SPINASTERNAL MUSCULATURE IN CERTAIN 

 INSECT ORDERS 



By L. E. CHADWICK 



Departvient of Entomology 



University of Illinois 



Urbaiw, III. 



PART I. THE SPINASTERNAL MUSCLES OF THY- 

 SANURA AND PTERYGOTE INSECTS 



Within the muscular system of hexapods, the spinasternal muscles 

 form a group which, though probably diverse in origin and develop- 

 ment, is relatively easy to distinguish and define. The spinasternal 

 muscles are widely distributed among modern species, and while they 

 are not overly numerous, their number has evidently been altered 

 greatly during the differentiation of the various orders. Thus, a com- 

 parative study should readily provide some notion both of the ancestral 

 status of the spinasternal muscles and of their principal evolutionary 

 trends. 



As will be seen, muscles that belong to the spinasternal category 

 have been found in one or another insect by many workers, yet few 

 have recognized in them an evolutionarily significant element of the 

 hexapod muscular pattern. I believe that these muscles are best 

 understood as relics of a part of the early arthropod musculature that 

 the insects are gradually abandoning. Reasons for this conclusion will 

 be set forth in the sections that follow, for one must reach a clear 

 decision on this point before one can apply data now available on 

 the spinasternal muscles successfully to phylogenetic problems. My 

 primary purpose here is therefore to review the distribution of the 

 several spinasternal muscles throughout the class of insects, insofar 

 as present information will permit. 



Included among the muscles to be examined are all thoracic muscles 

 with either or both attachments on the spinae or on their present mor- 

 phological equivalents. The spinae (sps) are median intersegmental 

 interneural apophyses, invaginated from the ventral integument. In 

 some Apterygota they are found in the cervical, thoracic, and abdomi- 

 nal intersegments (Maki, 1938; Barlet, 1951, 1953, 1954), but they 

 occur only in the first and second thoracic intersegmental regions of 

 pterygote insects, except in Grylloblatta, which is reported (Walker 



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