302 INSECTA. 



consolidated by degrees, ultimately acquire that density of struc- 

 ture which the external skeleton of the insect exhibits in its 

 perfect or imago state. But, while this extraordinary metamor- 

 phosis is going on externally, other changes not less important 

 are in progress in the interior of the body. The size of the 

 alimentary canal, and the shape, proportionate dimensions, and 

 general arrangement of the different parts composing it, are se- 

 cretly and imperceptibly undergoing variations in accordance with 

 the altered necessities of the animal. We have already seen a 

 conspicuous example of this in Lepidopterous insects, 340 ; and, 

 in other orders, equally striking instances might easily be selected. 

 One of the most remarkable is met with in many Hymenoptera^ 

 as, for example, in bees (Apis), wasps (Vespa), and ant-lions 

 (Formica- leo), as well as in most of the Ichneumonidtz. In all 

 these genera, the larva being concealed in a close cell during its 

 developement, under circumstances which would render the evacu- 

 ation of excreinentitious matter an obvious inconvenience, both 

 the larva and pupa (Jig. 132) are entirely without either intes- 

 tinal canal or anal orifice : what little excrement is produced by 

 the digestion of the highly nutritive substances wherewith these 

 larvae are fed being collected in a blind cavity or caecum placed 

 behind the stomach, until the accomplishment of the last change ; 

 at which period the insect, liberated from its confinement, becomes 

 provided with a pervious intestine, and able to get rid of feculent 

 matter. 



The fat-mass ( 340), which at the close of the larva state has 

 reached its maximum of developement, is gradually absorbed du- 

 ring the concealment of the insect in its pupa-case, its nutritive 

 portions being no doubt appropriated to the nourishment of the 

 pupa ; so that in the mature insect the fatty material has almost 

 entirely disappeared, nothing being left in its place but the dense 

 cellular web in which the fat had been deposited. 



The silk-secreting apparatus of such genera as possess the means 

 of spinning a silken thread is peculiar to the larvae ; and, after 

 the commencement of the pupa state, no traces of its previous exist- 

 ence are to be detected. 



(348.) But, while the above-mentioned organs disappear, others 

 become developed ; and the perfect insect is found to possess vis- 

 cera, for which a skilful anatomist might seek in vain in the earlier 

 stages of its existence. The generative system appears, at first, to 

 be absolutely wanting in the larva ; but Herold,* after much 



* Entwickelungsgeschichte der Schmetterlinge, 1815, 4to. 



