dye penetration and the results of several replicates were pooled (Table II) . They show that there is a strong prey effect but no syrup or water effects . Thus there is no doubt in this experiment, unlike that above, that starving larvae of prey increases their uptake when prey are later made available. Since workers supply larvae with less food when they are queenless, it is likely that they respond more to starved ones . Neither syrup nor water produce dif-
0. 187 NS 7 116
4 116 0 .861 NS
19 43
5 116
6 116 0.096 Ns
ferences but it is evident that sugar makes water more acceptable to larvae : less than 5 % took plain water, compared with 50 % that took 5 sucrose solution . Notice the very significant increase that unfed workers cause to larval prey consumption twice as many larvae received food . This presumably means that these workers over-collect and share their surplus with larvae . There are also signs of this being the case for syrup and water though the figures are not statistically significant ;. again it is clear that larvae drink very little plain water . It can be asked whether the extra larval intake induced by unfed workers goes to unfed larvae or to well-fed ones . The relevant figures are as follows : if both workers and larvae are fed, 4/38 larvae receive dye ; if workers are fed but larvae not, 14/30 ; if larvae are fed but workers not 14/26 ; if neither are fed 22/26 . This shows first that starved larvae can get dyed food from well-fed workers that would collect little for themselves ; and second, that well-fed larvae will accept surplus collected by unfed workers . If both workers and larvae are deprived before the experiment, food reaches the majority of individuals . Starvation of either larvae or workers increases larval uptake . In the third experiment mixtures of equal numbers of sugar-deprived and prey-deprived larvae (distinguishable by a cuticular mark) were reared by groups of workers that had experienced various pre-treatments . One group were all starved of sugar, another were all starved of prey and a third was made up of half one and half the other sort . After being set up they were each offered red-dyed flies and bluedyed sugar solution . During this period the number of flies eaten and the weight of sugar
1 050
ANIMAL BEHAVIOUR, 25, 4 Table III. Workers were Fed on Either Prey or Syrup for a Week and Then Set in Three Groups of which One was Composite. In Each Group Half the Larvae had Been Fed en Prey and Half en -Syrup and Such Type was Marked Distinctively . During the Experiment, the Amount of Food Collected was Measured and the Number of Larvae Dyed Red (From Prey) or Blue (From Syrup) was Recorded . There were 20 Larvae In Each of the Three Cultures Worker pretreatment
Syrup taken (mg)
Flies eaten (no.)
Larval pretreatment
All had syrup
10 . 5
10
Syrup Flies
0 0
10 8
Half had syrup, half had prey
16 . 7
7
Syrup Flies
0 0
9 9
All had prey
28. 3
1
Syrup Flies
0 4
9 4
solution drunk was measured ; finally the larvae were examined for dye . The results (Table III) show a close association between the sugar drunk and the proportion of sugar-starved workers in the group ; similar results were obtained for prey-starved workers . As expected from earlier results, workers well-fed on sugar still took quite a lot of their syrup but workers well-fed on prey took only one fly. It is striking that nearly all the larvae were given prey juices, though slightly fewer prey-fed larvae took this (21) than prey-starved ones (28) . This difference was due mainly to the fact that the workers were so busy drinking syrup after being deprived of it, that they not only failed to induce all the preyfed larvae to take more prey, but gave them syrup instead. Presumably their crops were so full of thin syrup that they had no room for prey juices . Thus these results demonstrate that sugarstarved workers give priority to supplying prey to prey-starved larvae even when they have a source of sugar available . Also, that the ingestion of sugar solution by workers can, in extreme cases, temporarily interfere with the distribution of prey juices but only to larvae that are already well supplied with these . Workers deprived of prey or water compensate by taking more . This appears to be true for sugar too, though workers take it readily even if it is constantly available . Larvae increase their prey intake if temporarily deprived, but not their sugar or water intake . Workers overcompensate after prey deprivation and pass the surplus on to larvae even to those already well-fed with prey . Larvae will take some sugary water but not plain water . The Nature of the Larval Stimulus to Workers One may first ask whether or not workers vary their prey collection in relation to the mass, or
No . out of 10 larvae receiving : Syrup Flies
surface area of the larval population . This would be consistent with the fact that Myrmica workers have a feeding bias towards big larvae (Brian & Hibble 1963) and that food collection is related to larval number (Brian 1973) . To test this hypothesis seven cultures of 20 workers with either 0, 2, 4, 6, 8, 10 or 12 large normal larvae were set up. After assessing fly consumption for 5 days, intake (Y) regressed positively on larval number (X) and gave the equation Y = 0 . 62X-J-0. 59
(P<0 .05)
The larvae were then decapitated and returned to the culture. No relationship between intake and larval number could be detected during the next 5 days . As a variant of this experiment, nine cultures were set with either 0, 5 or 20 decapitated larvae in three replications . Fly consumption was again measured and analysis of the results showed that the effects of treatment differences were insignificant. These two experiments show that a variable population of headless larvae does not raise prey consumption above the levels associated with workers alone. Yet it is well attested that big normal larvae attract more food than small ones and that many normal larvae attract more than few. One must presume therefore that size as an individual attribute, stimulates the workers indirectly. As third-stage larvae all have heads the same size (though the body size varies by a factor of 8) it is likely that their ability to take and store more food is the essential factor governing prey collection by workers . Larvae which are normal in every way except that they cannot swallow can be obtained by decerebration ; such larvae have heads which they can move, but they cannot eat . An experi-
BRIAN & ABBOTT : CONTROL OF FOOD FLOW IN MYRMICA RUBRA
ment was set with the following three treatments normal larvae, decerebrate larvae, and no larvae . There were five replications and the consumption of flies and 5 % sucrose solution was measured in the usual way. Results (Table IV) give highly significant treatment differences (P<0 . 01). Most of these are due to the differences between cultures with normal larvae and those with no larvae : the relative consumption is six times for flies but only 1 .8 times for fluid. However, workers collected more flies for decerebrate larvae than for themselves alone : averaging 4 . 2 more (a 49 % increase) and more than 2 SE of difference. Though this is not strictly an orthogonal comparison it is a sign that suggests that the head may be a shape stimulus to workers for prey collection. There is, by contrast no evidence at all that the head increases fluid collection . This analysis can be taken a little further, for it has been shown that workers cannot distinguish pharate pupae from third-stage larvae when they are collecting them from a light arena and taking them into the nest (Brian 1975b). Workers recognize larvae by means of a widely spread surface chemical reinforced by body rotundity and hairiness ; the head plays no part in this . Pharate pupae are pupae in the third-stage larval skin but they are immobile and cannot urinate, salivate or swallow food ; yet they have the old larval head (containing pupal antennae) projecting fixedly forwards. They are thus, at least externally, larvae with correctly shaped but inTable IV. The Flies and Fluid Collected by Workers with Either Normal Larvae, Larvae Without Brains or No Larvae : One Experiment with Five Replicates
Material collected Flies (no .)
Normal larvae
Larvae without brains
32 30 40 34 33 169 33 . 8
18 7 17 12 10 64 12 . 8
No larvae
6 12 7 7 11 Sum 43 Mean 8.6 sE Differences between means = 2 .03 flies
5 % Sugar (mg)
sE
682 1045 1102 894 1100 4823 965
457 551 558 555 498 2619 524
569 622 536 613 525 2865 573
Sum Mean Difference between means = 52 . 2 mg fluid
1 05 1
active heads and if they can stimulate as much prey collection as decerebrate third-stage larvae the effective clue must be the head shape not the head movement. To test these various possibilities in one experiment the following graded series from normal larvae through decapitate pharate pupae to no brood at all was tested for stimulatory power : (1) normal third-stage larvae, (2) as 1 but unable to swallow (decerebrate larvae), (3) as 1 but without a head (decapitate larvae), (4) as 1 but immobile and with a protruding head (normal pharate pupae), (5) as 4 but without head (decapitate pharate pupae), (6) no brood at all . Ten large larvae of each grade were placed with 30 workers for 3 days . Food was assessed in terms of the flies eaten, and the 5 % sugar drunk (after correction for evaporation) ; dyes were naturally useless with larvae that could not eat . Larvae were starved beforehand to maximize their potential uptake, though those in all treatments except the first continued to be short of food, for obvious reasons . Four experiments of this design including eleven replicates are summarized in Tables V and VI. The usual analysis of variance shows that the treatments differ significantly (P<0 . 001) . Workers by themselves ate, as expected, a considerable number of flies ; but those with normal larvae took 4 . 8 times as many, and there is no doubt that this difference accounts for most of the treatment variation . The only other two means that stand out are those for decerebrate larvae and pharate pupae ; both `larvae' with heads. If one pools them and then compares them with the pooled `headless' and 'nolarvae' treatments one gets means of 11 .30 and 8.23 flies respectively . This margin of 3 .07 flies (a 37 % increase) is more than twice the standard error of differences and, even allowing for the non-orthogonality of the comparison, can be taken to point once more to the role of the larval head as a shape stimulus . Nevertheless, the main factor is undoubtedly the ability of larvae to ingest prey juices . These results show that deprived larvae must obtain extra prey by taking what workers offer them . Even satiated workers respond to larval heads by collecting food . The data for syrup (Table VI) also show that the major part of the highly significant treatment variance arose from differences between no
1052
ANIMAL BEHAVIOUR, 25, 4 Table V. The Number of Flies Collected by Workers with Either Normal Larvae, Larvae Without Brains, Larvae With the Head Li A d Off, Pharate Pupae, Picrate Pupae with the Head Ligat ed Off or No Brood at Data from Four Experiments Indadtg 11 Replicates are Listed
Trial no.
Larvae without heads
Pharate pupae
Pharate pupae without heads
No . brood
1 2 3
43 45 53
2 6 2
12 9 10
12 8 6
1 4 8
5 2 3
4 5
13 11
3 1
2 5
2 2
2 1
3 1
6 7 8
61 67 49
29 26 20
9 24 16
18 20 8
13 15 7
13 17 13
9 10 11
51 40 37
17 10 6
10 3 1
9 27 14
9 4 9
22 11 8
470 42 . 7
122 11 . 1
101 9 .2
126 11 . 5
73 6.6
Sum Mean sE
Normal larvae
Larvae without brains
98 8.9
Difference between means (s2) = 1 . 42 flies .
Table VI . The Weight (g) of 5 °% Sucrose Solution Collected By the Same Workers in the Same Cir. cumstances as the Previous Table but After 2 Weeks Rest
No larvae
0 . 80 0 . 75 0. 71
1 . 02 0 . 90 1 . 07
1 . 00 0. 87 0.74
0. 65 0.31 0 . 75
0 .77 0. 90
0 . 79 0. 87
0 . 59 0. 76
0 . 66 0. 77
0.58 0.71
1 . 82 1-67 1 . 65
0. 83 1 . 03 1 . 14
0. 76 0.91 0.76
0.83 1 . 14 0.91
0 . 92 0 . 74 0 . 77
0 . 74 0 . 87 0 . 71
1 .71 1 .60 1 . 50
0 .88 0 .78 0 .79
0 . 87 0 . 84 0. 76
1 . 03 0 . 97 0 . 85
0. 85 1 . 03 0.85
1 .00 0.91 0. 86
16 . 10 1 . 46
9 .47 0. 86
8 . 81 0. 80
10 . 06 0. 91
9 . 19 0.84
8.04 0.73
Normal larvae
Larvae without heads
1 2 3
1 . 39 1 . 24 1 . 60
0 .77 0 .78 0. 79
4 5
0 . 81 1 . 11
6 7 8 9 10 11
Trial no .
Sum Mean SE
Pharate pupae
Pharate pupae without heads
Larvae without brains
Difference between means (sd) = 0 .031 g.
larvae and normal larvae : in fact workers with normal larvae took twice as much as workers alone. Larvae with heads come next in the array of means (average 0 .89 g) with 13 % more food taken than the rest combined (averaged 0 . 79 g) . The magnitude of this effect is more than 3 sE of difference .
Thus all these results are in accord and taken together indicate that a head even though no more than an immovable knob on one end of a larvae stimulates food collection by workers . Nevertheless, larval ability to ingest and assimilate food is the really important factor governing food flow from workers to larvae .
BRIAN & ABBOTT : CONTROL OF FOOD FLOW IN MYRMICA RUBRA
Food Communication Between Nurses and Foragers Nurses are strongly attracted by brood and struggle to get near it . They feed larvae more than the foragers though the latter do feed in some circumstances (Brian 1974b) . Nurses do not normally leave the nest at all (they are not equipped physically for exposure to sunlight and enemies) and they can therefore be expected to communicate food needs to foragers. A laboratory experiment to test this will now be described . Foragers and nurses were separated as previously described and marked distinctively by cutting off one epinotal spine . They were then fed or not fed, and set for experiment in all the four possible combinations, with 25 foragers and 25 nurses in each culture . All of the three foods were tested in turn. Starved growing larvae were present in all cases and dyed food of the type under test was provided . The dye was traced next day by dissection of the workers and three degrees of penetration as already described
1 05 3
were identified ; those without any trace (encoded 0), those with the dye as far as their crops but no further (encoded 1) and those with the dye perfusing the whole body (encoded 2) . The results are set out in Table VII . This table provides (in column 7 the totals) data for a comparison of nurses and foragers as far as the uptake of the three foods is concerned . Of prey, the nurses appear to take more than the foragers, but this difference is not significant (x2 = 1 . 36, 2 df) ; for water there is no clear difference (x2 = 0.729, 2 df) ; of syrup, which is the only food that all workers took, there is a slightly greater penetration in foragers than nurses and this is significant (x 2 = 4 . 06 P < 0 . 05) . This indication that nurses absorb less sugar than foragers is no doubt related to their different energy requirements ; they might also be expected to take more protein than foragers for they lay eggs as well as feed larvae . Examination of the manner of prey distribution (Table VII) shows that starvation of the nurses reduced the number that took none (from
Table VII. Foragers or Nurses were Either Fed or Unfed and then Put Together in the Four possible , 25 of Each Class. Next Day the Dye from Fresh Food had Either Not Entered (Encoded 0), Reached the Crop (Encoded 1), or Been Fully Absorbed (Encoded 2) . Statistical Significance of x2 Tests for Homogeneity Is Indicated as in Other Tables Nurses
Class of workers
Fed foragers
Unfed foragers
Degree of penetration of dye
Fed
Unfed
~0 1 2
4 21 0
10
1 2
x2 for fed versus unfed
Fed
Unfed
Total
16 4
4 7 14
2 7 16
15 51 34
30 .9**, nurses 1 . 62, foragers 1 .21, interaction
9 15 1
4 16 5
5 14 6
1 9 15
19 54 27
12. 10**, nurses 10-82**, foragers 1 . 65, interaction
(0 { 1 l2
0 7 18
0 9 16
0 10 15
0 10 15
0 36 64
0.694, nurses 0 . 174, foragers 0. 174, interaction
1 01 2
0 7 18
0 4 21
0 5 20
0 7 18
0 23 77
0. 056, nurses 0 .056, foragers 1 . 57, interaction
Nurses
~0 1 L2
4 14 7
9 11 5
0 14 11
1 10 14
14 49 37
14. 9***, nurses 3.60, foragers 1 . 84, interaction
Foragers
(0 { 1 ~, 2
7 13 5
3 13 9
0 19 6
2 10 13
12 55 33
6.25*, nurses 5. 47, foragers 4.75, interaction
Prey Nurses
Foragers
5
Syrup Nurses
Foragers
1
Water
1054
ANIMAL BEHAVIOUR, 25, 4
9 to 6) and increased the number that fully absorbed it (from 4 to 30) . The number of nurses with dye confined to their crops fell (from 37 to 14). Well fed nurses evidently either refrain from taking more, or if they do, hold it in their crops . These differences are statistically very significant (P<0 . 001), and accord well with expectation . Foragers show a similar trend ; starvation reduced the number that took no prey (from 14 to 5), increased the number that absorbed it (from 7 to 20) and decreased the number with prey juices in their crops only (from 29 to 25) . These differences between foragers, fed or not fed beforehand, even though they are slightly smaller than the same differences for nurses are nevertheless statistically significant (P<0.01) . Both these results, though predictable, were worth experimental exploration ; they show that well fed workers either hold food in their crop or do not take any ; whereas starved ones absorb it . Of much more interest, in the present context, is the effect of nurses on foragers and vice versa. First, one may look for an effect of forager pretreatment on nurses . (Compare columns 3 and 5 with columns 4 and 6 under `prey' in the nurse rows of Table VII .) The data show that depriving foragers of food, reduces the number of nurses that do not feed (by only one), decreases the number with full crops (by five), and increases the number that fully absorb food (by six) . This might be taken to suggest that foragers whilst satisfying their own prey needs, influence nurses to do the same but the x2 test is not significant . When communication in the opposite direction is considered, the changes are larger : starved nurses reduce the non-feeding foragers from 13 to 6 and the ones with full crop from 31 to 23 whilst those fully-absorbing food increase from 6 to 21 . These changes, in contrast to the above are highly significant statistically (P<0.001) . This result thus provides the evidence needed to show that nurses can stimulate prey collection by foragers, even in small laboratory cultures where it would be very easy for them to collect prey themselves . A point of further interest is that they cause foragers to absorb more juices rather than to carry more in their crops . Presumably nurses by taking prey juices from foragers effectively deprive them of their own food supply with the result that they collect and absorb more . However, crop filling frequently occurs independently of assimilation ; the crop in fact has long been known as a reservoir of socially available food . The interaction
between the effects of pre-treatment (fed or unfed) and the classes of worker so treated (nurses or foragers) can be tested by comparing the sum of the vectors for dye penetration in those cases where both classes are treated the same (whether fed or unfed) with the sum where both classes are treated differently (one starved and one not) . The vectors are : 6, 28, 16 and 9, 23, 18 respectively (where the figures represent 0, 1 and 2 degrees of dye entry) and the difference is not statistically significant . Syrup distribution (Table VII) provides no statistically significant effects at all . No degree of saturation with syrup could stop workers feeding when the opportunity presented itself . The only difference between the classes of workers was the slightly greater penetration in foragers than in nurses, already mentioned . Water (Table VII) has a strong influence on nurses : deprivation caused non-drinkers to fall from 13 to 1, and absorbers to rise from 12 to 25 (though crop carriers fell by only 1) . Foragers were also affected by starvation in the usual way but the actual changes are just too small to give a significant x 2 test . Foragers did not affect nurses, but they were affected by nurse pretreatment . The vector of foragers with fed nurses (10, 26, 14) compared with that of foragers with unfed nurses (2, 29, 19) is just statistically significant (P<0.05) . There is thus a weak communication of water needs as of prey needs from nurses to foragers but not in the opposite direction . In this water experiment, as for both the other two foods, interaction is not statistically significant. This experiment shows that depriving both nurses and foragers of either prey or water (but not sugar) increases their subsequent uptake . It also shows that though nurses can induce foragers to take more prey, or more water, the influence is not reciprocal ; in fact the society would gain nothing if foragers could extract prey juices, or water, from nurses . The causal sequence from larvae in the nest via nurses to foragers in the field thus leads to the collection of prey and water and its transport back to the larvae . Thin syrup is collected and distributed widely and foragers absorb more than either nurses or larvae ; they no doubt use it as a source of energy . Discussion This experimental analysis of the flow of food from outside the nest to the larvae inside has confirmed the existence of a `gradient of hunger' (Sudd 1967) which though generally accepted has
BRIAN & ABBOTT : CONTROL OF FOOD FLOW IN MYRMICA RUBRA
not previously been demonstrated . Larval acceptance of food has been shown to be influenced both by prior starvation and by the attention of nurses with food . Unless the larvae can swallow the food and dispose of it, very little is collected and there is no doubt that this is the main factor increasing flow of food inwards . However, evidence now exists that the larval head stimulates food collection slightly and, it can be presumed, guides its administration . Although movement is not necessary for the initial stimulation, extension of the head into line with the body brings the mouth to a forward projecting position where it can receive food more easily . Thus in ants other than Myrmica it is likely that the head itself and the act of swallowing are the generative stimuli not the movement. In this then it differs from wasps, whose larvae make similar movements which have been shown to attract food-laden workers . Vespa orientalis workers rasp the sides of the cell with their mandibles and the sound so produced if recorded and played back attracts workers (Ishay & Schwartz 1973) . Larvae of Vespula spp. also move, often in unison, with the same result (Akre et al . 1976) . Wasp larvae are thus able to project their needs in a way that Myrmica larvae appear unable to do. That prior prey-starvation of workers leads to an improved feeding of larvae, even well-fed ones, has been shown to be true for the ant fridomyrmex humilis by Markin (1970) . Presumably transfer in such cases depends on nurses manipulating larvae in a way similar to that described by LeMasne (1953) so that superficially refractory ones are aroused to take food. It is important that the food collected in excess as a result perhaps of a sudden glut can be absorbed by the society even if it is not immediately used as ants have very limited storage facilities outside their own bodies . Vespula spp . when glutted offer pieces of food to larvae and those which do not eat it quickly have it removed and offered to other larvae . Foragers on occasion also bring in more prey than nurses at the nest entrance can deal with and it is dropped into the nest cavity . They do not deliver it to larvae themselves in later stages of nest development (Akre et al. 1976) . Queens influence the passage of food in the Myrmica society in various ways . This work has shown that they can increase the uptake by workers if they themselves are deprived of food
1055
beforehand, but earlier work has shown that they have more subtle, passive ways . Dead ones in fact as long as they are mature queens, not virgins, can cause an increase in the feeding rate of small larvae by spring-type workers though not even live queens can do this with young autumn-type workers (Brian 1975b) . In spring too, queens are able to reduce the solid protein (not the liquid) given to those big female larvae that indicate an ability to grow into sexuals (Brian 1973) . A similar situation occurs in Formica polyctena for Lange (1958) has shown that dead queens can increase the amount of food given to workers in their vicinity as compared with the amount given to other workers . Thus queens can regulate food distribution without taking any part in it themselves . REFERENCES Akre, R. D ., Garnett, W. B ., MacDonald, J. F ., Greene, A. & Landolt, P. 1976 . Behaviour and colony development of Vespula pensylvanica and V. atropilosa (Hymenoptera : Vespidae) . J. Kansas Entomol. Soc., 49 (1), 63-84. Brian, M . V. 1973. Feeding and growth in the ant Myrmica . J. Anim. Ecol., 42, 37-53. Brian, M. V . 1974a. Caste differentiation in Myrmica rubra: the role of hormones. J. Insect. Physiol., 20, 1351-1365. Brian, M . 1974b . Brood rearing behaviour in small cultures of the ant Myrmica rubra L. Anim. Behav., 22, 879-889 . Brian, M . V. 1975a. Larval recognition by workers of the ant Myrmica. Anim. Behav., 23, 745-756. Brian, M . V. 1975b. Caste determination through a queen influence on diapause in larvae of the ant Myrmica rubra. Ent. Exp . & Appl. 18, 429-442 . Brian, M . V . & Hibble, J. 1963 . Larval size and influence of the queen on growth in Myrmica . Insectes soc., 10 (1),71-81 . Haskins, C. P. & Haskins E. F . 1950. Notes on the biology of social behaviour of the archaic ponerine ants of the genera Myrmecia and Promyrmecia. Ann. ent. Soc. Am ., 43, 461-491 . Ishay, J. & Schwartz, A . 1973. Acoustical communication between the members of the oriental hornet (Vespa orientalis) colony . J. accoust. Soc. Am., 53 (2), 640-649. Lange, R . 1958 . Der Einfluss der Konigin auf die Futterverteilung im Ameisenstaat . Natures ., 8, 196. LeMasne, G. 1953 . Observations sur les relations entre le couvain et les adultes chez les fourmis . Annis . Scil nat. Zool., 15, 1-56. Markin, G. P. 1970. Food distribution within laboratory colonies of the Argentine ant Irldomyrmex humilis (Mayr) . Insectes soc ., 17 (2), 127-158. Sudd, J . H . 1967. An Introduction to the Behaviour of Ants . London : Edward Arnold . Vowles, D . M. 1955 . The foraging of ants. Br. J. Anim . Behav ., 3, 1-13 . (Received 29 November 1976 ; revised 11 February 1977 ; MS. number : 1588)