Group effects refer to morphological, physiological, or behavioral differences between animals that are grouped versus those of the same species that are bereft of social contact. The prolongation of the juvenile growth period in isolated nymphs is the best-studied group effect in cockroaches, occurs in a wide range of species (Table 8.3), and is discussed in Chapter 9 in relation to its evolutionary connection to caste control in termites. One benefit of accelerated development in grouped nymphs is that it moves them quickly through one of the riskiest stages of their lifecycle. The number of cockroach species examined for group effects is extremely limited relative to the number of species available for study; especially interesting would be a study of those in which nymphs seem to disperse shortly after hatch, like Than. akinetum (Grandcolas, 1993a). Altered juvenile growth rates, however, are not the only effect of social interaction. Like some other insects that aggregate (reviewed by Eickwort, 1981), molting in grouped cockroaches tends to be synchronized (Ishii and Kuwahara, 1967). This may be an evolutionary response to the threat of cannibalism, as all nymphs are vulnerable at the same time, and are incapable of feeding on each other until their mouthparts sclerotize.
Adult cockroaches also show group effects, which are
Table 8.3. Cockroach species that exhibit group effects on development.
Blattidae: Blatta orientalis, Eurycotis floridana, Periplaneta americana, P. australasiae,P. fuliginosa (Willis et al., 1958)
Blattellidae:Blattella germanica,B.vaga,Supella longipalpa (Willis et al., 1958; Izutsu et al., 1970)
Blaberidae:Diplopterapunctata,Eurycotis floridana,Nauphoeta cinerea,Pycnoscelus surinamensis,Rhyparobia maderae (Willis et al., 1958;Woodhead and Paulson, 1983)
manifested in physiology and behavior, can be species specific, and have a complex influence on reproductive success. In B. germanica the presence of another adult has an impact on how fast a female reproduces and how much she eats, but the former is at least partially independent of the latter (Gadot et al., 1989; Holbrook et al., 2000a). Komiyama and Ogata (1977) found that isolated females of this species deposit a greater number of oothecae than group-reared females, but the hatching success of those oothecae was considerably lower. In Su. longipalpa, group effects were primarily behavioral, and group composition rather than isolation was more influential on reproductive events. Neither oocyte growth nor calling behavior was affected by isolating virgin females, but the onset of calling and its diel periodicity were advanced in virgin females housed with other virgin females relative to females housed with either mated females or males that were unable to mate (Chon et al., 1990). Several studies have shown that isolated male cockroaches show a decreased reaction to female sex pheromone (Roth and Willis, 1952a; Wharton et al., 1954; Stürkow and Bodenstein, 1966); the social history of male N. cinerea is known to influence the amount of sex pheromone they produce (Moore et al., 1995).
A number of other behavioral effects can be induced by isolating cockroaches: the normal flight reaction to disturbance may be lost (Hocking, 1958), circadian rhythm may be altered (Metzger, 1995), the ability to learn may be affected (Gates and Allee, 1933), and activity increased (Hocking, 1958) or decreased (Cloudsley-Thompson, 1953). Aggressiveness was delayed in isolated male N. cinerea (Manning and Johnstone, 1970), but isolation increased aggressiveness in Periplaneta (Bell et al., 1973), The. petiveriana (Livingstone and Ramani, 1978), and several cave-dwelling Blaberidae (Gautier et al., 1988). Raisbeck (1976) found an aggression-stimulating substance produced by isolated P. americana that is masked or suppressed by "aggregation pheromone" when the insects live in groups.
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