146 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



The Odonata, which Hke the Dermaptera have nine spinasternal 

 muscles, hold a somewhat anomalous position ; and that they have 

 long been separate from the rest of the Pterygota is as evident in their 

 spinasternal musculature as in other facets of their organization. If 

 our interpretation of the reported data is correct, the dragonflies 

 possess the muscle 2sps-^ils, which is found elsewhere only in Thy- 

 sanura, Megaloptera, and Coleoptera ; yet the Odonata have also icv- 

 cxiX, which the beetles and dobsonflies lack. Since the presence of 

 icv-cxxX in the Mecoptera suggests that this muscle was distributed 

 among primitive neuropteroids before the Megaloptera and Coleop- 

 tera became separated from them, the Odonata, as far as their spina- 

 sternal muscles are concerned, could have come from this same basic 

 neuropteroid stem. But the Odonata could equally well be traced to an 

 orthopteroid origin, since many present orthopteroids possess icv- 

 cxiX ; while, as we have seen, the immediate ancestors of these or- 

 thopteroid forms must have had a 2sps-^ils that had not yet disap- 

 peared nor been converted into the 2sps-Iils of some existing 

 orthopteroid species. We have mentioned on page 141 that a derivation 

 of the odonatan pattern directly from one like that of the Thysanura 

 is likewise possible. Our evidence thus leaves several choices, whose 

 common feature is merely that they all place the separation of the 

 Odonata from the other Pterygota well back in time. Simultaneously, 

 the facts afford proof that the Odonata share a distant common an- 

 cestry with other winged insects; but more than this they do not 

 tell us. 



The Mallophaga, too, present difficulties ; for their possession of 

 icv-cXiX among a total of six spinasternal muscles tends to cast them 

 either with the orthopteroid orders or with some offshoot of the postu- 

 lated primitive neuropteroid line, while their ownership of ^sps-^ils 

 is not paralleled in any existing orthopteroid group. On either alter- 

 native, the direct line of their descent would seem to go far back, and 

 their affinities to psocopteroid insects would therefore appear less 

 close than some students of other criteria have thought. 



Also unexpected is the high degree of similarity between the Mal- 

 lophaga and the phasmids. Not only do the Mallophaga possess all 

 four of the spinasternal muscles that have been found in the walking- 

 sticks, but they possess them with similar modifications. The resem- 

 blances extend further than the musculature, to the relative propor- 

 tions of the three thoracic segments, the odd position of each coxa 

 within its segment, and the unusual form of the respective segmental 

 sternal apophyses. The Phasmatodea are of course ordinarily placed 



