Aggregations as Defense

Although cockroaches are known to have a variety of predators and a large number of weapons in their arsenal to defend against them, most available information relates to predation on individuals. Diurnal aggregations of inactive cockroaches, however, have properties that differ from active, nocturnal individuals and thus change the parameters of the predator-prey interaction. Cues that lead predators to prey are multiplied when prey aggregate (Hobson, 1978), and the rewards of finding such a concentrated source of food are greater. Since cockroaches typically assemble in inaccessible places (crevices, leaves, hollow logs, under bark, among roots), their apparency is presumably low to predators that rely primarily on visual cues. Conversely, cockroach aggregations may offer a more intense signal to olfactory hunters. At least one parasite is known to specialize on cockroach aggregations: eggs of the beetle Ripidius pectinicornis are laid in a cluster near cockroach aggregations, and early larval stages then locate their host (Barbier, 1947).

The greater number of available sensory receptors in an aggregation increases group capacity to sense potential predators. There is anecdotal evidence that vigilance behavior by peripheral insects may occur in aggregations of P. americana. Ehrlich's (1943) description depicts older

Fig. 8.1 Aggregation of nymphs of Cartoblatta pulchra on a tree trunk in Kenya. The nymphs are both aposematically colored and produce a sticky exudate on the terminal abdominal segments. Note that heads are oriented toward the center of the group (cycloalexy). Photo courtesy of Michel Boulard.

individuals serving as sentries on the periphery of the group; when danger approaches they warn the young with body movements. A more realistic interpretation, however, may be that members of the aggregation react to the evasive maneuvers of the first insect to detect a predator. Alarm pheromones have been described in Eurycotis floridana (Farine et al., 1997), Thereapetiveriana (Farine et al., 2002), and cave-dwelling Blaberus spp. (Crawford and Cloudsley-Thompson, 1971; Gautier, 1974a; Brossut, 1983). The emission of these chemicals results in the rapid scattering of group members. Predators confronted by a confusing welter of moving targets presumably have trouble concentrating on individual prey.While defensive glands have been described in a large number of cockroaches (Roth and Alsop, 1978), whether the secretions of these glands function as weapons, signals, or both is in many cases untested. Certainly insects that exude or project defensive chemicals would benefit from an increase in point sources (Vulinec, 1990). One example of this type of defensive strategy is known among the Blattaria, although it may occur in others (e.g., Dendroblatta sobrina—Hebard, 1920a). Similar-sized nymphs of Cartoblatta pulchra (Blattinae) openly assemble on tree trunks in Tanganyika and Kenya (Fig. 8.1). One group, composed of 100-150 individuals, formed a rosette larger than a human hand. Individuals were polarized, with their heads facing the center of the group and their abdomens directed radially outward (cycloalexy). A brisk movement disperses the cockroaches, and they run into crevices in the tree trunk (Chopard, 1938). The insects are aposematically colored (black and orange), and each nymph displays a thick proteinaceous secretion on the terminal abdominal segments. This material originates from type 5 tergal glands (Fig. 5.11), is characteristic of many oviparous cockroaches (Fig. 4.7), and functions at least in part to protect them against ants (Roth and Alsop, 1978). Most known aposematic cockroach species are active during the day in relatively open areas and do not form conspicuous aggregations (e.g., Platyzosteria ruficeps—Waterhouse and Wallbank, 1967).

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