Id 



INVERTEBRATE PHYSIOLOGY 



more important role functionally than their antagonists ; the flexors are 

 the main postural (tonic) as well as the main active muscles, so a high 

 degree of control may be required for them. 



In regard to the "fast" fiber, the insect motor unit may be either the 

 whole muscle, as in the extensor tibiae, or only one or a few of the com- 

 ponent muscle units, as in the flexor tibiae. Very recently Tiegs ( 1955) has 

 demonstrated by staining methods the presence of two axons supplying the 

 thoracic muscles of several species of insects. The sound muscles of cicadas 

 are, however, supplied with only a single axon (Hagiwara, 1953 ; Pringle, 

 1954). 



a. 



c. 



Fig. 1. Diagram to illustrate the three principal types of 

 muscle organization encountered in insects. They are all shown 

 here as if doubly innervated, (a) Single-unit type, e.g., retractor 

 unguis, levator and depressor tarsi, (b) Multiple-unit, common- 

 innervation type, e.g., extensor tibiae, (c) Multiple-unit, separate- 

 innervation type, e.g., flexor tibiae, extensor trochanteris. 



We may conclude that a double innervation is probably the common 

 mode ; in a few instances there is an additional axon and some muscles are 

 suppHed by only one axon. From the point of view of neuromuscular trans- 



