Comparative Fine Structure of the Claw Sensilla of a Soft Tick, Argas (Persicargas) arboreus Kaiser, Hoogstraal, and Kohls, and a Hard Tick, Amblyomma americanum (L.)

Abstract

The (at the distal end of the tarsus) of the first leg of the adult tick, Argas (Persicargas) arboreus Kaiser, Hoogstraal, and Kohls, and the adult tick, Amblyomma americanum (L.), were studied by scanning and transmission electron microscopy. Six pairs of in the tick and 3 pairs in the tick are symmetrically located at the lateral and the medial side of the claw. Three different types of sensilla are described. Type 1 sensillum, a blunt-tipped thick-walled seta, is innervated by 2 sets of dendrites: one set of 2 mechanoreceptive dendrites ending in the socket and another set of 4 to 5 chemoreceptive dendrites ascending into the shaft. Seven canal openings (140 A) at the tip allow communication between the dendrites and the environment. Type 2 sensillum, a sharppointed thick-walled seta, also has 2 mechanoreceptive dendrites and 0 to 4 chemoreceptive dendrites in the shaft receiving chemical stimuli through a single subterminal slit opening (140 A). Type 3 sensillum, a sharp-pointed seta, having only 2 dendrites at the base, is a typical mechanoreceptive sensillum. In the type 1 and 2 sensilla, 2 sets of dendrites are separately encircled by their own internal enveloping cell but share a common external enveloping cell. An extra middle enveloping cell only surrounds the internal enveloping cell of the chemoreceptive dendrites. This is apparently a unique feature of tick sensilla. The first pair of legs of ticks are frequently waved in the air and serve a major role in the sensory perception of the animal. Recent investigations by scanning and transmission electron microscopy have revealed the fine structural details of the tarsal sensilla and Haller's organ of Amblyomma americanum (L.) (family Ixodidae, hard ticks) and Argas (Persicargas) arboreus Kaiser, Hoogstraal, and Kohls (family Argasidae, soft ticks) (Foelix and Axtell, 1971, 1972; Roshdy, Foelix, and Axtell, 1972; Axtell et al., in press). Similar sensilla types have been described on the palps (Foelix and Chu-Wang, 1972). From the morphology and permeability studies (Foelix, 1972) chemoand mechanoreceptive functions can be ascribed to particular sensilla. Additional arise from the tip of the tarsus and project distally in proximity to the claws. These claw setae were not included in previous investigations but due to their locations should also have a significant role in Received for publication 31 January 1973. * Department of Entomology, North Carolina State University, Raleigh, N. C. 27607, USA. This research was supported in part by the Office of Naval Research, Naval Biology Program, under contract no. N00014-70-A-0120-0001, NR 306-033. Paper No. 3945 of the Journal Series of the North Carolina State University Agricultural Experiment Station. sensory perception. In Ixodes persulcatus P. Sch. and Hyalomma asiaticum P. Sch. and E. Schl., these were shown experimentally to be involved in the detection of repellents and were individually classified as chemoor mechanoreceptors on the basis of shape and a gross electrophysiological response (Zolotarev and Elizarov, 1963, 1964; Zolotarev and Sinitsyna, 1965). No fine structure investigations using electron microscopy were conducted on those species, however. We investigated the fine structure of the of Argas arboreus and Amblyomma americanurn by means of scanning and transmission electron microscopy. Since these are innervated, it is appropriate to refer to them as sensilla. These results will contribute to a more complete basis for future behavioral and electrophysiological studies on the sensory perception of these representative species of ticks. MATERIALS AND METHODS Argas (Persicargas) arboreus were from a NAMRU-3 Medical Zoology colony originally collected from rookeries of Bubulcus i. ibis in the type locality of this tick species, near Cairo, and subsequently maintained on domestic pigeon hosts. The Amblyomma americanum were field-collected with a cloth drag in the vicinity of Jacksonville and Came Lejeune, North Carolina. For scanning electron microscopy the ticks were