Group effects

Conspecific aggregations serve to create and maintain local microclimates. The increased thermal mass leads to a more stable body temperature because metabolic heat is conserved and convective cooling is reduced. Water loss by evaporation or excretion creates a region of higher vapour pressure. Caterpillars may accentuate these effects by spinning communal webs. Three hypotheses concerning the benefits of aggregation behaviour were considered by Klok and Chown (1999) (see also Chapter 6): avoidance of predation by birds, overwhelming of plant defences by synchronous feeding, or physiological benefits (water and thermal relations). Mopane worms Imbrasia belina (Lepidoptera, Saturniidae) are strictly gregarious in the first three instars, with aggregations which may exceed 200 individuals; rates of water loss decrease as aggregation size increases, and body temperatures of aggregations are similar to those of large solitary caterpillars. Aggregation in this species compensates to some extent for the physiological effects of a 4000-fold increase in body mass during the six weeks of larval development (Klok and Chown 1999). Field measurements of haemolymph osmolality also demonstrate more stable water balance in caterpillars living in groups (Fig. 4.11, Willmer 1980).

Coccinellid beetles are known for aggregating in thousands during diapause, behaviour which compensates for modest tolerance of water loss in isolated lady beetles Hippodamia convergens (Coccinellidae) (Yoder and Smith 1997). Surprisingly, adult females (8.8 g) of the giant Madagascar hissing cockroach, Gromphadorhina portentosa (Blattaria, Blaberidae), also benefit from clustering, along with a mite, Gromphadorholaelaps schaeferi, which lives in small groups on its leg bases (Yoder and Grojean 1997). A detailed experimental analysis of the costs and benefits of aggregation was undertaken by Rasa (1997) on a tenebrionid beetle of the southern Kalahari desert, Parastizopus armaticeps

(0.26 g). This species aggregates in burrows during the day and emerges at sunset to feed on unpredictably available detritus of Lebeckia linearifolia (Fabaceae). During periods of drought, group size increases and evening emergence becomes highly synchronized. Beetles in groups have lower water loss rates than solitary ones, but this benefit is countered by the fact that competition for food is fierce in group burrows. The ideal strategy for these beetles, if they are to balance hygric and nutrititional constraints, is to alternate between grouped and solitary lifestyles, and Rasa's field data showed that marked individuals do behave in this way.

Many female insects choose oviposition sites which have microclimatic benefits (e.g. the under-surfaces of host plant leaves) and some lay their eggs in clusters. Egg clustering in butterflies results in aggregation of early instar larvae, with the potential benefits described above, but egg clustering may also be adaptive by reducing egg mortality due to desiccation. This hypothesis was tested in Chlosyne lacinia (Nymphalidae): eggs in large, multilayered batches had greater hatch success than those in smaller, monolayered batches, especially at low av (Clark and Faeth 1998). Multiple selective factors probably determine egg clustering, including cannabilism of eggs by siblings (Clark and Faeth 1998).

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