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.