[46i ] THE SENSORY PHYSIOLOGY OF THE HARVEST MITE TROMBICULA AUTUMNALIS SHAW BY B. M. JONES Department of Zoology, University of Edinburgh (Received 18 May 1950) (With Twenty-four Text-figures) INTRODUCTION 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 Solo- mon (1937) the red-legged earth mite, Halotydeus destructor, in relation to environ- mental 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 pre- liminary 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. JEB.27, 3*4 3° 462 B. M. JONES REACTIONS TO LIGHT Among members of the Acarina, unfed ticks and harvest mites closely resemble each other in showing a tendency to climb upwards, a movement which in some unfed insect larvae is associated with a positive response to light. However, the results of Totze (1933) and MacLeod (1935) on the response to light of eyeless species of ticks are contradictory. Lees (1948) found himself in agreement with MacLeod, who was quite unable to confirm the findings of Totze, who maintained that unfed ticks, Ixodes ricinus, in all stages are strongly photopositive. Lees, like MacLeod, tested unfed and engorged ticks which were all photonegative. Ticks show a strong inclination to climb up to the tips of the stems of rushes and grasses. This behaviour could be accounted for either by a response to light, or to the influence of some form of negative geotaxis. Krijgsman (1937), working with tick larvae, Boophilus annulatus, found they were indifferent to gravity. Lees (1948) stated that it was not easy to interpret the results of his gravity tests on Ixodes ricinus, but concluded that although negative geotaxis may be of some significance, the inclination to maintain a position at the tips of glass rods, serving as models of natural grass or rush stems, was partly a tactile response following arrival at the tip. If, as the evidence suggests, ticks are photonegative and independent of gravity, it is difficult to account for the persistent nature of their tendency to climb, upwards, which must invariably lead unfed ticks, in the first place, from the roots of vegetation and up the stems of grasses, before they experience arrival at the tip, which as Lees suggests only enhances this behaviour. The findings of gravity tests on harvest mites (see p. 482) showed that they assumed a random distribution upon a vertical rod. This apparent independence of gravity is also readily shown by the mites when they move about upside down upon the under surface of the window of a light trap (Jones, 1950 a). With the elimination of gravity as an influence, there remained the possibility of a response to light being responsible for the upward-climbing movement. Responses to beams of light Method. The tests were made in a dark room. A circle of filter-paper, 18 cm. in diameter, placed upon a circular glass plate, 20 cm. in diameter, served as the experimental field. Although a dark surface is usually prescribed, preliminary tests showed the reactions of mites to horizontal light to be similar upon either black or white filter-paper. White filter-papers were therefore used because it was easier to mark the paths traversed by the mites. Illumination was provided by 40 W. and 100 W. bulbs enclosed in light-proof containers with a circular aperture "z\ cm. in diameter, to allow the escape of a horizontal beam of light across the filter-paper. The light, before it escaped, was first cooled by passing it through a z\ in. tank containing an acidified solution of alum. Light intensities were measured with a G.E.C. photometer. Single horizontal beam. Mites were placed, usually three at a time, in the centre of the illuminated field. On exposure to a strong beam, with a light gradient extending Sensory physiology of the harvest mite Trombicula autumnalis 463 from 670 m.c. at the centre of the field to 6500 m.c. at the source, they usually moved away from the light before tracing a circular path either to the right or left, which brought them into a position facing up the gradient. This initial turning movement was a prelude to the mites tracing paths straight towards the source, but with a tendency to curve to one side of the beam (see Fig. 1B). The movements of mites exposed to a weaker beam, with a light gradient extending from 270 to 2600 m.c., did not show the same features. The initial turning, induced by the strong beam, was not evident, and the tracks towards the source were decidedly wavy, and only straightened near the source (see Fig. 1 A). Occasionally a mite moved away from the light and when transferred back to the centre of the field it repeated -670 m.c. Fig. 1. Tracks of harvest mites in a horizontal beam of light. A, in a beam graded from 2600 to 270 m.c.; B, in a beam graded from 6500 to 670 m.c. the negative response. Sometimes a photopositive mite when repeatedly brought back to the centre of the field orientated towards the light by taking almost the same path it had traced previously. Behaviour of this kind has probably been responsible for the suggestion that each animal inherits a specific response to an offered stimulus. Two horizontal beams. In a two-light experiment with beams of equal intensity arranged to intersect at about 900 at the centre of the field, the mites were inclined to follow a path some distance along the bisector, before moving towards one or the other light source (see Fig. 2B). When two unequal crossed beams were presented the mites moved directly up the gradient of the stronger beam (see Fig. 2 A). To discover the ability of the mites to detect a stronger light intensity behind them, they were allowed to move up the gradient of a single beam until they had approached within about 6 cm. of the source (2600 m.c), before an opposing light of 6500 m.c. at the source was switched on. The mites continued in the same direction for about a centimetre, stopped, made a complete turn, and then moved towards the stronger 30-2 464 B. M. JONES light. Occasionally a mite displayed continuous turning movements on reaching the centre of the field (see Fig. 3). Fig. 2. Paths followed by harvest mites in two horizontal beams of light which intersect at right angles. A, in two unequal lights, one of 6500 m.c. CZt)> the other of 2600 m.c. (-»-), B, in two equal lights of 2600 m.c. (-*•). Fig. 3. Tracks of harvest mites between two unequal lights. The opposing light of 6500 m.c. ( *) was switched on when the mites had moved within 2 or 3 cm. of a light of 2600 m.c. (->). Response to a laterally presented beam In this series of experiments the light source remained in the same position, whereas the filter-paper was turned about a centre point like a record on a gramo- phone disk. The mite was placed in the beam and allowed to move towards a light. The filter-paper and the mite were then turned clockwise through an angle of 90°. The light now fell laterally upon the mite so that one eyespot was more stimulated than the other. This asymmetry of stimulation caused the mite to re-orientate itself towards the light. By repeatedly turning the filter-paper in this way the mite was induced to trace a rectangular path which brought it back to the starting point (see Fig.