348 INSECT A. 



tii'st is the formation of a proctodseum (gh) by an invagination, which ends 

 blindly in contact with the rnesenteron. Shortly after this a thickening of 

 the epiblast (bsm") appears along the ventral surface, which gives rise mainly 

 to the ventral nerve cord ; this thickening is continuous behind with the 

 epiblast which is invaginated to form the proctodseum, and in front is pro- 

 longed on each side into two procephalic lobes, in which there are also 

 thickenings of the epiblast (fjsae), which become converted into supra- 

 O3sophageal ganglia, and possibly other parts. 



Towards the close of the second larval period the muscles (lm) become 

 segmentally arranged, and give indications of the segmentation which 

 becomes apparent in the third larval period. The third and last larval 

 stage (fig. 191 E) of Platygaster, during which it still remains in the tissues 

 of its host, presents no very peculiar features. The passage from the second 

 to the third form is accompanied by an ecdysis. 



Remarkable as are the larvse just described, there can I think be 

 no reason, considering their parasitic habits, for regarding them as ancestral. 



Metamorphosis and heterogamy. 



Metamorphosis. The majority of Insects are born in a condition 

 in which they obviously differ from their parents. The extent of this 

 difference is subject to very great variations, but as a rule the larvai 

 pass through a very marked metamorphosis before reaching the adult 

 state. The complete history of this metamorphosis in the different 

 orders of Insects involves a far too considerable amount of zoological de- 

 tail to be dealt with in this work ; and I shall confine myself to a few 

 observations on the general characters and origin of the metamorphosis, 

 and of the histological processes which take place during its occur- 



rence 1 . 



In the Aptera the larva differs from the adult only in the number 

 of facets in the cornea and joints in the antenna?. 



In most Orthoptera and Hemiptera the larvae differ from the 

 adult in the absence of wings and in other points. The wings, etc., 

 are gradually acquired in the course of a series of successive moult- 

 ings. In the Ephemeridae and Libellulidse, however, the metamor- 

 phosis is more complicated, in that the larvae have provisional tracheal 

 gills which are exuviated before the final moult. In the Ephemeridse 

 there are usually a great number of moultings ; the tracheal gills 

 appear after the second moult, and the rudiments of the wings when 

 the larva is about half grown. Larval life may last for a very long 

 period. 



In all the other groups of Insects, viz. the Diptera, Neuroptera, 

 Coleoptera, Lepidoptera, and Hymenoptera, the larva passes with 

 a few exceptions through a quiescent stage, in which it is known as 

 a pupa, before it attains the adult stage. These forms are known as 

 the Holometabola. 



1 For a systematic account of this subject the reader is referred to Lubbock (No. 

 4-20) and to Graber (No. 411). He will find in Weisrnanu (No. 430 and 431) a detailed 

 account of the internal changes which take place. 



