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 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." 
I was able to verify Mr. Trouvelot's observation on a cast-off skin of 
T. Polyphemus. However, in the nearly related species Att. 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 
