The Humidity Behaviour of the Mealworm Beetle, Tenebrio Molitor L. Iii. the Mechanism of the Reaction

IT has previously been shown that, under suitable experimental conditions, Tenebrio molttor reacts clearly to humidity (Pielou & Gunn, 1940) and that the humidity receptors are probably the pit-peg organs and peg organs on the antennae (Pielou, 1940). The present work is an attempt to analyse the mechanism of this reaction both qualitatively and quantitatively, in terms of elementary animal behaviour. The analysis of the mechanism of behaviour reactions of animals to diffuse molecular stimuli—odours, humidity and air temperature (as distinct from "radiant heat")—has been attempted only on a limited scale. This is no doubt due to the difficulties of controlling and measuring the conditions and of evolving a technique as accurate and precise as that which can be used when light is the stimulus. Such analyses as have been made, in general, attempt to carry over the theories and classifications—especially the classification of Kiihn (1919)—that have been developed mainly in relation to light. The modifications made by Gunn (Fraenkel & Gunn, 1940) in Kiihn's classification make it more suitable for the inclusion of reactions to diffuse stimuli. This system includes two main categories: kineses, or undirected reactions, in which the direction of movement of the animals is not precisely related to the direction of the source of stimulation, and taxes, the directed reactions in which the orientation of the axis of the animal's body is influenced by the direction of the source. In ortho-kinesis, the average velocity of locomotion or the frequency of activity depends on the intensity of stimulation, and aggregation occurs in the region of lowest velocity; in klino-Jnnesis (avoiding reactions) the amount of turning during locomotion depends on the intensity of stimulation and upon some other factor like sensory adaptation (Ullyott, 1936; Gunn, et al. 1937). In klino-taxis the animal compares the intensities of stimulation in its vicinity by alternative movements to the left and right (either movements of the whole body or of some receptor region such as its antennae) and the comparison is thus between two intensities which are successive in time. In tropo-taxis the behaviour is similar but orientation