BIOTIC ASSOCIATIONS OF COCKROACHES— ROTH & WILLIS 33I 



effect that large numbers of these insects seeking the same environ- 

 ment in a Hmited space would appear to be gregarious, whereas there 

 is probably no true social tendency (Rau, 1924). Reactions of cock- 

 roaches to certain stimuli in the environment undoubtedly do result 

 in aggregations of individuals. However, as Chopard (1938) has 

 pointed out, it is difficult to assign the respective parts played in as- 

 sembling by the attraction of the milieu and by gregarious instincts. 

 Chopard (1938) also stated that Orthoptera with a gregarious tend- 

 ency are found rarely isolated; one finds them, on the contrary 

 nearly always collected in the same shelters, close together, as if con- 

 scious of a need for contact between themselves. He continued 

 further that one can be tempted to attribute the assembling to taxes 

 but that interattraction equally plays an important role ; for example, 

 if one places a large number of cockroaches in a container and offers 

 them similar shelters composed of cardboard tubes, one finds that 

 nearly all the individuals will assemble in one of the tubes, ignoring 

 the others. Pettit (1940) claimed that in Blattella germanica gre- 

 gariousness seemed to depend on the mutual attractiveness of body 

 secretions as well as a thigmopositive behavior and love of warmth. 

 Ledoux (1945) has studied experimentally gregariousness and so- 

 cial interattraction in Blatta orientalis and Blattella germanica. He 

 also found that the cockroaches tended to collect in shelters containing 

 other cockroaches. He concluded that group formation is not the re- 

 sult of chance, but is a social phenomenon, and that interattraction is 

 mainly olfactory, conditioned by (i) positive chemotaxis to odors 

 emitted by the cockroaches themselves, (2) positive hygrotaxis, and 

 (3) thigmotaxis. He found also that large groups are not stable and 

 tend to break into smaller groups. 



Gregariousness in the Orthoptera varies in intensity according to 

 the species and within a species according to the age or physiological 

 state of the insects (Chopard, 1938). This is well exemplified by 

 several of the blattid species discussed below. 



Gregarious groupings of cockroaches have been observed most fre- 

 quently among the domiciliary species. A few examples will suffice. 

 Gal'kov (1926) observed heavy infestations of undetermined cock- 

 roaches in workers' living quarters in the Ural region : "In the cor- 

 ners near the stove, the cockroaches covered the walls in a dense 

 carpet." After fumigating he collected about 135,000 dead cock- 

 roaches from one barracks and about 475,000 from another. We 

 have reviewed a few other examples of heavy infestations in our 

 1957 (a) paper. 



