64 METAMORPHOSIS 



the prepupa is the sole vestige of the existence of more than a 

 single pupal instar, and even the ecdysis between the last larval 

 instar and this phase has been eliminated, except in very few 

 instances. In the metamorphoses of the fly Rhagoletis, for 

 example, Snodgrass (1924) showed that an extra-cuticular coat 

 intervenes between that of the pupa and the wall of the puparium. 

 Evidence of a prepupal ecdysis has also been noted by Raff (1934) 

 in the case of sawflies of the genus Perga, where the exuvias in 

 question are located near the caudal extremity in the pupal 

 cocoon. Such instances, however, are few, but the matter has 

 attracted very little attention. 



The pupal instar represents, therefore, the abbreviated recapitu- 

 lation of ancestral nymphal stages. Change of function, however, 

 has resulted in the evolutionary process and an acquired quiescent 

 condition has been assumed. This condition became necessary 

 in order to allow the larval body and its component organs to 

 become remodelled, in order to adapt them to the requirements 

 of the future imago. 



It is well known that, during the usual process of ecdysis between 

 any two nymphal or larval instars of a given insect the old cuticle is 

 cast off. Prior to the consummation of this event, the formative cells 

 beneath not only secrete a new cuticle, but also remodel the outer 

 configuration of the body and the appendages into the new form to be 

 assumed. This process goes on gradually from instar to instar, and in 

 many insects the changes involved are but slight, but in others they 

 are considerable. In the Anoplura, for example, they are so trivial as 

 to be scarcely evident, while in many of the Heteroptera remarkable 

 changes in body-form and the appendages may result. In the 

 Holometabola this same formative process is continued in the pupa, but 

 it lies concealed, and drastic changes are involved. The descendants of 

 the ectodermal formative cells, which modelled the larval appendages 

 from instar to instar, give rise to the imaginal buds, and it is from these 

 rudiments that the corresponding imaginal parts are derived. It 

 is too commonly implied that there is entire discontinuity during 

 pupation, and that the imaginal appendages are new developments, 

 apart from those of the larva. If a graded series of pupal types be 

 studied, it will be seen the change from the larval appendages into those 

 of the adult is a matter of degree only — there is no real discontinuity 

 in the process. 



Hormones and Metamorphosis. One of the most notable results 

 of recent work in experimental entomology has been the discovery 



