ANA TOMY AND MORPHOLOG Y OF INSECTS. 21 



the cuticle ; the wings are similarly developed in cater- 

 pillars.* 



In the hairy caterpillars of the Lepidoptera, as De Reaumur 

 showed, the hairs on the new integument are formed in the 

 same way, and not in the interior of those which clothe the 

 integument which is about to be shed. 



In the hymenopterous vermiform larva, according to 

 Dewitz,t the first rudiments of the legs of the imago are simi- 

 larly formed in the young larva, but before the ecdysis they 

 sink, as it were, beneath the hypoderm, which forms flask- 

 shaped involutions around them, so that they are not revealed 

 at the next ecdysis, but remain as embryonic rudiments until 

 the larva attains its full growth, and only appear externally at 

 its last ecdysis, when it passes into the nymph stage. In 

 general all the structures which are peculiar to the imago in 

 the Metabola are produced from such encapsulated rudiments, 

 which are known as imaginal discs. 



Imaginal Discs (Fig. j). — This term was first applied by 

 Weismann to certain encapsulated groups of embryonal cells 

 closely related to the central nervous system in the larvae and 

 young pupa; of certain Diptcra. They differ in no way materially 

 from the rudiments of the legs of the hymenopterous nymph 

 which appear in the larva, except that they arise in the embryo or 

 very young larva, and during the growth of the latter they remain 

 closely attached to the nerves and trachea;, and apparently lose 

 all connection with the hypodermis. This is, however, only 

 apparently the case, as later observers have repeatedly detected 

 the long drawn-out neck of the capsule of hypodermis in which 

 they are enclosed, passing into the subcuticular hypodermis of 

 the part from which they orginate. Indeed, Weismann saw 

 these bands, but did not correctly interpret them. No doubt, 

 however, can exist any longer on this point, as will be suffi- 

 ciently demonstrated in the course of this work. 



* E. Verson, ' La formazione dcUe all ncUa Larva del Bombyx mod.' 

 R. Stazione Bacologica speiimeiitale Pubblicazioni dal Ministero di Agticolt. 

 ind. E. Comm. With two plates. Padova, i8go. 



t Dewitz, ' Beitriige zur Kenntniss der Postenibryonalen EntwickUing dcr 

 Gliedmassen bei den Insecten.' Zeitsch. f. w. Zool., Bd. xxx., suppl. 1878. 



