We divide cockroach aggregations into two categories, on the basis of the mechanism by which they are formed: cohort aggregations and affiliative aggregations. Cohort groups are formed by the non-dispersal of neonates after the hatch of an ootheca, and represent kin groups. Whether a cohesive sib group results in a cohort aggregation or is incorporated into an affiliative aggregation depends on the oviposition behavior of the female. The placement of an ootheca in an area remote from con-specifics by an oviparous female, or oviposition by a solitary ovoviviparous female will result in a group comprised solely of siblings. There are currently few reports of this kind of aggregation. In Lanxoblatta emarginata, group size is the mean brood size or slightly less, suggesting that in this case, aggregation of nymphs results from non-dispersal of a sib group (Grandcolas, 1993a).We suspect that some species of forest cockroaches whose nymphs live in the leaf litter form cohort aggregations. Affiliative aggregations are multigenerational groups that may include all developmental stages and both sexes. They are fluid societies formed by both the incorporation of cohorts of nymphs hatched into the group and by immigration. No genetic relationships are implied for affiliative aggregations, but they are not ruled out. Cockroaches that are urban pests form affiliative aggregations, and, along with cave cockroaches, are the best characterized in terms of gregarious behavior.
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