THE NEUROMUSCULAR BASIS OF COXAL FEEDING AND LOCOMOTORY MOVEMENTS IN LIMULUS

¹ Department of Zoology, University of Massachusetts, Amherst, Massachusett 01002

1. Transverse feeding movements of a Limulus leg coxa can trace a simple repeating are of abduction and adduction around a dorsolateral pivot, or may take an oval path of depression, adduction, elevation, and abduction. This ingestive chewing path and the reverse sequence mediating egestion both require movement of the pivot.

2. The actions of the nine coxal muscles were determined by chronic electromyogram and movement recordings in intact animals. In all feeding patterns most muscles act during adduction, the four ventral plastrocoxals being the main adductors. None of the muscles actively abduct the coxa.

3. In the promotor-remotor coxal swing of locomotion the dorsal tergocoxal muscles predominate, although the plastrocoxal muscles are also active. Muscles attached to the posterior margin of the coxa are active during remotion, and those on the anterior margin are active during promotion (except muscle 40, which is active during remotion).

4. The oval paths characteristic of ingestive and egestive chewing result from action of muscles 29 and 27. These muscles act radially to the arc of adduction-abduction and displace the pivot of that are dorsolaterally. When activity in these muscles phase-lags adduction, the path of ingestive chewing results. When their activity phase-leads adduction, the reverse sequence of egestive chewing is produced. Thus a major behavioral alteration results from phase-shifting the action of two muscles in an otherwise stable pattern of motor output.