ABSTRACT
If the conclusions reached in the preceding lectures are correct, it is possible to sketch in rough outlines the probable course of evolution of the truly social insects belonging to the order Isoptera and the suborder Aculeata among the Hymenoptera. We have seen that the former arose among the Protoblattoids, possibly as early as Permian times and, after passing through primitive stages like those of existing Mastotermitidæ and Calotermitidæ, culminate in the specialized and exuberant Ethiopian and Oriental Termitidæ. That the character and direction of Isopteran evolution as a whole has been in the main determined by their peculiar food is obvious. A diet of cellulose or humus is responsible for their most striking peculiarities—their defenceless integument, their micro-phthalmy and photophobia, their architectural employment of their own fæces, their trophallactic relations with one another, the employment of symbiotic protozoa in the digestion of cellulose, etc. Only the highest termites have acquired a new habit, that of cultivating and eating fungi, a habit also intimately connected with their peculiar cellulose environment and diet. The choice of an abundant but rather indigestible food and confinement in small hard-walled cavities led to a prolongation of life as in many other wood-boring insects and a tendency to congregation of the adults and an affiliation of the parents and offspring. We observe similar, though feebler and subsocial developments among many Coleoptera of the families Ipidæ (Scolytidae), Platypodidæ and Passalidæ, in Phrenapates among the Tenebrionidæ and Parandra among the Cerambycidae.
