GENERAL 11 



larvae of the several groups. Sometimes there are two 

 distinct forms of larvae and the change from one to the 

 other is so great as to constitute a true hypermetamorphosis. 

 This occurs in the order Lepidoptera and will be discussed 

 and illustrated under that order in Chapter VII (see p. 136). 

 The ordinary larva, as every one knows, has a cylindric 

 worm-like body with a head capping its front end, and 

 mouth parts directed downward. It is, of course, an insect 

 with three main body divisions, head, thorax and abdomen; 

 but the last two are very similar in external appearance, 

 the three rings of the thorax being distinguished from the 

 more numerous rings of the abdomen by the possession of 

 minute jointed legs on each: even this distinction fails when 

 these legs disappear. The ordinary larva feeds downward 

 from the surface to which it clings. The leaf -miner, on the 

 contrary, must feed forward. The ordinary larva creeps 

 freely about, but the leaf -miner is confined within its narrow 

 gallery. To understand the modifications of form that go 

 with the leaf -mining habit we must bear in mind the condi- 

 tions imposed by the habitat. 



The principal needs of the miner in accordance with which 

 all its peculiarities of form have been evolved, are for thin, 

 flat forward-reaching mouth parts, and for holding apparatus 

 to keep them up against the mesophyll for their work. 

 Hence the mouth turns forward, and the head takes on the 

 form of a flat wedge. Walking legs tend to disappear and 

 a variety of stay-apparatus tends to be developed — spacing 

 humps, and tubercles and bristles and bands of setulae, 

 and sometimes an anal sucker. The miners that live in 

 the more solid leaves have the prothorax greatly enlarged, 

 filled with powerful muscles for moving head and jaws, and 

 covered above and below with flat plates of chitin, to take 

 the brunt of the pressure against the walls of the mine. The 

 rear of the head capsule has become more widely opened, 

 and the hind margins have been produced backward within 



