Ehrlich (1943) stated that individuals of Periplaneta americana that are feeding will ward off intruders by spreading their wings and pushing with their hind legs. However, the intruder will approach again and again biting the feeder in the legs and wings. Frequently the odor of approaching food was sufficient to cause the cockroaches to fight and bite each other. Biting and fighting also occurred when individuals of this species defended their daytime hiding places. A position of attack is assumed when two antagonistic individuals of P. americana meet (Ehrlich, 1943, fig. 14). The insects raise their bodies slightly above the ground, by extending their legs, and they stretch their heads forward horizontally so that their mouth parts protrude; when the insects jump at each other, they may wound each other severely in the soft parts of the body. Fighting between sexually excited males resulted in injury to their legs, wings, cerci, and other parts of the body. Frequently an insect that could no longer defend itself was killed. Lederer (1952) also made similar but less extensive observations on fighting in this species.
Pettit (1940) quoted Woodruff as stating that nymphs of Blattella germanica, apparently healthy and perfectly normal, would do battle for no apparent cause other than a chance meeting, and that occasionally the fight was to the finish, the loser being eaten. Pettit could not substantiate such voracious attacks, although he saw nymphs engage in fights lasting about two seconds during which one would be driven off by vigorous bites on legs or cerci. Small nymphs of B. germanica tended to ignore each other, but third-and later-instar nymphs would engage in "quarrels" of short duration when two met. Pettit noted that males of B. germanica that were crowded together quickly set upon, but did not always kill, other cockroaches introduced into their cage. When he isolated a dozen males in a small cage, they became quarrelsome and three of the group were killed and partly eaten. After several days the surviving males had taken positions so that each was equidistant from his neighbors. Some of these males attacked other males and a female that were introduced, by biting their legs and cerci. Females under similar conditions were much less aggressive, although Pettit saw some females that roved about biting all large members of the group that were within easy reach.
We have frequently observed aggressive behavior between males of Nauphoeta cinerea, which resulted in torn wings. The males would wrestle with each other rolling over and over.
INTERSPECIES COMPATIBILITY
We agree in essence with Chopard (1938) who stated that it is improper to speak of associations apropos of the ecological distribution of Orthoptera. He continued that it is clearly evident that different species of Orthoptera, which are found grouped on a territory more or less narrowly limited, have no interdependence among them. Their grouping results uniquely from almost similar reactions to the different factors which characterize this limited milieu. There is neither interdependence nor interaction; the grouping is a false biocoenose, born under the action of the environment, and does not survive a modification of this milieu.
However, as there are numerous examples of mutual toleration between different species as well as examples of incompatibility, the subject has more than academic interest even if no true ecological significance. On the other hand, further study may show that certain of these associations are definitely ecological, particularly among the feral species. As might be expected, most of the following examples pertain to domiciliary cockroaches.
Dozier (1920) occasionally found Periplaneta americana with Eurycotis floridana in decaying stumps, beneath loose bark of decayed trees, and beneath corded wood. Adair (1923) stated that in his house in Egypt Periplaneta americana, Blatta orientalis, and Blattella germanica were found together in a cupboard. Sambon (1925) found B. orientalis and B. germanica side by side but not fraternizing in a home in Italy. Gould and Deay (1938) observed that apartments over stores were infested with both B. germanica and P. americana, but did not indicate whether these occupied the same microhabitat. Gould and Deay (1940) observed that in the Purdue University greenhouse Periplaneta fuliginosa was found "under benches, boxes, pots and other objects in association with the American roach." Dr. L. A. Hetrick (personal communication, 1954) wrote us that several summers before he had had a mixed infestation of cockroaches, which included Periplaneta australasiae, Periplaneta fuliginosa, and Pycnoscelus surinamensis, in his chicken shed.
Eads (personal communication, 1955), in response to our inquiry about the mixed populations of cockroaches that he had reported infesting sewers in Texas (Eads et al., 1954), stated that "Each of the ten colonies of B. orientalis found in Tyler manholes were associated with larger colonies of P. americana. True breeding colonies of B. orientalis appeared to be present since all the developmental stages were taken. The same situation existed with the P. fuliginosa and the two species of Parcoblatta. Larger colonies of P. americana were associated with the other species in each case. From our limited observations the two species always appeared to be perfectly compatible." Eads et al. (1954) had found Periplaneta fuliginosa in three manholes, Parcoblatta bolliana in one manhole and Parcoblatta pensylvanica in one manhole. We assume that the groups of each species were spacially discrete so that they were recognizable as colonies. Dr. T. A. Olson (personal communication, 1958) has observed two or more species of cockroaches in a single structure but never in mixed colonies. Each species was separated physically from the others. Olson concluded that cockroaches of different species do not mingle freely unless forced to do so by some special environmental condition. Pettit (1940) found B. germanica and P. americana similarly separated in the same building or even in the same basement laboratory.
Perkins (1899) found Lobopterella dimidiatipes generally living in company with the young of Periplaneta australasiae in Hawaii. Rehn and Hebard (1914) in Florida found P. australasiae abundant with Periplaneta americana on a quarter-boat. They also noted that the forficulid Marava [= Prolabia] arachidis (Yersin) appeared in numbers in a kitchen after dark accompanied by swarms of P. americana. These workers also found Leurolestes pallidus in a fruit store in Key West "where the species was common in a pile of old burlap bags and in cracks under the stands which it shared with one fairly large colony of Blattella germanica, occasional specimens of Holocompsa nitidula, a few specimens of Periplaneta americana, and one specimen of Supella supellectilium." They also found H. nitidula with Blaberus craniifer "between old boards in a woodshed, where nymphs were more numerous than adults."
Rehn and Hebard (1914) stated of Supella supellectilium in Florida that "The females were all taken in cupboards where Blattella germanica was found in swarms." The association in human habitations of S. supellectilium and B. germanica has been reported also by Sein (1923), Puerto Rico; Shaw (1924), Australia; Mallis (1954): "German and brown-banded roaches were often found in the same crevice."; Anonymous (1958), Texas; and Anonymous (1958a), Georgia. Gould and Deay (1940) stated that other species of cockroaches, especially B. germanica, may be found with S. supellectilium in the same part of a building. Yet Shaw (1925) stated that "when Supella supellectilium Serv. invades places already occupied by Blattella germanica L., it tends to oust the latter."