loo THE INTEGUMENTAL SKELETON OF THE IMAGO. 



The skeletal structures of insects, both internal and external, 

 undoubtedly consist of indurated cuticle, both the epiostracum 

 and endostracum taking part in their formation. These 

 hardened or chitinized cuticular structures form plates, rings, 

 or complex solid or hollow parts, resembling small ossicles, 

 and are denominated sclerites. 



The internal sclerites which form an endo- skeleton are 

 either indurated inflections of the epidermis, or indurations of 

 the cuticular layer of the alimentary canal, or of the tracheal 

 vessels, yet the former are apparently replaced in some 

 arthropods, by a tissue closely resembling cartilage, as, for 

 example, the ento-thorax in the scorpions ; and this tissue is 

 regarded by Lankester* as of mesoblastic origin — a fact which, 

 taken with many others, indicates a similarity which is not by 

 any means a mere superficial resemblance between the carti- 

 laginous skeleton of a vertebrate and the indurated epidermal 

 skeleton of an insect. 



Further, in both sub-kingdoms it is indubitable that the 

 skeletal structures afford the most important morphological 

 characters, and preserve, more or less perfectly, the original 

 segmental character of the embryo in the adult organism. In 

 both sub-kingdoms the skeleton is modified in accordance with 



* ' On the skeleto-trophic tissues and coxal glands of Limulus, Scorpio and 

 Mygale.' Quart. Journ. Micros. Sc, vol. xxiv., new series, 1884. 



Articules et celui des Insectes Hexapodes en particuliere.' Ann. Sc. 

 Nat. Zool., torn, i., 1824. 



39. AUDOUIN, v., art. ' Insectes,' Dictionnaire Classique d'Histoire Na- 



turelle, torn, viii., Svo, Paris, 1825. 

 This article gives a very complete r<?sume of the views of Audouin, which 

 are the basis of the modern nomenclature of the thorax. 



40. Straus Durckheim, ' L'Anatomie Comparee des Animaux Arti- 

 cules. Auquelle on a joint TAnatomie descriptive du Hanneton,' 410, 

 Paris, 1828. 



One of the most masterly memoirs ever published. The nomenclature of 

 the parts is, however, entirely empirical, and founded on supposed analogies 

 with the parts of vertebrates. 



41. Mi.\LL .\ND Denny, ' The Structure and Life-History of the Cock- 

 roach,' Svo, London, 1886. 



This little work is an excellent introduction to the more extended study 

 of the Insecta, and should be read by every student. 



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