BIOTIC ASSOCIATIONS OF COCKROACHES — ROTH & WILLIS 335 



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 com- 

 plete 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 supel- 

 lectilium, 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 madcrae. 



Wharton et al. (1954) observed that virgin adult males of Peri- 

 planeta 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. Re- 

 moval 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 re- 

 sponse. 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 court- 

 ing 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 ap- 

 parently 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 



