Sylvia D. Gillett

Contests between larval damselflies: empirical steps toward a better ESS model

Abstract Some hypotheses about facultatively aggressive interactions among damselfly larvae (Odonata: Zygoptera) have recently been generated from a model (Crowley 1984) based on the theory of evolutionarily stable strategies (ESS). The present study tests some of the assumptions and predictions of this ESS model with larvae of the damselfly Ischnura elegans van der Lind. in two sets of laboratory experiments. In contest experiments, interactions between two larvae on a thin dowel were observed in all possible combinations of the final three instars. The tendency to maintain control of the site and to evict the opponent was positively related to hunger, relative size (instar difference), and aggressiveness (labial striking), but negatively related to advancing toward a similar-sized opponent along the dowel. Contests were longer and involved more physical contact when larvae were more similar in size. No injuries or mortality resulted from the observed encounters. In distraction experiments, individual larvae were placed in a small feeding chamber with daphnid prey. Experimental larvae were surrounded by last-instar larvae visible through clear plastic walls; controls had no surrounding larvae. Time spent staring at others carried a feeding cost. This cost, the observed negative-exponential distributions of contest duration, and the apparent rarity of dangerous aggression, suggest that contests between similar-sized larvae could be considered wars of attrition. Other possible interpretations and some implications of these behaviour patterns for vulnerability to other predators are noted.

Primer pheromones and polymorphism in the desert locust

Olfactory stimuli are known to alter the labile phase status of polymorphic locusts. This study looked at the way in which the gregarization pheromone, found in the faeces of crowd-reared nymphs, is perceived by nymphs of the desert locust, Schistocerca gregaria. It was necessary to expose isolated-reared nymphs to this stimulus for 7 days in order to see changes towards values typical of crowded locusts in nymphal social behaviour and body colour. Antennaeless nymphs were unable to perceive the stimulus. Faeces from crowd-reared adults, however, had an opposite effect; previously crowded nymphs, on isolation, lost their tendency to form social groups faster than controls, when exposed to the faeces of adult locusts. This solitarizing stimulus had an effect in 4 days. The stimulus was not strong enough to overcome the gregarizing effect of rearing nymphs in pairs. An experiment was carried out to discover if the antennae were also the organs of perception of the solitarizing stimulus. This was confounded by the fact that amputation of the antennae had a short-term solitarizing effect which masked the role of the antennae in perception of the solitarizing stimulus of the adult faeces.

Social determinants of aggregation behaviour in adults of the desert locust

The social aggregation behaviour of adults of the desert locust Schistocerca gregaria, was found to be determined by rearing density, in the same way but not to the same extent as that of nymphs. Adult locusts reared crowded since hatching showed more social grouping than adults reared in isolation. For adults, the least amount of grouping was shown by isolates, F1 of solitaria caught in the field. Both nymphs and adults were found to be attracted to the edge of the testing arena in inverse proportion to their attraction to each other. This edge effect could be used as one behavioural measure of phase polymorphism complimentary to the measure of social aggregation. Unlike nymphs, adults did not rapidly learn to group when forced into contact with one another. Grouping of isolated adults was not found to change with age.

Solitarization in the desert locust, Schistocerca gregaria (Forskål) (Orthoptera: Acrididae)

The rate at which gregarious nymphs of Schistocerca gregaria (Forskal) lose the tendency to form groups after different periods of isolation was measured. There was a marked loss of social grouping after isolation for 24 h. The fall in level of grouping was highly significantly correlated with the number of days the nymphs had been isolated. Further experiments showed that the extent of loss of grouping behaviour could be significantly modified by the length of time the nymphs spent in crowded conditions before they were isolated, but the density during previous crowding did not have a consistent effect on reduction in grouping. It had been previously considered that S. gregaria nymphs lose very little of their gregarious tendency once it is acquired; this is now shown not to be the case. The behaviour of this species is very responsive to the social environment, whether the locusts are learning to form groups or are losing that tendency.

The response of predators to varying densities of Gregaria locust nymphs

Abstract The response to varying densities of prey was investigated by presenting predators (four human volunteers and a lizard, Lacerta lepida Daudin), in an arena, with from 5 to 120 fifth instar phase gregaria nymphs of the desert locust (Schistocerca gregaria Forskal). The activity of the nymphs was also varied by varying the feeding and temperature conditions. Above a certain prey density (which differed between predators) human catching efficiency declined, while the lizard became totally incapable of catching any locusts. Efficiency of all predators was reduced by increasing activity of the prey.

Locust as prey: Factors affecting their vulnerability to predation

Abstract Experiments were carried out to investigate four factors which could affect the vulnerability of nymphs of the desert locust, Schistocerca gregaria to predation. Three different vertebrate species of predator were used, humans, lizards, and a chicken. The factors studied were the colour and size/age of the locusts, the background on which the nymphs were presented and the numbers presented (singly or in groups of five). These variables were each found to be important to at least one of the species used, in modifying predation; different variables being important to different predators. The green nymphs surprisingly did not appear to be better protected on the green background than were the yellow and black nymphs.

Changes in the social behaviour of the desert locust, Schistocerca gregaria, in response to the gregarizing pheromone

Abstract Experiments were carried out on second instar nymphs and immature adults of the desert locust, Schistocerca gregaria , to observe the effect of a pheromone, produced by crowded locusts, on the social behaviour of locusts reared in visual and tactile isolation. The three parameters of social behaviour measured (numbers touching, grouping, and at the arena edge) were not affected by the pheromone to the same extent or in the same direction. The pheromone facilitated the grouping behaviour of nymphs. Isolated nymphs themselves did not produce the pheromone; or if they did, it was in insufficient amounts to alter each others behaviour. The increase in grouping observed also in adults was not due to a direct effect of the pheromone on the adult stage, but was probably a residual effect from exposure during the nymphal stage. Exposure of isolated adults to the pheromone caused an altered distribution of touching locusts; compared with fully crowded and fully isolated adults, more were found in small groups (two locusts only) than in larger groups (three or more). The pheromone did not alter the rate of learning to group of nymphs or adults, either in the 2-hr test period or with further tests on adults. Isolated adults exposed to the pheromone showed a fall in grouping after some experience of social crowding. Choice experiments carried out on individual adults gave complex and inconsistent results.

Social aggregation of adult Schistocerca gregaria and Locusta migratoria migratorioides in relation to the final moult and ageing

Laboratory experiments, were carried out on nymphs and adults of the desert locust, Schistocerca gregaria, and the African migratory locust, Locusta migratoria migratorioides, in view of the field observation that locusts become temporarily less gregarious after the final moult. The experiments showed no reduction in grouping after the final moult but did show a reduction in grouping in mature Schistocerca. There was little change in the numbers of locusts found in small groups but a dramatic reduction in the number found in large groups.

The role of integumental colour pattern in locust grouping

Abstract Experiments were carried out on normally pigmented and albino locusts to measure their social behaviour. Albino nymphs and adults grouped less than normal locusts. This is unlikely to be due to their providing each other with insufficient stimulus to grouping, as in mixed-animal tests, albino locusts showed no increase in grouping and normal locusts showed no fall in grouping.