42 ON THE ORIGIN AND [CHAP. 



labrum, labium, and, in fact, all the mouth parts of a 

 perfect insect. 



The caterpillars of Lepidoptera are generally classed 

 with the vermiform larvae of Diptera and Hymen- 

 optera, and contrasted with those of Orthoptera, 

 Hemiptera, &c. ; but, in truth, the possession of tho- 

 racic legs places them, together with the similar larvae 

 of the Tenthredinidae, on a decidedly higher level. 

 Thus, then, the period of growth (that in which the 

 animal eats and increases in size) occupies sometimes 

 one stage in the development of an insect, sometimes 

 another ; sometimes, as for instance in the case of 

 Chloeon, it continues through more than one ; or, in 

 other words, growth is accompanied by development. 

 But, in fact, the question is even more complicated 

 than this. It is not only that the larvae of insects at 

 their birth offer the most various grades of develop- 

 ment, from the grub of a fly to the young of a grass- 

 hopper or a cricket ; but that, if we were to classify 

 larvae according to their development, we should have 

 to deal, not with a simple case of gradations only, but 

 with a series of gradations, which would be different 

 according to the organ which we took as our test. 



Apart, however, from the adaptive changes to which 

 special reference was made in the previous chapter, 

 the differences which larvae present are those of gra- 

 dation, not of direction. The development of a grass- 

 hopper does not pursue a different course from that 

 of a butterfly, but the embryo attains a higher state 

 before quitting the egg in the former than in the 

 latter: while in most Hymenoptera, as for instance 

 in Bees, Wasps, Ants, &c., the young are hatched 



