[46i ] THE SENSORY PHYSIOLOGY OF THE HARVEST MITE TROMBICULA

The ectoparasitic habit of the hexapod larva of Trombicula autumnalis is the cause of much discomfort to residents of infected localities in the British Isles, between late June and the beginning of October. The mite is a member of the Trombiculid group which includes species known to transmit disease in some parts of the world. The unfed larvae are found either upon the soil or climbing upon low-lying vegetation. Under suitable conditions they aggregate into clusters and are then more easily detected as orange patches. Development to the nymphal stage cannot take place unless the larvae obtain a meal from the superficial tissue of a vertebrate host to which they must securely attach themselves. The nymphs and adults are non-parasitic and lead a hypogeal existence at a depth of about 12 in. below the surface of the soil (Cockings, 1948). The hairs of a mammal, or the feathers of a bird, as they brush against infected soil or low-lying vegetation, are admirably suited for picking up the mites, but the question arises, to what extent are sensory perceptions of environmental stimuli of the mites directed towards the acquisition of a host. The chief aim of the present work has therefore been to investigate (a) the responses of the mite to stimuli most likely to have value with respect to the problem of acquiring a host, and (b) the nature of the sensory organs. Few workers have studied the orientation mechanisms of members of the Acarina. Henschel (1929) described the reactions of Tyrolichus casei, the cheese mite, to chemical stimulation. Totze (1933) studied the sheep tick, Ixodes ricinus, and Solomon (1937) the red-legged earth mite, Halotydeus destructor, in relation to environmental conditions. Lees (1948) reinvestigated the reactions of Ixodes ricinus with respect to those stimuli the tick will encounter in its natural environment. As to the behaviour of harvest mites there exist only a few scattered and very incomplete references to observations in the field. It was found desirable to carry out a preliminary ecological study of Trombicula autumnalis in 1947 to provide information upon which to base the present work. The observations of the mite in the field were invaluable for suggesting lines of investigation in the laboratory, and for assessing the significance of the sensory perceptions of the mite with respect to acquiring a host.