Unusual fine structure of sensory hair triad of the millipede, Polyxenus.

Three sensory hairs are inserted at right angles to teach other at the temporal region of the head of Polyxenus. They display structural features characteristic of trichobothria and of scolopidia as well. They also manifest several unusual details: 1) The dendritic cilia are enclosed within a capsule formed by enveloping cells. 2) The dendritic cilia are interconnected by desmosome-like junctions. 3) The 9 X 2+0 organization of the dendritic ciliary microtubules is maintained over the entire length of the cilia. 4) Neither a tubular body nor even so much as an inflation of the dendritic cilia develop. 5) Pores and pore tubules occur in the lower halves of the hairs. This uncommon combination of structural details renders the modality of the adequate stimulus uncertain. Though most features suggest a mechanoreceptor, the hairs could function as chemo-, hygro- and/or thermoreceptors as well.