218 THE ENTOMOLOGIST. 



tbe distension of the membrane uniting the prothorax with the 

 head on one side and the mesothorax on the other, and to some 

 extent by the compression of the prothorax itself. My command 

 of terminology is not sufficient to enable me to express myself 

 briefly, and yet avoid (from a purist point of view) talking 

 nonsense. Thus, to talk of the old head and the new head is, 

 strictly speaking, nonsense. There is only one head ; what is 

 old and new is the hard chitinous covering or cuticle — hard on 

 the mature head, soft at, and for a short time after, each moult. 

 This hard covering is cast at each moult, and the first stage of 

 the exuviation is this retreat of the head proper into more 

 expansible quarters. The mouth-parts only, remain in the old 

 head, the space in which, that they do not occupy, being filled 

 with fluid, the same fluid that bathes the larva under the old 

 skin. I believe that when the skin is actually cast the larva 

 laps up the remains of this fluid, as the head is full of it just 

 before, but is quite empty, if not dry, as soon as it has taken 

 place. 



In the moult to pupa the head always splits up, but in larval 

 moults it remains entire. There is much variation as to one 

 detail, bearing closely on the accident or malformation under 

 consideration. 



In some species the head remains attached to the cast skin, 

 more or less fixedly. This may be readily seen in the cast skins 

 of the larvae of the gregarious Vanessas as they remain attached 

 to the larval webs. In other cases they are so lightly attached 

 to the skin that they fall away from it almost at once. In the 

 majority of cases, amongst Noctuae, &c., the head is separately 

 detached. In the breeding-cage these heads will be found lying 

 on the floor, whilst the skin proper remains attached to the 

 larval resting-place, except when the larva eats it, as often is 

 the rule (the head is never eaten). In this section the old head 

 is not drawn off by any traction of the old skin when it slips 

 backwards, but seems to be quite loose, and falls off. Whether 

 it be the rule in certain species, or only an exception occurring 

 in some individuals, one often notices the old head has to be 

 shaken off by the larva, or even rubbed off against the surface 

 the larva rests on. Whether this is the rule in some species or 

 not, it is usually, I think, pathological, and is a step in the 

 direction of Mr. South 's specimen. In this instance the old 

 head remains in the position it occupied, after the new head had 

 taken up its quarters in the prothoracic skin, the moult in all 

 other respects being successfully accomplished. This accident 

 is, in my experience, always fatal ; the larva is effectually 

 muzzled, but more than this, the muzzle is of a special char- 

 acter. It can, in nine cases out of ten. be readily removed by a 

 little force, but the new head has hardened in the muzzle, and 

 the circular opening has impressed itself round the base of the 



