GROWTH AND TRANSFORMATION 187 



characteristic of the more generalised and the latter of the 

 more specialised orders of winged insects. From this, as 

 well as from the probability that there has been mutual 

 divergence between larva and imago among the meta- 

 morphic insects, it may be presumed that the hidden method 

 of wing-growth has in these orders superseded the primitive 

 open method. It is by no means easy, however, to under- 

 stand why the wing-rudiments which are evident on the 

 outer aspect of a young grasshopper early in its life-history, 

 do not in a beetle or moth pupa become externally visible 

 until the last larval cuticle has been shed and the pupa 

 revealed. We have seen, from the origin and growth of 

 the imaginal wing-buds in a caterpillar, how this state of 

 things is brought about ; the problem that confronts the 

 student is to find a reason why they sink into apparently 

 internal pouches instead of growing outwards. The fact that 

 they grow outwardly in the more primitive orders of insects 

 indicates that the hidden type of growth must be regarded 

 as secondary ; the problem may therefore be stated as the 

 mode of derivation of the one type of wing-growth from the 

 other, the origin of the endopterygote from the exopterygote 

 life-history. 



In elucidation of this problem it may be instructive to 

 notice examples of the abnormal appearance of outward 

 wing-rudiments on the larvae of certain metamorphic insects. 

 This was observed in " mealworms," grubs of the beetle 

 Tenehrio molitor^ by R. Heymons (1896), and has recently 

 been studied with some detail in that same species by 

 H. Singh-Pruthi (1924). Abnormal outward wing-rudi- 

 ments on mealworms (Fig. 53) have been usually noticed on 

 well-grown specimens, but Singh-Pruthi has demonstrated 

 them on comparatively early larvae and has shown that their 

 appearance is facilitated by submitting the insects to a high 

 temperature, which has the effect of retarding or preventing 

 the final transformation into beetles. Most of these 

 abnormal mealworms fail to pupate ; the pupae that do 

 result are also abnormal, and those individuals that succeed 

 in reaching the pupal stage rarely develop into beetles. 



