Structures dendritiques des cônes antennaires apicaux de diplopodes (Myriapoda)

A comparative study of four antennal apical cones of diplopods is undertaken in two julids, Typhloblaniulus lorifer Brölemann and Cylindroiulus punctatus (Leach), and in a penicillate, Polyxenus lagurus (Linnaeus). The sensory cones, with an apical pore, show a comparatively similar structure. Sensory units converge on each of them. Each unit consists of a group of neurons, the dendrites of which are surrounded by two superposed sheath cells; all the units are also surrounded by other sheath cells overlapping one another at the base of the sensory cones. According to their structure the dendrites seem to belong to two types of neurons: chemoreceptors and mechanoreceptors. The chemoreceptive dendrites are nearly always uniciliate and reach the cone tip. The mechanoreceptive dendrites are always biciliate. At the base of each antennal cone of P. lagurus the two tubular bodies of the single mechanoreceptive neuron surround the chemoreceptive distal segments of one sensory unit. In the sensory cones of the two julids, one mechanoreceptive neuron per sensory unit can be observed, i. e. more than 8 neurons per cone in the specimens studied; their tubular bodies occur in the cone lumen, at 3/4 of its length. Besides, in julids, ducts and a group of sensory cells can be observed, their four ciliary extensions being “free” inside the cone; the cone wall is provided with pores on 3/4 of its length. The possible double function, gustatory as well as tactile, is discussed.