METACHEMOGENESIS IN INSECTS — ROCKSTEIN 265 



from the apparent implication of his own experimental data and those 

 of others interested in the biochemistry and physiology of such insects 

 at different stages of their life histories. Dr. Snodgrass's response to 

 a preliminary draft of this thesis included a comment that this sug- 

 gested a "new line of research" in the matter of the continuity of 

 development from the pupal stage into adult life (Snodgrass, personal 

 communication, 1955). The scattered and sometimes obscure data 

 presented herein will serve to emphasize the need for serious reex- 

 amination of the more classical, restricted concept of metamorphosis, 

 as a climactic event by which the adult form is completed (aside from 

 sexual maturation) at the moment of or very shortly after emergence 

 of the imago. Indeed, there appears to be increasingly convincing 

 evidence for the universal existence of postemergence maturation 

 changes (such as alteration in biochemical properties) in adult holo- 

 metabolous insects, which must be reflective of the maturation of 

 specific body functions of the adult, such as flight ability, for example. 



THE MEANING OF METAMORPHOSIS 



In his classical treatment of holometaboly, Poyarkoff (1914) in- 

 ferred that the pupa of holometabolous insects is not a recently 

 acquired additional stage, interposed between the larva and adult, but 

 is rather a subdivision of the imaginal stage, resulting from the inter- 

 position of an additional moult. It is interesting that he compared the 

 pupa of the Holometabola with the subimaginal form of the Ephem- 

 erida and suggested that they both represented incomplete adults. 

 Hinton (1948) in confirming Poyarkoff's view also emphasized Poy- 

 arkoff's suggestion that this additional moult is essential to the com- 

 pletion of development of the adult in providing a new cuticle to which 

 the muscles of the imago, newly formed, can attach themselves. Snod- 

 grass (1954), in reaffirming this position, presents firm morphological 

 evidence in the form of various examples of pupal resemblance to the 

 adult. Wigglesworth (1954) also states that the obvious function of 

 the pupa is to bridge the morphological gap that exists between the 

 larva and the fully developed adult, for which he cites experimental 

 embryological data (such as the fact that irradiation of the egg of 

 Drosophila or of Tineola up to a particular point in embryonic devel- 

 opment affects only larval characters ; after such times only the im- 

 aginal characters may be affected). This is further evidence for the 

 continuity of development of the adult from the larval stage, in the 

 very presence of the imaginal discs in the general epidermis of the 

 larva, frequently in the early instars, even in the prelarval embryo. 



