EXTERNAL ANATOMY OF INSECTS. 403 



in tegmina. It appears to consist, in many cases, of se- 

 veral layers of a whitish membrane, and generally breaks 

 into fibres. In some elytra of the larger Dynaslidcs, 

 towards the sides the exterior layer is separated from the 

 rest by a kind of cellular substance. The fibrous struc- 

 ture of this inner skin (which I call the Esoderma) seems 

 to give it some affinity to the skin of vertebrate animals a . 

 In many parts of the body, however, it appears to be 

 merely a thin pellicle. A medical friend, to whom I 

 showed specimens of it, thinks it a kind of cellular mem- 

 brane. 



2. A few words are next necessary with regard to the 

 articulation of the integument, or the mode by which the 

 several pieces of which it and its members consist, are 

 united to each other. In some, as in several of the parts 

 of the head, the occiput, vertex, temples, cheeks, &c. — 

 the line of distinction is merely imaginary ; in others an 

 impressed line separates a part from its neighbours, as is 

 the case with the nose in Vcspa, &c. the head in the 

 Arachnida. But m the majority of instances the parts 

 are separated by a suture, or form a real joint. The 

 kinds of articulation observed by anatomists in vertebrate 

 animals do not all occur in insects, and they seem to 

 have some peculiar to themselves. Thus, for instance, 

 they have no proper suture ; for though they exhibit the 

 appearance both of the harmonic and squamose [ecail- 

 leuse Cuv.) sutures b , yet these parts being all limited by 



a Anat. Compar. ii. 557. 



b A harmonic suture is when the margins of two flat bones simply 

 touch each other, without any intermediate substance; and a squa- 

 mose, when the thin margin of one covers that of the other. Anat. 

 Com far. i. 124. With regard to the flat portions of the integument of 



2 D 2 



