FIELD AND FOREST. 77 



their transformations, as well as to the cases constructed by larvse for 

 their habitation during the larval period, and wliich they transport 

 from place to place. The great majority of the Lepidoptcra have in 

 the larva state two elongated sacs or vesicles, doubled once or twice 

 upon themselves, and occupying each side of the space between the 

 alimentary canal and the tracheal system, and extending along the 

 posterior two-thirds of the bociy. From the forward end of each of 

 these glands a tapering duct passes directly to the lower lip of the 

 larva, where they unite and terminate in a minute projecting teat or 

 tubule. A few rudimentary muscles are frequently found controlling 

 the contraction of the sacs, in which a viscous syrup or secretion is 

 being elaborated, and a minute sphincter muscle is frequently found 

 surrounding the mouth of them. After the fourth or fifth moulting of 

 the larvae these glands are generally found present, but in miniature, 

 on dissection ; but as the end of the season approaches, and the ninth 

 or tenth moulting completed, they will be found to have reached a 

 nearly full size in those species which never " spin" until the approach 

 of the pupal metamorphosis. In the CUsiocaiipas, (the web-worms of 

 the apple tree and wild cherry,) a few other allied species of the 

 BombycidcB, and in many of the Phalaenide and Tortricide groups the 

 silk glands are well developed at an early age, and the slightest touch 

 of the leaf upon which they are feeding will cause them to attach their 

 thread and drop suddenly some feet before they check their descent. 

 The extreme rapidity with which the gummy secretion hardens is 

 finely illustrated in this action. The attenuated thread, in many cases 

 not exceeding 1-200,000 of an inch in diameter, loses its water imme- 

 diately on exposure to the air. 



The thread, after it has issued from the body of the larva, is com- 

 mercially termed " silk" (serica,) but when the larva has been soaked 

 in acetic acid, then split open longitudinally, and the glands, with 

 their contents, removed, drawn out to a length of 18 or 24 inches and 

 dried, it is known as "gut," and is used in the manufacture of fishing 

 tackle, for attaching hooks to lines, and in delicate machinery. The 

 material for this latter purpose is generally obtained from the silk- 

 worm proper, (Bombyx mo?-i,) the alianthus-worm, [B. Cynthia,') and 

 one or two other allied species. Most patient and elaborate experi- 

 ments have been made, especially within the last fifty years, to utilize 

 our native American speciesj but \yith very limited success, Cecropia 



