420 INSECTA. 



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 larvae 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 detail 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 antennae. 



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 

 moultings. In the Ephemeridae and Libellulidse, however, the 

 metamorphosis is more complicated, in that the larvae have 

 provisional tracheal gills which are exuviated before the final 

 moult. In the Ephemeridae 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, Neuro- 

 ptera, 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. 



In the Diptera the larvae are apodous. In the true flies (Muscidae) they 

 are without a distinct head and have the jaws replaced by hooks. In the 

 Tipulidae there is on the other hand a well-developed head with the normal 

 appendages. The pupae of the Muscidae are quiescent, and are enclosed in 

 the skin of the larva which shrinks and forms a firm oval case. In the 



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

 420) and to Graber (No. 411). He will find in Weismann (Nos. 430 and 431) a detailed 

 account of the internal changes which take place. 



