The common North American ant-lion, Myrneleon immaculatus DeGeer, is generally distributed throughout the United States and has extended its range northward into Canada at various places. Throughout its range, the regular, cone-like pits of larvae of this species inay be found in dry, sandy stretches that in an ecological sense constitute small, isolated desert regions.

Temperature is one of the most important of rhe physical factors with which desert species must contend. It is the purpose of this paper to present the results of gn investigation of the effects of temperature upon the behaviour and activity of larvae of M. immaculatus in their natural habitat.