402 EXTERNAL ANATOMY OF INSECTS. 
in ¢egmina. 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 Dynastide, 
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 Hsoderma) seems 
to give it some affinity to the skin of vertebrate animals *. 
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 Vespa, &c. the head in the 
Arachnida. But in 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 °, yet these parts being all united by 
* Anat. Compar. ii. 557. 
» 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. 
Compar. i. 124. With regard to the flat portions of the integument 
