NERVOUS CONTROL OF INSECT MUSCLES 11 



mission the flexor tibiae and similarly constructed muscles need not be re- 

 garded as having more than the usual double innervation, but only as com- 

 pound or multiple-unit muscles, the individual units of which conform to 

 the single- or double-innervation pattern. On this basis insect limb muscles 

 can be divided into three categories (Fig. 1). These are the single-unit 

 type (Fig. l,a), e.g., the retractor unguis, levator, and depressor tibiae 

 muscles; the multiple-unit, common-innervation type (Fig. l,b), e.g., the 

 extensor tibiae ; and the multiple-unit, separate-innervation type ( Fig. 1 ,c ) , 

 e.g., the flexor tibiae and the extensor trochanteris. 



Neuromuscular Junctions 



A wide variety of endings has been described for the terminations of 

 motor axons on insect muscles. Filiform branches with no definite junc- 

 tions have been observed (Montalenti, 1928 ; Morison, 1928) and this is the 

 kind usually encountered in the Crustacea (Van Harreveld, 1939). The 

 finer branches have been described as actually entering the muscle sub- 

 stance (Marcu, 1929; Tiegs, 1955). However, there are strong physio- 

 logical grounds for not accepting this picture of axons penetrating the 

 muscle fibers and also for being rather skeptical about the fibrillar type. 

 This caution is strengthened by the fact that in a few instances end plates 

 of definite structure have been clearly observed (Tiegs, 1955; Hoyle, 

 1955a.) There are many early references to endings of end-plate type, 

 often large enough to warrant description as Doyere eminences. The 

 histological observations are rendered extremely difficult by the presence 

 of enormous numbers of fine tracheae, tracheoles, and tracheal end cells. 

 The tracheoles often penetrate right into the muscle-fiber substance. In this 

 case there is no doubt about the accuracy of the observations, and these 

 diffuse surface and penetrating intracellular tracheoles with their asso- 

 ciated cells could easily be responsible for the reports of fibrillar-type nerve 

 endings. 



The end plates are very refractory to staining. In locusts, where it is just 

 possible to microdissect down to the level of end plates, to make out their 

 outline, and even to pull them off the muscle fibers, no technique has been 

 found which will stain them at all satisfactorily. The fine nerve twiglets 

 enter the end plates at the immediate point of contact with the muscle fiber. 

 There the nerve sheath appears to become confluent with the muscle-fiber 

 sheath, the fine sarcolemma. At the point of contact there is often a large 

 nucleus or cluster of nuclei of the sheath, and it may be this cluster of nuclei 

 rather than the end plate proper which gives rise to the appearance of the 

 Doyere eminences. A fine trachea is also usually associated with this point. 

 The end plate is composed of about half a dozen fine tongues of granular 

 cytoplasm. These may be embedded in a matrix of finely granular sub- 



