PERFECT INSECTS WITH THE LARVAL HEAD. 17 



Stettin. Ent. Zeit. (1849), p. 365. The segments of the abdomen possess 

 on each side an apical spine. By a continuous movement of the abdo- 

 men to the right and left, those spines press successively against the 

 loosened skin, forcing forward the transforming insect. Probably simi- 

 lar arrangements will be found in other insects. 



The causes why such a rush of blood originates just at the time of the 

 moult. I find nowhere recorded. I think it not sufficient to consider it 

 as a simple consequence of an action of the nervous system, especially 

 as I believe myself able to give a more plausible explanation. 



The crust of insects consists of the external chitinized epidermis, and 

 the internal soft hypodermis. Above the latter, which becomes some- 

 what separated from the epidermis, the new skin is to be formed, at 

 first without impediment to the functions of the insect. As long as the 

 more or less isolated parts of the new skin allow a free circulation of 

 the blood around and between them to feed the old epidermis, the 

 action of the dorsal vessel follows its regular way. By and by the iso- 

 lated parts of the new skin become larger, and partly united, until 

 finally the whole new skin is already formed and chitinized. The circu- 

 lation is at first only disturbed ; later, it is impossible for it to flow in 

 the old way and to the old skin, and the blood, obliged to turn in 

 another direction, rushes naturally in the easiest one, to the dorsal 

 vessel. This is the moment of the beginning of the rush of the blood to 

 the head ; of course the nervous system, irritated by the rush, will help 

 to accelerate more the action of the dorsal vessel. 



It is obvious that the new skin, at least in some parts of the body, 

 must exercise a more or less strong pressure against the old skin. I 

 am of the opinion of Dr. Gerstaecker, that the moult is not alone a con- 

 sequence of such pressure ; but in some parts, for instance in the head, 

 the pressure is obviously prevalent, and must originate a partial resorp- 

 tion of the old epidermis, as that of the thicker sutures. At least, thus 

 the splitting of the sutures in many insects could be explained ; I say 

 purposely in many insects, because a large number transform in a dif- 

 ferent way. In some Lepidoptera the skin of the head does not split. 

 Mr. Trouvelot (Americ. Natur., I, p. 37) records for Telea Polyphemus 

 that the skin splits transversely under the neck just at the end of the 

 head, and perhaps in some way laterally, and probably behind or 

 through the whole prothorax. " When about one half of the body 

 appears the shell of the old head l'emains 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." 



I was able to verify Mr. Trouvelot's observation on a cast-off" skin of 

 T. Polyphemus. However, in the nearly related species Aft. Cecropia, Pro- 

 methea, Yama-mai, I found that the sutures of the head always split in 

 the regular manner. According to Kirby and Spence. the manner of 



