GREGARIOUSNESS
Casual statements that cockroaches are gregarious are often encountered in the literature. There has been some argument to the effect that large numbers of these insects seeking the same environment in a limited space would appear to be gregarious, whereas there is probably no true social tendency (Rau, 1924). Reactions of cockroaches 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 assembling by the attraction of the milieu and by gregarious instincts. Chopard (1938) also stated that Orthoptera with a gregarious tendency are found rarely isolated; one finds them, on the contrary nearly always collected in the same shelters, close together, as if conscious 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 gregariousness 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 social 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 result of chance, but is a social phenomenon, and that interattraction is mainly olfactory, conditioned by (1) 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 frequently among the domiciliary species. A few examples will suffice. Gal'kov (1926) observed heavy infestations of undetermined cockroaches in workers' living quarters in the Ural region: "In the corners near the stove, the cockroaches covered the walls in a dense carpet." After fumigating he collected about 135,000 dead cockroaches from one barracks and about 475,000 from another. We have reviewed a few other examples of heavy infestations in our 1957(a) paper.
Periplaneta americana was observed by Gould and Deay (1938) in an old meat-packing building in Indiana. Adult cockroaches were present in large numbers between closely placed beams, but the nymphs were more common in cracks between bricks. Clusters of several hundred cockroaches were seen on the open walls of the cold, dark hide room. Gould and Deay stated "American roaches of all sizes live together in perfect harmony. Young nymphs have been noted in clusters underneath adults and crawling over the adults as they wander about in rearing jars." In the monkey house of the Hamburg zoo, P. americana spent most of the day in the cellars resting on the walls in groups of about 200 individuals (Brecher, 1929). Lederer (1952) noted that in closed, dark, heated spaces under the aquarium at Frankfurt am Main, P. americana rested in groups of 20 to 30 individuals; he stated that it was remarkable that the "herd" divided itself into groups each of which usually contained insects of the same age or stage of development. Eads (1954) found P. americana in 40 percent of 762 sewer manholes in Tyler, Tex.; 13 percent of 670 of these manholes were heavily infested with 100 or more cockroaches in each. Other heavy sewer infestations have been reviewed in our 1957(a) paper.
Ehrlich (1943) has stated that Periplaneta americana exhibits social behavior. For instance, cockroaches of various ages inhabit a fairly large space jointly; the adults and older nymphs sense approaches with their antennae and warn and protect the young by a beating of wings and by body movements. There is complete utilization of the available living space; the imagos drive older nymphs from their resting places, and the older nymphs drive out the younger ones, until all cracks, depending on their size, are occupied by various age groups of different sizes. In his experiments Ehrlich observed that in cages with no hiding places the cockroaches would group together; when given a choice of small and large shelters, P. americana hid only under the larger ones that could shelter more insects. Finally, the cockroaches ceased to bite and fight each other when they crowded together in the face of danger.
Of Blatta orientalis Marlatt (1915) stated "This species is notably gregarious in habit, individuals living together in colonies in the most amicable way, the small ones being allowed by the larger ones to sit on them, run over them, and nestle beneath them without any resentment being shown." Haber (1919) also observed that this species is often noticed "huddled together, the younger ones crawling over, around, and beneath the older ones."
Wille (1920) observed that nymphs of Blattella germanica remained almost constantly in groups during the first and second instars, but less so during the third instar. He believed that the aggregations of young occurred because they could occupy narrow crevices where the larger insects could not penetrate. At usual room temperatures the older nymphs and adults lived completely isolated, but at certain temperatures they gathered together in large, tightly pressed groups.
Supella supellectilium is said to be gregarious (Gould and Deay, 1940). The smaller nymphs aggregate in small groups in rearing containers, but the older ones remain separate from one another (Hafez and Afifi, 1956). Leucophaea maderae is sociable and rarely found alone; in their favorite hiding places, hills of these cockroaches can be seen hanging together (Seín, 1923). Wolcott (1950) also stated that L. maderae is gregarious. Annandale (1900) observed that in the "Siamese Malay States" large colonies of Periplaneta australasiae conceal themselves in hollows of bamboo logs from which houses are built. Moulton (1912) stated that he was astonished at the large numbers of P. australasiae and Symploce cavernicola that he saw swarming on the sides of caves of Mt. Jibong, Borneo.
Rehn and Hebard (1905) stated that in Key West, Fla., Eurycotis floridana fairly swarmed under the coquina boulders in the woods, in groups of a dozen containing both young and adults; Pycnoscelus surinamensis was very abundant in the same type of habitat. Caudell (1905) also found the young of E. floridana with the mature individuals. Hebard (1917) in his discussion of Lattiblattella rehni again mentioned finding frequent colonies of E. floridana in Florida. He also found many specimens of Blaberus craniifer under boards on the ground at Key West. He found Parcoblatta lata numerous under bark of dead pine trees in Alabama. However, Dowdy (1955), in an ecological study of oak-hickory forest in Missouri, stated that "Parcoblatta [sp.] were never recorded as being gregarious, in fact they were mostly solitary. However, in some cases two were found together." Yet Blatchley (1895) stated of Parcoblatta pensylvanica that in the winter in Indiana "One cannot pull the loose bark from an old log without dislodging a colony of from ten to a hundred of the nymphs of various sizes." Males of Parcoblatta virginica were said to be often gregarious beneath loose bark and under chunks and rubbish (Blatchley, 1920).
Rehn and Hebard (1927) quoted observations made earlier by Hebard on Byrsotria fumigata in Cuba: "I found the specimens under flat stones, sometimes in colonies of 3 or 4 mature specimens and numbers of immature individuals in all stages of development." These observers also reported that Aspiduchus borinquen was found in Puerto Rico in a limestone cavern by thousands in the grass and on the walls. J. W. H. Rehn (1951a) stated that a related species, apparently Aspiduchus cavernicola, was seen in great numbers on the side walls and roof of a cave in Puerto Rico, but it was not possible to collect any of these and, we infer, confirm the species. Rehn and Hebard (1927) in their account of Simblerastes jamaicanus reported finding it in numbers in a termite nest. Pemberton and Williams (1938) stated that Diploptera punctata is of gregarious habits in Hawaii. Saupe (1928) observed a strong "herd instinct" in all age groups of Blaberus craniifer. Bunting (personal communication, 1956) stated that large nymphs and adults of Blaberus discoidalis "congregate in narrow cracks or on the underside of some low object. The younger nymphs keep in close communities of approximately the same age." Sonan (1924) stated that in Formosa(?) Salganea morio is usually found in groups of six or seven in decayed trees. Species of the genus Litopeltis may be found in small groups as they are somewhat gregarious (Rehn, 1928).
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.