The Sensory Physiology of the Sheep Tick, Ixodes Ricinus L

Abstract

1. Posture The unfed sheep tick when at rest adopts either a ‘questing’ attitude with the forelegs extended or an attitude of repose with the forelegs folded. While walkingthe hungry tick waves the forelegs in the manner of antennae. 2. Sensory responses (a) Gravity. A tick walking on a 24 cm. vertical glass rod, which serves as a satisfactory model of natural grass or rush stems, usually climbs from the base to the tip, quests, then walks up and down near the tip before finally coming to rest there. This response involves a taxis (upward-turning near the tip) and a kinesis. Negative geotaxis may be of some significance, but reasons are given for supposing that this pattern of behaviour is partly a tactile response following arrival at the tip. (b) Humidity. The humidity behaviour is greatly influenced by the physiological state. When their water balance is normal, unfed ticks avoid higher humidities but come to rest with equal readiness in moist or dry air. The avoiding response (taxis) disappears after desiccation and is replaced by a kinesis: the tick is then intensely active in dry air, but soon comes to rest in moist air. On remaining in damp air the water balance is restored by water uptake through the cuticle. The first response then reappears. The operation of the kinesis does not seem to depend on sensory stimulation. The taxis is weakly developed in normal engorged ticks, but the kinesis is again strongly manifested after desiccation. (c) Tactile responses. Unfed ticks with their legs folded respond to vibrations by questing instantly: they readily cling to any object which brushes against them. The formation of densely packed larval clusters is a further response to contact stimulation (d) Temperature. Hungry ticks orienting to an odourless tube at 37°C. either approach it eagerly and climb on, or avoid it. This stimulus is usually attractive to larvae and nymphs, but repellent to adult ticks. The response is to a gradient of air temperature and not to radiant heat. Objects at any temperature higher than that of the surroundings may prove attractive, but avoiding responses are always elicited by temperatures higher than 42°C. In a linear gradient extending from 8 C.to 45°c. ticks aggregate rapidly in the coldest region after previous exposure to 25°C. Fewer cold-adapted ticks are trapped in this way; those remaining outside the coldest region show no definite ‘preferendum’ within the range 11-41° C. (e) Smell. Below 20°C. ticks are indifferent to the odour of sheep wool alone; above 20°C. wool is slightly attractive. In the presence of a temperature gradient (wool wrapped round a tube at 37° C.) this odour becomes highly attractive. Thus, hungry adults which are repelled by temperature alone are attracted when both stimuli are presented together. They respond with equal vigour to the scents of dog, rabbit, cow and horse hair. (f) Light. Unfed ticks at rest on a glass rod respond to a sudden fall in light intensity by questing (shading section). Engorged ticks are strongly photonegative; newly moulted ticks also avoid a directed filumination, but become indifferent as they age 3. Sense organs The following sensory structures are borne on the forelegs: (a) Huller's organ. This composite organ includes groups of sensilla in the anterior pit and posterior capsule which are morphologically distinct and have an independent innervation. The sensilla of the anterior pit are the humidity receptors responsible for the taxis while the peg-like chemoreceptors of the capsule are olfactory. (b) The temperature sensilla are short, thick-walled hairs borne on the dorsal and lateral aspects of the leg. (c) Tactile bristles are restricted to the ventral surface of the distal articles; their stimulation by vibrations of the substratum leads to the questing response. The palpal organ, situated at the tip of the palps, is a chemoreceptor with a high threshold of stimulation. Its function lies in the attachment response rather than in orientation. Temperature sensilla and tactile bristles are also present on the other legs and on the palps. The reactions to light are due to a dermal sense. 4. Attachment By offering warm or cold membranes of various types as sites for attachment, temperature, smell and a factor of the fleece which is perceived as a contact stimulus by the palpal organ, were found to provide the necessary stimulation for inducing attachment. Most ticks still attach to the natural host if the forelegs or the palps are amputated; none attach if both are lacking. 5. Orientation mechanisms A questing response to light or vibration is often followed by movement. This mechanism (orthokinesis) is also very important in the humidity behaviour of the desiccated tick. Random changes in the direction of locomotion (klinokinesis) rarely occur. The usual response to favourable or unfavourable stimuli (e.g. temperature, smell and humidity) is directed and involves successive comparisons of intensity by the sensilla borne on the forelegs (klinotaxis). There is no tropotactic component, for ticks lacking one foreleg can still locate such stimuli accurately. 6. Orienting responses in the natural environment An important physical feature of the rough moorland grazings which form the main habitat of the sheep tick is the steep humidity gradient within the vegetation layer. Near the roots, where the tick remains quiescent for long periods, the atmosphere is permanently moist; at the tips of the vegetation, where the tick comes to rest during the ‘active’ period, humidities are variable but generally lower. All the sensory perceptions are of value in promoting surpival or host-finding. (a) First, certain reactions guide the tick to a situation--the vegetation tips--favourable for encountering the host. Of particular value in this respect is the upward-turning response summarized under § 2 (a). The tendency to remain near the tips is assisted at first by the humidity response, for ticks walking on the stem lattice avoid the high humidity near the roots. After an unsuccessful period of waiting the tick becomes desiccated and the kinesis comes into play: walking downwards the tick comes to rest at the roots, takes up water from the damp atmosphere, and is then prepared for a further period of activity at the tips. (b) Secondly, responses to tactile stimuli (unusual movements of the vegetation) and to light (shading) provide warning of the imminent approach of the host.cThirdly,orientations ot the warmth and scent of the skin take place when the host is very near: the tick then catches hold and climbs on. After dropping from the host the movements of the engorged tick down into the deeper layer of the vegetation are guided by the avoidance of directed illumination and to some extent by sensations of contact.