Morphology and Phylogeny of Insects. 439 



accepted as a proof of the secondary nature of an organ, since 

 in the appearance of the organs in the embryonic development 

 of closely allied animals no definite sequence whatever is 

 observable ; it is possible for one and the same organ or 

 system of organs to appear in one animal very early but in 

 another very late. Thus, for instance, in Apis the pores for 

 the stigmata are almost the very earliest differentiations of 

 the segmented germinal streak, while in Blatta germanica 

 they are not observable until after the segmentation of the 

 extremities has begun. But does it follow from this that the 

 stigmata are primary in Hymenoptera but secondary in 

 Orthoptera ? Witli regard, iiowever, to the discontinuity in 

 the series of abdominal appendages and their appearance only 

 on those segments on which pro-legs are present in the cater- 

 pillar, this fact, which has hitherto only been observed in the 

 embryo of one moth, does not yet prove in itself that the 

 pro-legs are secondary structures. Instead of setting up the 

 hypothesis that the caterpillars had acquired their abdominal 

 appendages in post-embryonic development, and that subse- 

 quently the first appearance of these organs was transferred 

 to early (embryonic) stages, we are just as much justified in 

 assuming that in the embryo, which originally possessed a 

 continuous series of abdominal appendages, later on rudiments 

 of only those appendages began to appear which also per- 

 sisted in post-embryonic development ; further, that the 

 suppression of tlie rest of the abdominal extremities resulted 

 from the same cause as that which in the embryo of other 

 Insects is responsible for the non-appearance of the whole of 

 the abdominal feet, i. e. in consequence of disuse. The 

 second hypothesis is even a priori not less probable than the 

 first ; but, by the fact that in certain lepidopterous embryos 

 a continuous series of abdominal legs is actually present, it is 

 completely confirmed and is certainly the only correct one. 

 When discussing my paper upon the external development of 

 Blatta germanica. Prof. Emery, inter alia, writes as 

 follows * : — " The abdominal legs of lepidopterous cater- 

 pillars may quite well have developed afresh through reversion 

 in phylogeny from the embryonic rudiments which quickly 

 disappear in the case of other insects, and those caterpillars 

 may with much probability be derived from primitive Cam- 

 podea-\'\k.G, forms. It is probable that cases of atavism of this 

 kind })lay a much more important part in phylogeny than is 

 generally supposed. Primitive structures are developed 



* Emery, " Neiiere Arbeiten iiber die Phylogenie der lusekten," Biol. 

 Centrulbl., Bd. ix., 1889-00, pp. ^0(3-405. 



