244 LECTURE XVIII. 



sect, the remains of the salivary apparatus are limited to the thorax, 

 and the common duct ojDens beneath the tongue. 



The epithelial lining of the alimentary canal of (he larva is shed at 

 each moult ; that of the closed stomach in the bee-maggot is evacu- 

 ated in the pupa state through the new formed anus. 



The superabundant nutriment prepared by the voracious larva is 

 stored up in the condition of masses of fat which surround the viscera 

 and occupy their interspaces. 



The parasitic Ichneumons introduce their ova beneath the skin of the 

 larvae of Lepidoptera. When hatched the Ichneumon larvae subsist 

 ujoon the fat of the caterpillars, which they infest. They avoid pene- 

 trating the alimentary canal, but evidently destroy many of the minute 

 branches of the trachea which ramify in the adipose tissue. Such 

 wounded tracheae probably permit the escape of sufficient air for the 

 respiration of the parasitic larvae ; for though the caterpillars so in- 

 fested survive and go into the pupa state, they are uneasy, and evi- 

 dently diseased ; the loss of the adipose store of nutriment prevents 

 the completion of the metamorphosis, and instead of a butterfly, a 

 swarm of small Ichneumons emerges from the cocoon. 



With respect to the outward form and integuments of the ver- 

 miform larva, these are contracted lengthwise, and partially dilated 

 during the pupa state. The longitudinal muscles contract, and are 

 permanently shortened by interstitial absorption : they shorten the 

 body by sheathing the segments one within the other, the intus- 

 suscepted portions being afterwards modified or removed. 



The dorsal vessel i^fig. 104, s), which is developed above the 

 intestine, and begins to pulsate before the larva quits the egg, un- 

 dergoes a corresponding change with the common integument in the 

 pupa state. It seems to be contracted by a series of intus-susceptions ; 

 the abdominal part is slightly expanded, more definitely divided into 

 chambers, and better provided with valves: the thoracic portion is 

 simplified, shrunk in diameter, and is more distinctly defined as an 

 aorta sent off from the heart. {Jig. 105.) 



The respiratory system undergoes still more remarkable modifi- 

 cations. The branchiae of the aquatic larvae either disappear or are 

 developed into wings: the long pneumatic tubes of those which, 

 living in water, breath air, shrink and disappear. The partial dilat- 

 ations of certain tracheae to form reservoirs of air for diminishing the 

 specific gravity of the body, begin to be formed in the pupa state of 

 the flying insect. 



Hei'old has shown that germs of the generative organs exist in the 

 larvae of the Lepidoptera : the testes appear on each side as four 

 nucleated cells in a longitudinal series, which, by progressive coa- 



