MYOGENIC RHYTHMS 107 



their mesothoracic tergal muscles. Part of this muscle has the normal 

 longitudinal arrangement and is functionally a wing depressor. But an- 

 other part, more laterally situated, has an oblique orientation due to the 

 carrying down of its posterior attachment on the very well-developed 

 mesothoracic phragma ; this muscle is the main wing levator. Such a re- 

 versal of the function of a muscle would be almost impossible to understand 

 if functional continuity is to be preserved in a 1 :1 excitation mechanism ; it 

 would imply a change over of ganglionic connections of an unprecedented 

 nature. If, however, the cicadas have passed in their evolution through a 

 myogenically rhythmic stage, there is no difficulty in understanding such 

 a change in the timing of the muscular contractions ; in a myogenic system 

 the timing of contractions is determined by the mechanical conditions, not 

 by the ganglionic connections of the motor nerves. The fact that Homoptera 

 have not evolved very far towards an isometric contraction of their flight 

 muscles has thus allowed the cicada muscles to revert to the 1 : 1 excitatory 

 mechanism after the oblique tergal muscle had assumed its modern orienta- 

 tion and role. 



Tiegs (1955) has described and illustrated some further features of the 

 histolog}^ of fibrillar flight muscle which are relevant to this evolutionary 

 story. He has resolved the problem of the sarcolemma, described by recent 

 workers as being absent in the bee and in Drosophila. It is, in fact, present 

 in all cases and is a true cell membrane ; the discrepancy has arisen from 

 the fact that many apparent "cells" in the transverse section are merely 

 areas delimited by intracellular tracheae within a very large cell. He has 

 established that the fibrils which can be isolated even from fresh muscle 

 are bundles of myofibrils for which he proposes the term "sarcostyle" ; each 

 sarcostyle in Diptera is formed ontogenetically by the incorporation of a 

 nucleated myoblast into the syncytial muscle cell. Finally he has shown 

 that there is an important difference in the histology of the motor nerve 

 ending as between normal insect muscle and the dipteran fibrillar type ; in 

 normal muscle the motor nerve ends in "Doyere cones" (apparently ana- 

 logous with the vertebrate end plates) on the surface of the fiber, but in 

 dipteran fibrillar muscle the nerve actually penetrates the giant muscle cells 

 and there are no end plates. How this last observation is to be correlated 

 with the muscle spikes recorded by Pringle (1949) and Roeder (1951) is 

 not clear, but it may represent yet another peculiar feature of these very 

 remarkable tissues. 



Myogenic Rhythms in General 



The occurrence of a myogenic rhythmic system in many different orders 

 of insects suggests, as has already been stated, that it has evolved many 

 times. It is always, so far as is known, correlated with the presence of large 



