422 INSECTA. 



become covered by the freshly formed cuticle, and appear as 

 small projections. At every successive moult these projections 

 become more prominent owing to a growth in the epidermis 

 which has taken place in the preceding interval. Accompanying 

 the formation of such organs as the wings, internal changes 

 necessarily take place in the arrangement of the muscles, etc. of 

 the thorax, which proceed pari passu with the formation of the 

 organs to which they belong. The characters of the metamor- 

 phosis in such forms as the Ephemeridae only differ from the 

 above in the fact that provisional organs are thrown off at the 

 same time that the new ones are formed. 



In the case of the Holometabola the internal phenomena of 

 the metamorphosis are of a very much more remarkable cha- 

 racter. The details of our knowledge are largely due to Weis- 

 mann (Nos. 430 and 431). The larvae of the Holometabola have 

 for the most part a very different mode of life to the adults. 

 A simple series of transitions between the two is impossible, 

 because intermediate forms would be for the most part incapable 

 of existing. The transition from the larval to the adult state is 

 therefore necessarily a more or less sudden one, and takes place 

 during the quiescent pupa condition. Many of the external 

 adult organs are however formed prior to the pupa stage, but do 

 not become visible on the surface. The simplest mode of Holo- 

 metabolic metamorphosis may be illustrated by the development 

 of Corethra plumicornis, one of the Tipulidae. This larva, like 

 that of other Tipulidae, is without thoracic appendages, but 

 before the last larval moult, and therefore shortly before the 

 pupa stage, certain structures are formed, which Weismann has 

 called imaginal discs. These imaginal discs are in Corethra 

 simply invaginations of the epidermis. There are in the thorax 

 six pairs of such structures, three dorsal and three ventral. The 

 three ventral are attached to the terminations of the sensory 

 nerves, and the limbs of the imago are formed as simple 

 outgrowths of them, which as they grow in length take a spiral 

 form. In the interior of these outgrowths are formed the 

 muscles, tracheae, etc., of the limbs ; which are believed by 

 Weismann (it appears to me without sufficient ground) to be 

 derived from a proliferation of the cells of the neurilemma. 

 The wings are formed from the two posterior dorsal imaginal 



