80 INVERTEBRATE PHYSIOLOGY 



into the end plates themselves. This was first demonstrated by Mangold 

 ( 1905) for Decticus and has recently been confirmed for Locusta by Hoyle 

 (1955a) and Cyclochila, Erythronema, and other species by Tiegs (1955). 

 Anatomically (Tiegs) and physiologically (Hoyle) it has been shown 

 that, although one (usually the larger) fiber probably sends a branch to 

 every end plate, the others (smaller ones) do not. Each end plate on a par- 

 ticular fiber is, however, probably similarly innervated, though there may 

 be exceptions to this rule. This means that, whilst every muscle fiber re- 

 ceives a uniform supply from one axon, only a proportion of the fibers 

 receive supplies, again of similar kind, from the others. A diagram illustra- 

 ting the general pattern of the innervation of a two-axon muscle is shown 

 in Fig. 3. Many thoracic muscles are, however, said to have only a single 

 ending per muscle fiber, e.g.. Apis (Morison) and Erythroneura (Tiegs). 

 These will be discussed later. 



Mechanical Responses 



A consequence of the paucity of motor innervation is that special neuro- 

 muscular mechanisms must be used in order to efifect graded contraction 

 of the muscles. In spite of the demonstration of this economy of nerve 

 supply and the physiological exploration of the rather similar crustacean 

 system, workers in the insect field did not seem to appreciate this point. It 

 was not until 1939, when Pringle studied the motor mechanisms of the 

 cockroach leg, that the presence of special mechanisms in insect muscle was 

 adequately realized. Earlier workers (especially Kahn, 1916; Friedrich, 

 1933; Solf, 1931) had treated insect preparations as if they worked like 

 the frog gastrocnemius preparation. We may now interpret their results 

 in the light of recent knowledge as if they had stimulated only the "fast" 

 nerve fibers of their preparations. The adjective "fast" refers to the relative 

 speed of the resulting contraction and not to the velocity of conduction along 

 the motor axon. Actually the two fibers of doubly innervated muscles are 

 sometimes markedly difl:'erent in diameter, e.g., in Geotrupcs (Marcu, 

 1929) and in Cyclochila (Tiegs, 1955), and so probably have different 

 conduction velocities, in which the thickest and fastest axon is almost cer- 

 tainly also associated with the greatest speed of contraction. But the two 

 motor axons supplying the extensor tibiae of Locusta and Schistocerca 

 metathoracic legs are very similar in diameter, conduction velocity, thresh- 

 old, etc. The corresponding fibers of the mesothoracic legs, although pro- 

 ducing comparable mechanical responses, are different in thickness, 10-1 I/a 

 for the "fast" and only 6jx for the "slow," with conduction velocities of 

 2.2 and 1.5 meters per second respectively. Tiegs found some cases, e.g., 

 the dorsal longitudinal muscles of Erythroneura, in which the fiber diame- 

 ters were about equal. 



