INSECTA 459 



abdominal legs), and Oligopod (thoracic legs only) stages respectively. 

 It is noteworthy that among larvae there are forms resembling these 

 different stages. Thus the first stage larva of Platygaster is in a Proto- 

 pod condition (Fig. 3 19 E). The first stage larva of Phaenoserphus is in 

 a Polypod state. The first two stages in the larval life history of the 

 Cynipid Figites resemble Protopod and Polypod embryonic stages 

 respectively (Fig. 319 F, G). The Campodeiform larvae of Carabid 

 beetles, of certain Trichoptera, and of the Neuroptera, resemble the 

 final Oligopod stage of embryonic development (Fig. 319 H). Because 

 of facts of this nature, it has been suggested by Berlese in a theory 

 which now carries his name, that the moment of eclosion from the 

 ^gg> perhaps decided by the amount of yolk, is one of significance in 

 determining the form of the larva. Thus in the Holometabola, the 

 stage in which hatching occurs corresponds with one or other of the 

 embryonic phases alluded to. Some insects hatch as veritable em- 

 bryos, i.e. as protopod, others as polypod, or oligopod larvae. A 

 fourth larval form, the apodous grub of Diptera, of many Hymenop- 

 tera and of some Coleoptera may be derived by degeneration from 

 the preceding oligopod stage. 



The theory further maintains that in Heterometabolous insects, the 

 above stages, with the exception of the Apodous, are always passed 

 through in the egg, and emergence from the egg in these insects 

 occurs as a nymph which has thus reached in embryonic life a higher 

 stage of differentiation than any larva. 



The natural corollary of this theory is that certain if not all of the 

 nymphal stages of the Heterometabola correspond to the prepupal 

 and pupal instars of the Holometabola. 



The development of adult appendages in the larva is only one of 

 the many aspects of metamorphosis. The wings which suddenly 

 appear in the pupa of the butterfly grow gradually through each of 

 the five larval instars, but instead of growing externally as in the 

 Heterometabola (Exopterygota) they arise as outgrowths from the 

 bottom of intuckings of the body wall. In other words an accom- 

 modating fold of the body wall forming a sac, opening at the surface 

 by a minute pore, hides the growing wing bud within it and this is 

 the main difference between endopterygote and exopterygote develop- 

 ment. 



At pupation the sac carrying the wing disc or bud at its base be- 

 comes straightened out by contraction of its walls and the wing bud 

 is thereby brought to view. Similar limb buds are to be found for the 

 adult legs and mouth parts, which always grow in association with the 

 corresponding larval organs. Such buds are known collectively as 

 imaginal discs and their existence characterizes all endopterygote 

 insects (Fig. 320). 



