INSECT SPINASTERNAL MUSCULATURE — CHADWICK 121 



PsocoPTERA. Badonnel, 1934 (Stenopsocus) ; Maki, 1938 {Psocus). 

 Raphidoidea. Czihak, unpublished thesis (Raphidia). 

 Thysanoptera. Maki, 1938 (Machatothrips). 

 Trichoptera. Maki, 1938 {S I eno psyche). 



Throughout arthropod phylogeny, various muscles are subject to 

 replacement by endoskeletal parts; and numerous examples of this 

 typical trend are to be found among the spinasternal muscles of in- 

 sects. The process has taken a variety of pathways, which this is not 

 the place to discuss ; the point of interest here is that the existence of 

 such endoskeletal structures permits one in favorable instances to in- 

 fer the place originally occupied by a muscle in some ancestor of the 

 form under investigation. In the course of the present review, I have 

 noted for certain species that endoskeletal structures, including liga- 

 ments, are the present equivalents of spinasternal muscles in others : 

 attention is called to these examples in the discussion of the individual 

 muscle types. 



To designate the individual muscles, or their equivalents, I have 

 used the topographical system adopted earlier (Chadwick, 1957) ; this 

 method has its defects, but only some such scheme is applicable in the 

 present context, and I have found none that is definitely superior. 

 Each muscle recognized as a morphological entity is identified by a 

 symbol that is formed by hyphenating the accepted abbreviations for 

 the skeletal parts between which the muscle is stretched. The abbre- 

 viations are for the most part those given currency by Snodgrass 

 (1929, etc.). Numerical subscripts, arabic for the thorax and roman 

 for the abdomen, indicate the proper segmental affinity of an attach- 

 ment. Correspondingly, intersegmental attachment sites are preceded 

 by the appropriate numeral, beginning with for the cervical inter- 

 segment ; however, the customary designations lev, 2cv ... for the 

 cervical sclerites and lax, 2ax ... for the axillary sclerites, the latter 

 with segmental subscripts, are retained. A glossary of the abbrevia- 

 tions used follows in table i. 



THE SPINASTERNAL MUSCLES 



The place of the spinasternal muscles in the general muscular pat- 

 tern of insects invites further study, for which we must look princi- 

 pally to the comparative embryologists of the future. Here one can 

 only indicate the nature of the problem. 



In addition to the characteristic outer circular and inner longitudinal 

 layers into which the bulk of the somatic musculature of hexapods 

 and related animals can be resolved, a third class of muscles is repre- 



