74 INVERTEBRATE PHYSIOLOGY 



A complete study of the phenomena of control in insects would shed 

 light on the functioning of the insect central nervous system, on which 

 there is at present almost no information, and hence on the properties of 

 neuropile in general. There seems every reason to hope that a detailed study 

 of the nervous control of insect muscle should be quite feasible and of gen- 

 eral value. 



It is to be expected that the insect mechanisms may be found to be some- 

 what similar to those of the crustaceans, many of which have already been 

 fairly completely elucidated; but there is no longer any need for insect 

 physiologists to borrow ideas from the crustacean field in interpreting the 

 insect phenomena. Modern techniques of investigation, particularly the 

 use of intracellular capillary microelectrodes, make it possible to give 

 unambiguous information about the functioning of individual muscle fibers. 

 Techniques are available for the stimulation of single nerve fibers even 

 when these cannot be prepared separately, and they have already proved 

 of value in insect work (Hoyle, 1955b, partly based on a method by Kuffler 

 and Vaughan Williams, 1953). 



Although this paper will be concerned with the limb muscles of insects, 

 occasional reference will be made to thoracic or wing muscles and to the 

 sound-producing muscles of cicadas, many of which have been evolved 

 from muscles operating limbs ; so far all the evidence shows that, although 

 the histology and metabolism of these thoracic muscle fibers has been 

 greatly altered in some orders, there has been no great change in either 

 their pattern of innervation or their neuromuscular mechanisms. At the 

 present stage of the investigation being undertaken by the author, it is not 

 possible to go much farther than a description of the neuromuscular 

 mechanisms, although a promising start has been made in the direction of 

 studying natural nervous control in the body ; some of this unpublished 

 work will be described briefly. A cursory examination of the problem shows 

 that tiny muscles cannot be satisfactorily operated along vertebrate lines, 

 where graded tension is produced by varying the number of units each in 

 one of two alternative states, i.e., rest or "all-or-nothing" contraction. 

 There are just not sufficient muscle fibers available for executing smooth 

 contractions by this method, even if space were available for the large 

 number of nerve fibers and their cell bodies which would be required to 

 control them. Hence an elucidation of the anatomy of the innervation must 

 play just as important a part as a study of the physiology in contributing 

 to an understanding of the nervous control of insect muscle. 



The Innervation of Insect Muscle 



Insect muscles are supplied with only a very small number of motor 

 nerve fibers; Mangold (1905) demonstrated a double nerve-fiber inner- 



