The physiological or psychological effects of gregariousness, or lack of it, are interesting aspects of the basic phenomenon. Landowski (1937) studied in Blatta orientalis the effect on development and growth of the transition from life in complete isolation to life in groups. He kept nymphs in groups of 1, 2, 4, 8, and 16 in jars of identical size and shape. Landowski found that (1) mortality increased with the size of the group and with age, as each animal occupied more of the available space. [Presumably these factors are less detrimental in nature where the group is unconfined.] He further found that (2) life in complete isolation extended the time required to produce an adult insect; and (3) the mean weight of the adult insect was, generally, in inverse proportion to the number of nymphs raised together; isolated insects usually attained the greatest adult weight.
Similarly, Griffiths and Tauber (1942a) found that isolation extended the period of nymphal development in Periplaneta americana. As most of their isolates died before reaching maturity, these workers concluded that the American cockroach does not thrive when individually isolated and that several individuals must be together for optimum development to occur. Pettit (1940, 1940a) observed that isolated nymphs of Blattella germanica take longer to mature than those reared in groups. Wallick (1954) found indications in B. germanica that there is an inverse relationship between population density and individual weight; as the population decreased the weight increased. He also noted an inverse relationship between population density and life expectancy in this species.
We (Willis et al., 1958) have confirmed the above observations that Blattella germanica, Blatta orientalis, and Periplaneta americana complete nymphal development in less time when reared in groups rather than individually. We (loc. cit.) also found that nymphs of the following additional species matured more quickly when reared in groups: Eurycotis floridana, Periplaneta fuliginosa, Supella supellectilium, Nauphoeta cinerea, and Pycnoscelus surinamensis; only a very slight decrease in the average length of the developmental period was found in grouped nymphs of Leucophaea maderae.
Wharton et al. (1954) observed that virgin adult males of Periplaneta americana that had been individually isolated upon emergence were almost wholly unresponsive to the sexually stimulating, female odor for a test period of four weeks. Similar males of comparable age that were kept in groups reacted strongly from the sixth day on. Removal of reactive males from the group inhibited the reaction in these isolates, but the response returned when the insects were regrouped. We (1952) had similarly observed that no isolated male of Blattella germanica was ever seen to give a courting response without having received some form of external stimulation. Yet when numbers of males were kept together isolated from females, on several occasions the males became active and a few individuals gave a courting response. As the sexual stimulus is received by the male of B. germanica through contact rather than odor, as in P. americana, presumably it was mutual contact between the grouped males that released the courting activity.
Cloudsley-Thompson (1953a), in his studies of diurnal rhythms in Periplaneta americana, observed a steady decline in total activity in successive 24-hour cycles: "When two cockroaches, even of different species (P. americana and P. australasiae) were kept together, this depression did not appear to set in so readily." The associates apparently kept each other active.
Isolated females of Periplaneta americana can be conditioned to run a simple maze with less time and fewer errors per trial than when paired or when a member of a group of three (Gates and Allee, 1933). There was less activity, and accordingly fewer errors per minute, among cockroaches tested as pairs and groups of three than as isolated individuals. This observation should not be contrasted with that of Cloudsley-Thompson (1953a), cited above, because the intervals during which activity was observed were quite different.
In the above account we have presumed that aggregations of some species are indications of gregariousness. However, until gregariousness has been proved experimentally for each species, we concede that reactions to environmental stimuli might be sufficient to bring about some of the observed groupings without any interaction between individuals.
In concluding this section we note that Tepper (1893) stated that carnivorous cockroaches in Australia lead more or less solitary lives, and that one rarely meets several together in close proximity. Takahashi (1940) observed that in Formosa Blattella humbertiana does not have a tendency to throng together. Rau (1947) stated that the adults of Ischnoptera deropeltiformis showed no tendency toward gregariousness, but in the laboratory newly hatched young lived close together under bark and remained together throughout the nymphal stages. We wonder whether this gregariousness was not imposed by the restricted quarters of the cage. As mentioned above, Dowdy (1955) did not find Parcoblatta sp. to be gregarious in the field.
INTRASPECIES FIGHTING
Fighting occurs among cockroaches of the same species over food or shelter or between males. Saupe (1928) observed late-instar nymphs of Blaberus craniifer attack each other and even adults. Additional records cited in the section on intraspecies predation (p. [322]) imply fighting within a species. Rau (1924) saw a male of Blatta orientalis attack another male in copula and bite away a large portion of its wing. Two other males in the container had their wings badly torn overnight, presumably as a result of fighting.