INSECT SPINASTERNAL MUSCULATURE — CHADWICK I45 



The postulated composite patterns and the changes in them that are 

 needed for the differentiation of the other more primitive neuropteroid 

 groups may be determined by comparing tables 2 and 3, and do not 

 require special comment except to note that, compared with the or- 

 thopteroid series, relatively large changes, involving several muscles, 

 are usually necessary. 



As stated on page 142, the Trichoptera and Lepidoptera could be de- 

 rived directly from the Megaloptera or Coleoptera ; whereas the pres- 

 ent Neuroptera, through their loss of isps-fun which the Trichoptera 

 and Lepidoptera still retain, can no longer stand in the direct line of 

 descent of these two groups. If, as most authorities agree, the Tri- 

 choptera are offshoots of the neuropteroid stock, they must therefore 

 have branched from it at a time before the differentiation of the 

 Neuroptera from the Coleoptera. So far as their limited spinasternal 

 musculature is concerned, there is nothing in either order to prevent 

 derivation of the Lepidoptera from the same line that gave rise to the 

 Trichoptera. The numerous other possible derivations of the two 

 orders that the spinasternal musculature would permit may be dis- 

 missed as probably of no genetic significance. 



The Hymenoptera, though they have only five spinasternal muscles, 

 have, like the Mecoptera, preserved icv-c.x\X. If they are to be traced 

 to a neuropteroid source, they must be associated with a group that 

 had retained this muscle after its disappearance from the lines that 

 led to the Megaloptera, Coleoptera, and Neuroptera. The Hymenop- 

 tera possess two spinasternal muscles that the Mecoptera lack, if one 

 accepts Weber's (1927) interpretation of the muscles of tenthredi- 

 nids, and could therefore be continued in close association with the 

 mecopteran line only down to the point where it lost isps-cxx and 

 2sps-fns. The present Mecoptera also show the muscle isps-fux in 

 the form fui-fui, a specialization that has not been reported for any 

 hymenopterous species. As noted on page 142, the spinasternal muscu- 

 lature of Hymenoptera is also derivable from orthopteroid sources; 

 and it offers us no facts that would permit a decision as to their true 

 affinities. 



We may mention here that the larval musculature of Trichoptera, 

 Lepidoptera, brachycerous Diptera, and Hymenoptera, and of most 

 beetles, does not include spinasternal muscles, so that it contributes 

 nothing to the problem with which we are here concerned. Spina- 

 sternal muscles are present, however, in the larvae of the other holo- 

 metabolous orders that have been examined ; and in the nymphs of 

 most Hemimetabola. Culicid larvae also are said to have transverse 

 muscles that may be remnants of a spinasternal musculature. 



