PHYSIOLOGY AND BEHAVIOR Of THE ARTHROPODA 



333 



precision can be achieved, even when one axon goes to more than one 

 muscle. The four muscles of the hand and claw of the crayfish are rep- 

 resented diagrammatically in Figure 17.8. The claw opener and hand 

 extensor share a single excitor axon, but have different inhibitory axons. 

 The hand extensor muscle shares its inhibitor with the claw closer. The 

 claw closer has two excitor axons, one for rapid, strong contraction and 

 one for slow, sustained contraction. The hand flexor has three axons, one 

 of each type, unshared by other muscles being considered. 



In the behavior of the pincers there is only one activity that re- 

 quires the instantaneous activity of two muscles, and that is a sudden 

 thrust or reach toward an adversary. Analysis of the nerve pattern shows 

 that only one axon need be active to produce this response; it stimulates 

 both the extension of the hand and the opening of the claw in a single 

 operation. At the end of the thrust the claw can be clamped shut by 

 stimuli in the rapid excitor axon of the claw, whether or not the claw 

 opener is inhibited, since the closer is a much more powerful muscle. 

 Hence, the whole maneuver of thrust and grab can be accomplished by 

 activity in two axons. 



In more gentle manipulatory movements an opener inhibitor is 

 probably useful, to permit gentle and sustained activity in the claw 

 closer. Obviously, if during manipulation the claw is to be opened, the 

 much stronger claw closer muscle must be relaxed. Interestingly enough, 

 the inhibitor of the claw closer also inhibits the extensor, so that open- 

 ing of the claw can be accomplished as a simple unhampered motion 

 by activity in two axons. 



This pattern of connections between nerves and muscles, which is 

 comparatively simple anatomically by vertebrate standards, permits re- 

 markably fine control and rapid activity. It appears likely that this 

 pattern occurs generally in arthropods and comparable studies in other 

 forms will further our understanding of arthropod activity. It may even 



Opener 



■>Ex^<z,nsor 



I 

 I 

 I 

 I 



Closer 



Fle>cor' 



Figure 17.8. Nerve supply to the last four muscles of the crayhsh pincers. Each 

 vertical line represents a single axon, which supplies all the fibers of the indicated 

 muscle(s). Dotted lines show inhibitory axons, heavy lines show rapid, strong excitors, 

 and thin lines show slow, sustained excitors. Arrows on the pincers indicate the directions 

 of movement. 



