88 BRITISH LEPIDOPTERA. 



in the pleural membrane of either side backwards, represents the 9th 

 and 10th somites of the larva and pupa, whilst the terminal cone 

 is a new formation within the area of the 10th somite. The pupal 

 cremaster, it may be added, undergoes complete atrophy, and is not 

 represented in the imago (Jackson). As to the actual formation 

 of this structure he writes : " The terminal papilla of the abdo- 

 men of the imago is formed by the growth of two folds of hypo- 

 dermis, a right and a left, inclosing a small portion only of the area 

 of the 10th somite. Accordingly it does not represent the 9th somite 

 as is generally held ; the whole 9th somite and the rest of the 10th 

 somite of the pupa are not clearly marked off from the 8th somite of 

 the imago. The apparent 8th tergum of the imago probably repre- 

 sents the 8th and 9th pupal terga and the 10th, if any portion of it is 

 persistent. The pleural region of the 9th pupal somite and of the 

 10th, so far as it exists, is represented by the soft membrane lying 

 behind the ridge extending dorsally from the eighth sternum of the 

 imago. As the bursal aperture lies near the centre of the eighth 

 sternum, this region may possibly include in the imago a portion of 

 the 9th sternal region as well. The softer integument behind, con- 

 necting it to the base of the terminal papilla, would, in that case, 

 coincide with a portion of the 9th sternum, and the ventral portion of 

 the 10th somite of the pupa anterior to the place of origin of the 

 terminal papilla." 



CHAPTER V. 



THE PHYLOGENY OF THE LEPIDOPTEEOU'S PUPA.'' : 



The essential cause of metamorphosis in insects is the differentiation 

 of function between the earlier and later stages. In the Orthoptera, the 

 insect has the same form and the same functions throughout its exist- 

 ence, differing only in size and the possession of wings. Although repro- 

 duction is especially the function of the adult winged form, even this 

 is not always so, but nutrition is equally attended to, and in the same 

 manner at all ages. Specialisation occurs in the Metabola, in the 

 direction of feeding being especially the function of the early stages, 

 the dispersal and reproduction of the species of the last. This leads 

 to differences of structure in the early and last stages, only to be 

 bridged over by an intermediate, quiescent (the pupal) stage. 



We find this stage fully established in the Coleoptera and 

 Hymenoptera. In these orders, the pupa is absolutely quiescent and 

 helpless, and, therefore, in need of protection from enemies and acci- 

 dents of all sorts. This protection is secured by the pupal stage being 

 passed in a cavity prepared by the larva. As we are only incidentally 

 interested in the pupae of these two great orders, we pass by, as com- 

 paratively rare and unimportant, the cases in which the pupa is 

 exposed, and in which it is protected in other ways, instructive though 

 these cases are as instances of further pupal evolution. Nor need we 

 go into detail as to the character of the pupal nidus, whether it be a 



* This chapter has been entirely written by Dr. T. A. Chapman, to whom we 

 express our warmest thanks. 



