258 INSECT TRANSFORMATION 



pupal stage in the life-history. And from this it may be 

 reasonably inferred that the stage in which these rudiments 

 appeared became later and later in the history of the more 

 highly-developed orders. The change therefore may be 

 supposed to have begun in an early stage by the small wing- 

 buds sinking into the body so that they could not be covered 

 by the next-formed cuticle, and thus their outward appearance 

 was postponed to successively later stages and finally to the 

 pupal stage. The cases of the beetles just mentioned illustrate 

 the previous condition of their appearance in the pre-pupal 

 stage. And it may be assumed that this difference in the 

 mode of wing-growth was correlated with the increasing 

 divergence between adult and larva to which reference has 

 so often been made in these pages. 



In this connexion it is suggestive to notice that among 

 many exopterygote insects, in which the newly-hatched young 

 is a larva differing in aspect from its parent the Aleyrodidae 

 and male Coccidae (pp. 86-89), among the Hemiptera, for 

 example the wing-rudiments grow beneath the cuticle of 

 the resting larva, like those of the pupa in an endopterygote 

 life-history. In these and in other insects, such as the Thy- 

 sanoptera, the final nymphal form is passive, prefiguring the 

 pupal condition among the higher insects. 



The pupa is so distinctive a feature in the transformations 

 of insects of the more specialized orders, that it is of especial 

 interest to find, in the life-histories of simpler types, stages 

 that may be compared with it. Those just mentioned illus- 

 trate how, in correlation with special conditions of the life- 

 history, an exopterygote insect may be passive in the stage 

 immediately preceding the adult ; this quiescent, pre-imaginal 

 instar may occur in cases where the young differs but little 

 from the adult (Thysanoptera), or in cases where the young is 

 hatched in a distinctive larval form (Cicadidae, Aleyrodidae, 

 Coccidae}. On the other hand the mayflies (Ephemeroptera) 

 furnish the unique example of a sub-imago, capable not only 

 of motion but of flight, preceding immediately the adult stage. 

 The comparatively large size of the wings in many endoptery- 

 gote pupae suggests that the condition now found only in the 

 mayflies may possibly have been general among the primitive 

 insects of early times. And on the other hand it is instructive 



