402 THE INVERTEBRATA 



A grub (Fig. 300 C) is an apodous larva which in other respects re- 

 sembles the eruciform type, e.g. certain Diptera 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. 320). In ohtect pupae (Fig. 317), 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 retained 

 as a barrel-shaped ^wpanwm over the pupa within. Such protected 

 pupae are called coarctate (Fig. 326). 



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. 299). All the Heterometabola have 

 such a wing development and therefore the alternative name 

 Exopterygota is often given to the group. 



Larvae of the Holometabola on the other hand possess, for the 

 most part, mouth parts having a form and mode of working different 

 from that of their adults, their legs are reduced in size and complexity 

 or even absent, and they show no sign of external wing growth. It 

 is in the pupal stage that adult appendages appear for the first time 

 on the surface. 



The development of adult appendages in the larva is only one of 

 the many aspects of metamorphosis. The wings which suddenly 

 appear in the pupa of the butterfly grow gradually through each of 

 the five larval instars, but instead of growing externally as in the 

 Heterometabola (Exopterygota) they arise as outgrowths from the 

 bottom of intuckings of the body wall. In other words an accom- 

 modating fold of the body wall forming a sac, opening at the surface 

 by a minute pore, hides the growing wing bud within it and this is the 

 main difference between endopterygote and exopterygote development. 



At pupation the sac carrying the wing disc or bud at its base be- 

 comes straightened out by contraction of its walls and the wing bud is 

 thereby brought to view. Similar limb buds are to be found for the 

 adult legs and mouth parts, which always grow in association with the 

 corresponding larval organs. Such buds are known collectively as 

 imaginal discs and their existence characterizes all endopterygote 

 insects (Fig. 301). 



Fossil record. Though the insects form an undoubted natural group 

 — all its members being referable to some generalized form, possessing 

 among other things mouth parts similar to those of the cockroach, 

 efficient for chewing solid food, an ii-segmented abdomen, a 3-seg- 

 mented thorax and a 6-segmented head, and two pairs of membranous 



