456 THE INVERTEBRATA 



Hymenoptera, Diptera, and Aphaniptera. The young stages of these 

 are known as larvae and differ from their aduUs in body form, mouth 

 parts, and the absence of compound eyes. So great is the difference 

 between the larva and the aduh that an instar known as the pupa 

 has been specialized to bridge the gulf (Fig. 341). This stage, one of 

 apparent rest, is actually one of great physiological and developmental 

 activity, and it is here that many larval tissues, e.g. the muscles and 

 the alimentary canal, are broken down by phagocytic or other action 

 and the new adult tissue is built up from many growth centres, 

 generally known as imaginal discs. A less obvious prepupal instar is 

 also present, enabling the change from larva to pupa to be effected. 



It may reasonably be assumed that metamorphosis of the Holo- 

 metabola has arisen through larval and adult specialization going on 

 concurrently but in opposite directions, and it is not surprising to 

 find among the orders composing this group, as for instance in many 

 Coleoptera, larvae which are rather nymph-like in that they are well 

 chitinized and possess well-developed legs, and mouth parts re- 

 sembling those of the adults (Fig. 318 A). 



The forms of larvae vary considerably and indicate to a great extent 

 the degree of metamorphosis passed through. A campodeiform larva 

 (Fig. 3 18 A) is one strongly resembling certain members of the 

 ametabolous Thysanura and possesses well-developed legs, biting 

 mouth parts, antennae and cerci, e.g. many Coleoptera. An eruci- 

 form larva (Fig. 318B) is fleshy and thin-skinned, its legs are often 

 in the form of supporting struts rather than organs of active loco- 

 motion, and there are no cerci. Further, prolegs are often found on 

 the abdomen, e.g. caterpillars of Lepidoptera and sawflies (Fig. 344). 

 A grub (Fig. 318 C) is an apodous larva which in other respects re- 

 sembles the cruciform type, e.g. certain Diptera, Coleoptera and 

 Hymenoptera. 



Pupal modifications are also found; thus the exarate type, cha- 

 racteristic of the Hymenoptera, Mecoptera, Neuroptera, is that in 

 which the cases, in which the adult appendages lie, are free of any 

 attachment to the body (Fig. 341). In obtect pupae (Fig. 338), wing 

 and leg cases are fused to the body wall, e.g. most Lepidoptera and 

 Diptera. In the most specialized Diptera the last larval skin is re- 

 tained as a barrel-shaped puparium over the pupa within. Such pro- 

 tected pupae are called coarctate (Fig. 349). 



In the Heterometabola the development of adult form is a gradual 

 process and the appendages, including mouth parts, antennae and 

 legs, grow directly into those of the adult. Wings in such forms 

 develop gradually as external dorsolateral extensions of the meso- and 

 metathoracic body wall (Fig. 317). All the Heterometabola have 

 such a wing development and therefore the alternative name Ex- 

 opterygota is often given to the group. 



