Light and electron microscopical techniques were used to examine the morphology and structure of certain sensory receptors on the tarsus of leg I, the palps and chelicerae of the adult cattle tick, B.microplus. Although there were variations, the general arrangement and structures found followed patterns common to other ixodid ticks although new receptors were found on the chelicerae. On the basis of the observed structural results electrophysiological procedures, for testing either gustatory or olfactory receptors, were used to study the responses of certain sensilla to potential stimuli. An in vitro feeding technique was developed to analyse feeding patterns. Possibilities for further research in each aspect of this work are discussed.
Tarsus I was distinguished from the tarsi of the other three legs by Mailer's organ which consisted of two components, the anterior pit and the posterior capsule. In the anterior pit, the six thick-walled sensilla, ap 1-ap 6 were found to be multi-innervated (2-15 neurones each) and could be differentiated into four categories. Sensillum ap 1 had plugged pores and was innervated by neurones with branching dendrites. Sensilla ap 2-4 were distinguished by a "spoke-wheel" arrangement of their cuticle walls and by canals which originated from grooves on the surface and ran through the spokes towards a central lumen containing the dendrites. Sensillum ap 5 had a smooth surface whilst that of ap 6 was corrugated, no pore system was found in either sensillum 5 or 6 although they might have openings at their tips, not seen in the present study. The posterior capsule of Haller's organ contained four thin-walled sensilla and a variable number of non-sensory pegs. The cuticular walls of the capsular sensilla had "plugged" pores and their neurones had branching dendrites; there were 29 neurones innervating the four capsular sensilla.
Electrophysiological data showed that both regions of Haller's organ had sensilla sensitive to olfactory stimuli. Coupled with the ultrastructural evidence, this supports the view that Haller's organ subserves an olfactory function as postulated by earlier workers. Apart from the distal sensilla "dd", demonstrated to be contact chemoreceptors, the functions of numerous other tarsal sensilla are still obscure, although some were found to have the structural characters of chemoreceptors.
Each palp had an organ bearing nine sensilla at its distal end. These sensilla were of two types: the A-type had two lumina, one of which contained four unbranched dendrites, whilst the B-type sensilla were characterised by a single circular lumen containing 7-12 unbranched dendrites. Some of the sensilla were sensitive to contact chemostimulants but the function of most of them was not clear.
The distal end of each chelicera had an outer and inner cheliceral digit. The outer digit had a terminal papilla innervated by two neurones which appeared to be mechanoreceptors and two sensory pits, one pit was innervated by eleven neurones some of which were found to be chemoreceptors, the other by a single neurone, containing an unusual vacuolar organelle. A pair of neurones, which appeared to be mechanoreceptors, innervated the shaft of the inner digit where it joined the cheliceral hood membranes. Thus a total of 16 neurones were located in each inner digit. Each outer digit was found to have a complement of 13 neurones of unknown function.
Characteristic chemoreceptor responses to sodium chloride, adenosine triphosphate, reduced glutathione and bovine plasma were recorded from the pit sensilla of the inner digit by electrophysiological techniques. This added a sensory function to the chelicerae of ticks, appendages that were generally regarded as mere cutting tools. The identification of chemosensory sensilla responding to blood chemicals similar to those within the feeding lesion may have implications for work on tick-host specificity and the phenomenon of host resistance to ticks.
In the in vitro feeding method, newly moulted adult females were allowed to attach on a modified Baudruche membrane, incorporated in an apparatus designed to permit the media offered below the membrane to be kept at 37°C and to be changed easily. Patterns of feeding activity were recorded within two hours of placing ticks on the membrane by monitoring changes in electrical resistance between a tick and the medium with a high input-impedance electric circuit. Differences in patterns of sucking and salivation were related to the chemical composition of media presented below the membrane. These observations suggested that inputs from the newly discovered cheliceral taste receptors of B.microplus might be able to mediate changes in feeding patterns, in response to stimulation by different chemical components in the feeding lesion. Incorporation of phosphorus-32 into the medium allowed the volume ingested by ticks to be measured. The techniques described here may facilitate future studies on host factors that influence attachment, engorgement and detachment of the cattle tick.