610 TEXT-BOOK OF ENTOMOLOGY 



Sphinx ligustri, and his detailed description will apply to most 

 lepidopterous larvae. 



The whole body is wrinkled and contracted in length, and there are occasion- 

 ally powerful contractions and twitchings of its entire body ; the skin becomes 

 dry and shrivelled, and is gradually separated from the new and very delicate 

 one of the next instar beneath. After several powerful efforts of the larva the 

 old skin cracks along the middle of the dorsal surface of the mesothoracic seg- 

 ment, and by repeated efforts the fissure is extended into the Island 3d seg- 

 ment, while the covering of the head divides along the vertex and on each 

 side of the clypeus. " The larva then gradually presses itself through the open- 

 ing, withdrawing first its head and thoracic legs, and subsequently the remainder 

 of its body, slipping off the skin from behind like the finger of a glove. This 

 process, after the skin has once been ruptured, seldom lasts more than a few 

 minutes. When first changed the larva is exceedingly delicate, and its head, 

 which does not increase in size until it again changes its skin, is very large in 

 proportion to the rest of the body." (Art. Insecta, etc.) 



Trouvelot's account is more detailed and an advance on that of Newport's 

 view. He explicitly states, and we know that he was a very close observer, 

 that the old skin is detached by "a fluid which circulates between it and the 

 worm." His account is as follows: The polyphemus worm, like all other silk- 

 worms, changes its skin five times during its larval life. The moulting takes 

 place at regular periods, which comes around about every 10 days for the first 

 four moultings, while about 20 days elapse between the fourth and fifth moult- 

 ing. The worm ceases to eat for a day before moulting, and spins some silk 

 on the vein of the under surface of a leaf; it then secures the hooks of its 

 hind legs in the texture it has thus spun, and there remains motionless ; soon 

 after, through the transparency of the skin of the neck, can be seen a second 

 head larger than the first, belonging to the larva within. The moulting gener- 

 ally takes place after four o'clock in the afternoon; a little before this time the 

 worm holds its body erect, grasping the leaf with the two pairs of hind legs 

 only ; the skin is wrinkled and detached from the body by a fluid which circu- 

 lates between it and the worm ; two longitudinal bands are seen on each side, 

 produced by a portion of the lining of the spiracles, which at this moment have 

 been partly detached ; meanwhile the contractions of the worm are very ener- 

 getic, and by them the skin is pulled off and pushed towards the posterior part ; 

 the skin thus becomes so extended that it soon tears just under the neck, and 

 then from the head. When this is accomplished the most difficult operation is 

 over, and now the process of moulting goes on very rapidly. By repeated con- 

 tractions the skin is folded towards the tail, like a glove when taken off, and the 

 lining of the spiracles comes out in long white filaments. When about one- 

 half of the body appears, the shell still remains like a cap, enclosing the jaws ; 

 then the worm, as if reminded of this loose skull-cap, removes it by rubbing it 

 on a leaf ; this done, the worm finally crawls out of its skin, which is attached 

 to the fastening made for the purpose. Once out of its old skin, the worm makes 

 a careful review of the operation, with its head feeling the aperture of every 

 spiracle, as well as the tail, probably for the purpose of removing any broken 

 fragment of skin which might have remained in these delicate organs. Not only 

 is the outer skin cast off, but also the lining of the air-tubes and intestines, to- 

 gether with all the chewing organs and other appendages of the head. After 

 the moulting, the size of the larva is considerably increased, the head is large 

 compared with the body, but 8 or 10 days later it will look small, as the body 

 will have increased very much in size. This is a certain indication that the 



