New Social Coleoptera¹

Abstract

IF we regard as truly social only those insects in which the parent, or parents, live with their offspring, protecting and feeding them, there have been known hitherto but three groups of beetles which come into this category, namely, the Platypodidæ, Scolytidæ, and Passalidæ. Prof. W. M. Wheeler has recently added to the list two genera of Silvanid beetles belonging to the family Cucujidæ, discovered by him in British Guiana. The beetles in question are Coccidotrophus socialis and Eunausibius wheeleri both of Schwartz and Barber. The bulk of his observations concern the first-mentioned insect, the other species being apparently rare. The beetles enter the hollow petioles of a Leguminous tree Tachigalia and either bore their way in, or gain admittance through perforations previously made by other insects. They enter either as a single pair, or one beetle enters and is very soon joined by an individual of the opposite sex. Upon taking possession of their future habitation the beetles remove any loose pith, or the remains of previous tenants, by pushing this debris into the pointed ends of the cavity by means of their flattened heads. This behaviour brings the insects into contact with the outermost layer of pith adhering to the walls of the cavity, and certain strips of nutritive parenchyma. The latter tissue forms the food of the beetles, and is also shared by young Coccidas of the species Pseudococcus bromeliæ, which soon begin to enter the petiolecavity. By means of the feeding action of the beetles, the strips become converted into grooves, the coccids stationing themselves in a row in each groove. The beetles carefully avoid soiling their food material and store their frass in the areas between the grooves. They lay their eggs along the frass ridges, and the larvæ which hatch out feed upon the same nutritive parenchyma astheir parents.When mature, they construct brown cocoons within which pupation occurs. The beetles emerging from the latter remain in the petiole with the original pair; they mate and produce eggs and larvae in turn, thus leading to the climax stage of the colony, which may eventually consist of several dozens of beetles of both sexes, and many larvæ and pupae in all stages of development. The Coccidæ also increase in number, so that the cavity of the petiole sometimes becomes crowded with inmates. In the meantime,the old and exhausted beetles die off, and their bodies are consigned to the refuse accumulations already mentioned. When this crowded condition is reached, beetles begin to leave the colony either singly or in pairs and, seeking other petioles, thus found new colonies.