432 LECTURE XVIII. 



during the metamorphosis, except in the genus Hister, in which some 

 traces remain in the perfect insect. 



The salivary vessels of the caterpillars of the Lepidoptera are of 

 two kinds : one pair is short and broad, sometimes vesicular, as in 

 the Cossus ligniperda ; and their ducts terminate at the base of the 

 maxilla?. Those of the second pair are very long and slender, occupy- 

 ing, with their longitudinal coils, the sides of the abdomen, and sending 

 their slender ducts forward to unite together and terminate upon 

 a peculiar prominence upon the under lip, which is called the spinneret. 

 (Prep., Nos. 2985 to 2988.) These tubular glands, though classed 

 with the salivary apparatus, are peculiar, in their full development, 

 to the larvae, and are called " sericteria," or silk-tubes, because they 

 prepare the glutinous material, or silk, which the larva spins to form 

 its cocoon. In the perfect insect the remains of the salivary appara- 

 tus are limited to the thorax, and the common duct opens beneath the 

 tongue. 



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

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

 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 larvoa 

 subsist upon the fat of the caterpillars which they infest. They avoid 

 penetrating 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 

 80 infested survive and go into the pupa state, they are uneasy, and 

 evidently diseased ; the loss of the adipose store of nutriment pre- 

 vents the completion of the metamorphosis, they perish, and in- 

 stead of a butterfly, a swarm of small Ichneumons emerge from the 

 cocoon. 



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

 form larvae, 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 {Jig. 159, s) which is developed above the intes- 

 tine, and begins to pulsate before the larva quits the Qgg, undergoes 

 a corresponding change with the common integument in i\\Q pupa 



