Aggregation Effects on the Growth of the German Cookroach, Blattella germanica (L.)(Blattaria : Blattellidae)

Nymphs of the german cockroach, Blattella germanica (L.), reared in groups grew faster than those in isolation. This acceleration of growth was found to be sufficiently exhibited in a group of two individuals of B. germanica without regard to their age, and also in a group paired with the nymphs of Periplaneta cockroaches, and even with a gryllus, Homoeogryllus japonicus. There was no critical period responsible for the growth acceleration during the nymphal period. The delay in the growth of the isolated nymphs was the result of cumulative effect during the nymphal period when reared in isolation. Visual and olfactory senses were not concerned in the perception of the stimuli from other individuals. Tactile sense seems to affect and appears that the antennae may play an important role in the perception of certain stimuli from other individuals through muttual physical contact. Factor(s) respondible for growth acceleration was found neither in the body surface nor in the faeces. Moreover, it was not emanated from the insect body. It seems that this growth acceleration is concerned rather with a psychological factor, which effects the regulation of the endocrine system in the insect.