No. 625] TRE EVOLUTION OF ABTIIROPODS 



171 



The different theories concerning the origin of the 

 wings of pterygotan insects were discussed in a recent 

 paper (Crampton, 1916) in which it was pointed out that 

 it is possible to consider that the wings of insects were 

 derived from paranotal outgrowths of the tergal region of 

 apterygotan forms, Crustacea, etc., which are ultimately 

 homologous with the paranotal outgrowths of the trilo- 

 bites, without attempting to derive the wings from these 

 trilobitan structures without the intermediation of other 

 ancestral forms. Not only do the lei)isnrids exhibit para- 

 notal structures (lateral torgnl onturowtli.<) which are 

 homologous with the precursors of wiims. hut the lepis- 

 moid forms {Lepisma, Nicolefia, ^[^(■Itili>\ etc.) approach 

 remarkably closely to the pterygotan type in many re- 

 spects, and may be considered as annectent between the 

 remainder of the Aptorygota and the lower Ptorygota. 



The lowest re]n-esentatives of the Pterygota, or winged 

 insects, constitute the Perlid-Ephemerid gi-ou]). com]io-^ed 

 of the Plecoptora, Kphemerida, and their imiiu'diale rela- 

 tives. The modern representatives of the gi'ou]) are in 

 many respects fully as j)riinitive as certain of their fossil 

 relatives, nlthoimh It is iic(m'~-:ii-_\ to turn to some such 

 extinct rorins as the " Prolcpln'tHci'oidea " and Pahneo- 

 dictyo])tera to find the connecting forms annectent be- 

 tween the Plecoptera and the ephemerids. The imma- 

 ture r^lecoptera are remarkably Similar to lepismids in the 

 nature of the head outline, mouth parts, tlioracic sclerites, 

 etc. (Crampton, 1917a), and even in regard to their 

 terminal abdominal structures the lepismids are very like 

 Plecoptera (Crampton, 1918«), but tlie Plecoptera have 

 lost the median terminal filament, which, however, is still 

 retained in the ephemerid members of the group. The 

 ephemerids, and the Odonata, represent somewhat aber- 

 rant types of development which branched off at an early 

 date to follow their own patlis of specialization, although 

 they have not i)roeeeded very far n'oiiu" tlr- road. The 

 Plecoptera, on the other hand. Iiav*^ can I.m] over in their 

 line of inheritance a great many (/liaiaeters which were 

 to beconu^ furtliei- developed in the hiffhei' groups of in- 



