The Use of Mixed Colonies of Termites in the Study of Host-Symbiont Relations

Abstract

The termite-protozoa complex is interesting to the student of evolution as an illustration of the evolution of a sexually reproducing group, the termites, paralleled by the asexually reproducing protozoa. These protozoa were present in the intestines of their insect hosts well before the Cretaceous, for they occur in the wood roach, Cryptocercus punctulatus Scudder as well as in the more primitive families of termites. In general, each species of host termite has its own distinctive faunule of protozoa. The present study analyses the behavior of the termite host as an isolating mechanism for the protozoa. A termite colony in nature is composed of a single species of termite. It often happens that the galleries of two separate colonies interdigitate in the same log, but they never run into each other. In all the observations in nature, no colony has been found containing more than one species of termite. The typical reaction when members of different species of termites are brought together in nature and in the laboratory is a violent, mutual attack. The normal behavior patterns of the insect hosts were modified in these experiments to permit the formation of laboratory colonies consisting of two or more species of termites. Laboratory colonies are kept in thin plywood between panes of glass to permit observation of the termites at all times. The plywood is drilled at the center to provide space for the insects in starting the colony. After several days the galleries extend out from the center through the plywood. A sheet of paper towelling between the wood and one pane of glass serves to hold sufficient moisture. The glass is bound with tape, and the whole colony fits into a metal cigarette tin measuring 4.5 x 5.75 x 0.5. This method of keeping termite colonies is similar to the one suggested by Adamson (1941). All the colonies were kept in the same type of plywood for several months before mixing. On the day of an experiment, the stock colonies (each consisting of a single species) are thoroughly chilled in the refrigerator to immobilize the insects. The desired number of chilled termites of the foreign species are introduced into the colony. As the temperature rises the insects become active. If fighting is noted at this time, the mixed colony is returned to the refrigerator. Several successive chillings are usually sufficient to permit the establishment of a mixed colony. Once the additions to the colony have been accepted no distinction can be noted in the behavior of the termites towards members of their own species as compared with members of the introduced species. Species recognition among termites is probably an odor-taste phenomenon. The following combinations of two species in the same colony were achieved: