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undergo a complete histolysis, and arise de novo from hypodermal cells, the so- 

 called spiral threads originating from elongated peritracheal nuclei. (See p. 449, 

 Fig. 412.) This is undoubtedly also the case with the salivary ducts, which are 

 strengthened and rendered elastic by tsenidia like those of tracheae. As the 

 urinary tubes are diverticula of the proctodseum, itself an ectodermal invagina- 

 tion, they may also, though not lined with a chitinous intima, be renewed. With 

 little doubt the intima of the ducts of poison, spinning, and most, if not all the 

 other glands, though certainly the dermal glands, is exuviated. We have found 

 that the lobster in moulting sheds, besides the skin with the most delicate setae, 

 the lining of the proventriculus, and the apodemes of the head and thorax, hence 

 it is most probable that the tentorium of the head of insects as well as the apo- 

 demes and phragmas of the thorax are exuviated. 



The formation of the inner skin, or that of any succeeding stage 

 (instar), is due to the secretion of the structureless chitinous layer 

 by the cells of the hypodermis, during the process of histogenesis. 

 These cells at this time are very active, and the formation of the 

 new layer of chitine arrests the supply of nourishment to the old 

 skin, so that it dries, hardens, and with the aid of the fluid thrown 

 out at this time separates from the new chitinous layer secreted by 

 the hypodermis. 



Mention of this fluid, which Newport was the first to observe, and which he 

 says causes the separation of the old from the underlying fresh integument of 

 the caterpillar, recalls a passage in Hatchett-Jackson's Studies in the morphology 

 of the Lepidoptera, which we quote on a succeeding page, where he calls atten- 

 tion to the formation of such a liquid, which in the reptiles facilitates the process 

 of moulting, adding, " Whether such is the case with the moult of the cater- 

 pillar, I do not know." Is it not also possible that the growth of the setse or 

 tubercles on the cuticle of the caterpillar may likewise serve to loosen and 

 detach the overlying skin about to be cast off ? After writing the foregoing, we 

 find that Miall and Denny have suggested that the setaa of the cockroach prob- 

 ably serve the same purpose as the casting-hairs of the crayfish and reptiles. 



It is well known that in the crayfish and in lizards the skin is first loosened 

 by the growth of temporary hairs or setae, which locally grow inward from the 

 old cuticle and push the skin away when it is shuffled off by the movements of 

 the body, jaws, and limbs, as well as the body in general. 1 



Such spines arise in the pupa of many insects, for Verhoeff finds 

 that the spines and teeth of pupal fossorial and other Hymenoptera, 

 as well as Coleoptera, function as moulting-processes for loosening 

 and pushing off the last larval skin, rather than for locomotion. He 

 also claims that the spines of the pupa of the dipterous Anthrax are 

 both for locomotion and for boring, especially the spines on the head 

 and tail. He therefore divides these pupal spines into helcoder- 

 matous (boring or tearing) and locomotor spines. 



1 See Max Braun's article entitled Ueber die histologisehen Vorgange bei der 

 Haiitung von Axtarnx Jliirintilis, with a full bibliography, in Semper's Arheiten aus 

 dein Zool. zoot. Institut in Wilr/bnrg. ii, pp. ILM-Kili. Also Semper's Animal Life, 

 p. 20. Trouvelot also discovered the moulting tluid. (Amer. Nat., i, p. 37.) 



