704 



TEXT-BOOK OF ENTOMOLOGY 



existence on a plane scarcely higher than that of an active ciliated 

 gastrula. 



Further observations, difficult though they will be, are needed to 

 enable us to account for the singular prematurity of the embryo of 

 these parasites. That these stages are reversional and a direct in- 

 heritance from the vermian ancestors of these insects is not proba- 

 ble, but the forms are evidently the result of adaptation in response 



at 



FIG. 653. Third larva of Poly- 

 nema : at, antenna ; fl, imaginal bud 

 of wing ; I, rudimentary legs ; tg, 

 buds of one of the three pairs of 

 styles of the ovipositor ; fk, fat- 

 body ; eg, ear-like process. 



b ut md 



FIG. 654. A-D, development of Teleas ; A, stalked 

 egg; B, C, D, the 1st larval stage: at, antenna: 'ml, 

 hook-like mandibles; mo, mouth ; b, bristles; m, intes- 

 tine ; 810, the tail ; ul, under lip or labium. E, larva of 

 another parasite, Ophioneurus. This and Figs. 650-653 

 after Ganin. 



to a series of stimuli whose nature is in part appreciable but mostly 

 unknown. 



It may be noted, however, that the appearance of a primitive band 

 in the second larval stage suggests the origin of these forms, as well 

 as that of insects in general, from a Peripatus-like, and again from 

 an earlier leech-like Annelid ancestor. Hence the first larval or 

 Cyclops stage is due to a precocious development caused by the 

 unusual environment, and is simply adaptational, and not of phylo- 

 genetic significance. 



