INSECT SPINASTERNAL MUSCULATURE — CIIADWICK I4I 



Thus we see that there has been a continuing reduction in the number 

 of these muscles with the differentiation of the more recent orders. 

 Apparent also is the likelihood that the individual muscular elements 

 are homologous throughout the numerous species in which each occurs, 

 for the presence of each muscle in several only distantly related forms 

 makes any other explanation very improbable. Nowhere has evidence 

 been found for the late acquisition or development of new spinasternal 

 muscles, except through modification of those already existing. One 

 sees, instead, that spinasternal muscles have sometimes disappeared 

 in phylogeny, or have been replaced with skeletal parts, whereas the 

 reverse seems not to have occurred. Similarly, an attachment once 

 lost or dissolved is not regained in the descendants of that stock. 



These observations and inferences from them encourage one to pro- 

 ceed with an analysis of relationships, on the assumption that any 

 group of insects that today possesses a certain muscle cannot have in- 

 herited it from one lacking the muscle in question. The converse of 

 this proposition is of course false : common possession of a given 

 muscle or pattern cannot guarantee any close genetic tie between the 

 two groups concerned. The approach from this angle is therefore 

 essentially negative, but it is nevertheless useful in showing that some 

 relationships proposed are clearly impossible while others are still open 

 for consideration. While it is improbable that any existing order of 

 insects is the direct offshoot of any other group now living, bearing 

 this in mind evidently does not exclude the possibility that, as far as 

 mere numbers are involved, a present pattern of the spinasternal 

 musculature could have been derived from one like that now manifest 

 in some other order. 



Examination of our data from the viewpoint set forth above shows 

 the following : 



1. At least 9 orders are surely of independent origin, in the sense 

 that none of them is traceable directly to any other order now extant. 

 These are the Thysanura. Blattariae, Megaloptera, Orthoptera, Cole- 

 optera, Psocoptera, Mantodea, Grylloblattodea, and Neuroptera. 



2. The spinasternal pattern of Dermaptera, Odonta, and Mallophaga 

 could have been derived directly from a pattern like that now repre- 

 sented in the Thysanura, but not from any other existing group. 

 However, certain elements interpreted by me as spinasternal occur in 

 modern Thysanura as ligamentous portions of the endosternum (see 

 table 2, notes). Since in pterygote insects the presumed homologues 

 of these parts are in many instances contractile muscles, these must be 

 assumed to have been inherited from a prethysanuran form in which 



