350 IN SECT A. 



appear as small projections. At every successive moult these pro- 

 jections 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 metamorphosis in such forms as the 

 Ephemerida3 only differ from the above in the fact that provisional 

 organs are thrown off at the same time that the new ones are 



o 



formed. 



In the case of the Holometabola the internal phenomena of the 

 metamorphosis are of a very much more remarkable character. The 

 details of our knowledge are largely due to Weismann (No. 430 and 43 1). 

 The Iarva3 of the Holometabola have for the most part a very different 

 mode of life to the adults. A single 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 ex- 

 ternal 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 Tipulidse. This larva, like that of 

 other Tipulidse, 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, trachege, etc., of the limbs ; which are believed by Weis- 

 mann (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 discs. The hypodermis of the 

 larva passes directly into that of the imago. 



The pupa stage of Corethra is relatively very short, and the 

 changes in the internal parts which take place during it are not con- 

 siderable. The larval abdominal muscles pass for the most part un- 

 changed into those of the imago, while the special thoracic muscles 

 connected with the wings, etc., develop directly during the latest 

 larval period from cords of cells already formed in the embryo. 



In the Lepidoptera the changes in the passage from the larval to 

 the adult state are not very much more considerable than those in 

 Corethra. Similar imaginal discs give rise during the later larval 

 periods to the wings, etc. The internal changes during the longer 

 pupa period are somewhat more considerable. Important modifica- 



