6o WHEELER. [Vol. VOL 



placed over against the Pterygota, which are characterized by 

 the possession of an amnion (Hexapoda Amniota). There is 

 a gap between these two groups of insects similar to the gap 

 between the amniote and anamniote vertebrates. Whether it 

 will be filled by the future study of such orthopteroid forms as 

 Machilis, Lepisvia and Forficiila remains to be seen. For the 

 present I am inclined to believe that the amnion first made its 

 appearance in the ancestral Pterygota. Even if it be contended 

 that the amnion was once present in the Apterygota and subse- 

 quently lost, its origin could not consistently be pushed further 

 back than the Hexapoda, since this envelope is lacking in the 

 Myriopoda, which, there is reason to believe, lie in the direct 

 line of descent. The proof that the so-called amnions of Peri- 

 patus, Scorpions and Pseudoscorpions are the homologues of 

 the insect amnion is not forthcoming. Judging from the few 

 descriptions of their formation, they appear to have arisen in- 

 dependently within their respective groups. 



Just as many of the Pterygota develop only rudiments of 

 wings or have altogether ceased to develop these organs in the 

 adult state, so the embryos of the Pterygota in some cases 

 develop only rudimental envelopes or none at all. It is reported 

 that the amnion is lacking in the Proctotrupid Hymenoptera 

 (Ayers, '84) and rudimental in Muscidae (Kowalevsky, '86 ; 

 Graber, '89) and viviparous Cecidomyidae (Metschnikoff, '66). 

 Certain ants of Madeira are incidentally mentioned by Met- 

 schnikoff as having the envelopes represented only by a small 

 mass of cells in the dorsal region. The absence or abortion 

 of the amnion is almost certainly a secondary condition. The 

 Proctotrupidae are egg-parasites and undergo an extremely ab- 

 errant embryonic and larval development. Both these and the 

 other insects mentioned belong to groups characterized by 

 high specialization. This is notably the case with the ants 

 and with the Muscidae which show considerable aberration in 

 their embryonic and larval stages. The paedogenesis of the 

 Cecidomyids studied by Metschnikoff stamps them also as ab- 

 errant. Moreover the embryos of other Orthorrhaphous Dip- 

 tera (Simulidae, Chironomidae, Tabanidae) have perfectly normal 

 envelopes. 



