Central connections of receptors on rotated and exchanged cerci of crickets.

The cerci of crickets, paired abdominal appendages bearing sound-sensitive filiform hairs, can be removed and grafted back so that their morphological axes acquire various relationships to those of the body. We have studied both the morphogenetic consequences of such surgery and the central connections made by the regenerating axons of the cercal sensory neurons. If a cercus is rotated and grafted back into its own socket, it back-rotates towards its original orientation in succeeding molts. If left and right cerci are exchanged, with or without rotation, back-rotation does not occur and super-numerary cerci are formed in predictable locations. There are two sub-populations of filiform hairs: those that vibrate transversely to the cercal shaft (T-hairs) in dorsal and ventral sectors, and longitudinally vibrating hairs (L-hairs) in lateral and medial sectors. Two giant interneurons are excited by T-hairs of their own side but not by L-hairs. If cerci are grafted so that they assume various orientations relative to the body, a consistent physiological result is obtained: T-hairs always appear to be the source of excitatory input to the giant interneurons, no matter where they are caused to be located by prior surgery. The phenomena of back-rotation, formation of supernumerary cerci, and formation of connections selectively by T-hairs, can be interpreted on the hypothesis of morphogenetic gradients.