360 INSECTA. 



render the evacuation of excrementitious matter an obvious inconveni- 

 ence, both the larva and pupa (fig. 176) are entirely without either 

 intestinal 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 per- 

 vious intestine, and able to got rid of feculent matter. 



(928.) The fat-mass ( 919), which at the close of the larva state 

 has reached its maximum of development, is gradually absorbed during 

 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. 



(929.) 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 existence 

 are to be detected. 



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

 come developed ; and the perfect insect is found to possess viscera 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 patient investigation, 

 succeeded in detecting the undeveloped rudiments of the future sexual 

 organs both of the male and female. It is during the maturation of 

 the pupa that these important parts expand ; and before the disclosure 

 of the imago they are found to have attained their complete propor- 

 tions, so as to be ready to perform their functions as soon as the ex- 

 pansion of the wings endows the insect with means of locomotion suffi- 

 ciently perfect to ensure the due dispersion of the species. 



(931.) It is in the nervous system, however, that the most interesting 

 phenomena are observable ; and in the lessons afforded by watching the 

 correspondence between the state of the animal during the several 

 phases of its existence and the development of the nervous ganglia, the 

 physiologist cannot fail to recognize those great and general principles 

 upon which our arrangement of the animal creation is based. In the 

 worm-like larva the ganglia are numerous, but of small dimensions 

 too feeble to be capable of animating powerful limbs, or of appreciating 

 impressions from the organs of the higher senses ; the animal is, in fact, 

 precisely in the condition of an ANNELIDAN, which it would seem to 

 represent. External limbs are therefore absolutely wanting in many 

 larvae ; in others they are represented by short and stunted appendages ; 

 and even in the most perfect, or hexapod larvag, they are feeble instru- 

 * Entwickelungsgeschichte der Schmetterlinge. 1815, 4to. 



