If specific pheromones are not involved in many species, how do groups form? Aggregation in cockroaches is generally mediated by visual, acoustic, tactile, and/or olfactory stimuli (Grasse, 1951). The complication is that these often are not the only causes. Environmental factors, including light (Gunn, 1940), temperature (Gunn, 1935), and air movement (Cornwell, 1968) also play an important role. Humidity is a factor, although the degree to which it exerts an influence may be species specific (Roth and Willis, 1960). In some cockroaches, the lower the humidity, the stronger the tendency to aggregate (Sommer, 1974; Dambach and Goehlen, 1999). Response to these, as well as other environmental stimuli, results in the initial selection of a harborage, which is consequently marked with bodily secretions (Pettit, 1940); these then help mediate immigration into the group. In laboratory tests, 82% of B. germanica choose harborages previously inhabited by conspecifics (Berthold and Wilson, 1967). As the size of an aggregation increases, the collective signal ofthe mass should serve as an increasingly more powerful attractant to unassociated individuals. Blattella germanica will migrate from a less to a more colonized refuge; new refuges are colonized stepwise, with males (Denzer et al., 1988) or mid-size nymphs (Bret and Ross, 1985) as the first to arrive.
Kavanaugh (1977) suggested three mechanisms by which a group may assemble: (1) independent, individual responses to environmental gradients, leading to aggregation in an abiotically optimum location; (2) individual response to stimuli provided by other individuals, leading to group formation at a common location; (3) some combination of the two. Cockroaches, like many other animals, appear to employ the third mechanism, with the first and second involved sequentially. This approach was recently formalized by Deneubourg et al. (2002) and Jeanson et al. (2005). These authors conclude that cockroach aggregations are self-organized systems, resulting from interactions between individuals following simple rules based on local information. First, similar species-specific responses to the physical environment increase the probability that cockroaches converge in the same vicinity. Positive feedbacks and the modulation of individual behavior dependent on the proximity of con-specifics then result in group formation. Short-range volatiles, contact chemicals, physical contact, alterations in local microclimate, and perhaps sonic communication (Mistal et al., 2000) may all signal the presence of con-specifics and serve as cues for an individual to slow or stop locomotion. The response to these cues may be modulated by heterogeneities in the environment. Garnier et al. (2005) used a group of micro-robots modeled after cockroaches to demonstrate that the aggregation process is based on a simple set of behavioral rules. The robots were not only able to form aggregations, but could also make a collective choice when presented with two identical or different shelters. These broader approaches to cockroach aggregation behavior help account for much of the ambiguity in the literature on the subject, and aid in integrating cockroaches into the existing literature on grouping behavior in other animal systems.
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