Class 3. Insecta. 247 



the body have attained their definite form. In some hemimetabolous 

 Insects the differences between larva and adult are^ however^ 

 much greater^ attributable to essential differences in habits. These 

 distinctions are very well marked in the Libellulidse and the 

 Ephemeridge, which are aquatic during larval life, but terrestrial as 

 adults ; in the larvae of these forms the tracheal system is closed, 

 and they breathe by means of tracheal gills {see p. 241); in the adult, 

 on the contrary, the usual relations obtain. Considerable differences 

 may also be noticed in several other points, e.g., in the dispositions 

 of the mouth-parts of the Libellulids : these differences hold 

 throughout the whole larval life until the last ecdysis, but, just as in 

 ■other forms, the wings develop gradually. At the last ecdysis all the 

 special larval characters disappear as at one stroke, although, as a 

 matter of fact, the changes have gone on gradually within the cuticle 

 •during the last phases. When the wings are completely developed, 

 and functional, the insect moults no more, and growth ceases. 



The Ephemeridse, wliicli, as already mentioned, are aquatic as lai'vae, are peculiar 

 in that the insect on leaving the water has feeble, though fnnotional, wings, but 

 immediately after, the final ecdysis occui-s and it appears with completely 

 developed wings. At this stage, in which it is capable of flight, but not 

 perfectly developed, it has been termed a sub-imago. 



Holometabolous Insects (Coleoptera, Hymenoptera, Lepi- 

 doptera, and Diptera) differ from the hemimetabolous forms, in that 

 there is a complete dissimilarity between the larva and the adult; 

 in that the larval stages exhibit, externally, no gradual approach 

 to the adult form; lastly, and this is the most important characteristic 

 of a complete metamorphosis, in that between the larval and imaginal 

 stages, a special period of pupation intervenes, during which the 

 animal doe's not feed and is generally quiescent. During this resting- 

 stage a series of significant changes occurs in the body. 



The larva of a holometabolous insect is distinguished from the 

 imago in the following points : the small ocelli situated medianly on 

 the head are absent, and the compound eyes are replaced by a group 

 of ocelli on each side of the head (these may, however, be altogether 

 wanting) : the antennae are almost always short, and consist of 

 a small number of joints : the mouth-parts constitute a biting 

 apparatus, even if those of the adult are suctorial, and if the latter 

 has biting mouth-parts they are always essentially different in form: 

 the legs are short, with fewer joints, and more uniform than in the 

 adult ; they have usually only a single claw : wings are altogether 

 absent : the thorax is small, the abdomen large : the cuticle, 

 ■except over the head, is generally more delicate than in the 

 imago : the nervous system usually consists of a number of separate 

 ganglia even in those forms in which it is later much concentrated 

 (cf. Fig. 210) ; the alimentary canal is often very different, and this 

 to a striking degree where the larval habits differ much from those 



