The scolopidial organs in the first antennal segment in Allacma fusca (Collembola, Sminthuridae)

Summary

The proprioceptors of the musculature moving the second antennal segment in the collembolan Allacma fusca were examined with the electron microscope. There are five muscles, the most important of which appear to be a levator and a depressor muscle. Only these two muscles are monitored by proprioceptors. These proprioceptors, the scolopidia, are referred to here as the levator (l-) and the depressor (d-) scolopidium. The former contains two, the latter one sensory cell.

Both scolopidia deviate from the usual pattern in that they have unmodified dendritic outer segments (no dilatations, no electron-dense material) and are enveloped by only one cell. But l- and d-scolopidia also differ from one another. The dendritic inner segments of the l-scolopidium are branched to an h-shaped pattern, with one branch ending on the levator muscle and the other running to the antennal nerve where the perikarya are located. In the d-scolopidium a muscle fiber of about 1 μm diameter (140–320 myosin filaments) accompanies the scolopidium for a distance of about 0.5 μm.

On the basis of the structural features it is hypothesized that (1) the mechanical forces possibly act on the membranes of the dendritic inner segments, (2) the small muscle parallel to the d-scolopidium is a receptor muscle, and (3) both scolopidia are highly derivative.