A variety of nonexclusive criteria have been used to delineate cockroach aggregation behavior. These include their arrangement in space (are they in physical contact?), mechanisms that induce grouping (is a pheromone involved?), and the outcome of physical proximity (do group effects occur?). Aggregations have been described as mandatory, nonobligatory, strong, weak, and loose, without further detail. To most entomologists, mutual attraction is considered the primary criterion of aggregation behavior (Grasse, 1951; Sommer, 1974); group membership involves more than co-location, with individuals behaving in ways that maintain proximity to other group members. In practice, the distinction is not easily made, because in most cases both environmental and social influences play a role (Chopard, 1938). Many cockroaches predictably seek dark, humid, enclosed spaces as shelter, and live in close association with nutritional resources. The functional basis of a nonrandom distribution is especially vague for the vast majority of cockroaches regarded as crevice fauna: those found in small groups in small shelters, for example, under logs, in leaves, under stones, under loose bark. Eickwort (1981) suggested testing aggregation behavior by supplementing the resources of a group to see if it results in dispersion of the insects. Tsuji and Mizuno (1973) and Mizuno and Tsuji (1974) gave Periplaneta americana, P. fuliginosa, P. japonica, and B. germanica excess harborage and found that while adults and older nymphs shelter individually, young nymphs seek conspecifics. The results are difficult to interpret, because all these test species are commonly found in multigenerational aggregations.
What, then, are necessary and sufficient criteria for calling a cockroach gregarious? Are two nymphs found together considered a group? Do they have to be the same species? Are neonates that remain near a hatched ootheca for an hour before dispersing gregarious? What if they remain for 3 days? Do aggregation pheromones have to be involved? Do the insects have to be touching? The literature provides no easy answers. A broad range of variables influences the degree to which individuals are positive, neutral, or negative with regard to joining a group. These include genetics, physiology, informational state, geographic region, and the experimental protocol used to test them (Prokopy and Roitberg, 2001). Behavioral observations, distance measures, and association patterns in the field are all appropriate (Whitehead, 1999), but an explicit description of the criteria used in arriving at a social description is the logical first step.
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