STRUCTURE OP THE INTEGUMENT OF INSECTS. 555 



tail, and is, moreover, an extremely common inhabitant of our 

 ponds and streams. This insect passes two or even three years 

 in its larva state, and during this time it repeatedly throws ofl" its 

 skin ; the cast-skin, when perfect, is an object of extreme 

 beauty, since, as it formed a complete sheath to the various 

 appendages of the body and tail, it continues to exhibit their 

 outlines with the utmost delicacy ; arid by keeping these larvae 

 in a Vivarium, and by mounting the entire series of their cast 

 skins, a record is preserved of the successive changes they 

 undergo. Much care is necessary, however, to extend them 

 upon their slides, in consequence of their extreme fragility ; and 

 the best plan is to place the slip of glass under the skin whilst it 

 is floating on water, and to lift the object out upon the slide. 



378. Structure of the Integument. — In treating of the sejDarate 

 parts of the organization of Insects, which furnish the most in- 

 teresting objects of Microscopic study, we may most appropriately 

 commence with their integument and its appendages (scales, 

 hairs, &c.) The body and members are closely invested by a 

 hardened skin, which acts as their skeleton, and affords points 

 of attachment to the muscles by which their several parts are 

 moved ; being soft and flexible, however, at the joints. The 

 skin is usually more or less horny in its texture, and is consoli- 

 dated by the animal substance termed chitine, as well as, in some 

 cases, by a small quantity of mineral matter. It is in the Coleop- 

 tera that it attains its greatest development ; the dermo-skeleton 

 of many beetles being so firm, as not only to confer upon them 

 an extraordinary power of passive resistance, but also to enable 

 them to put forth enormous force, by the action of the powerful 

 muscles which are attached to it. It may be stated as a general 

 rule, that the external layer of this dermo-skeleton is always 

 cellular, taking the place of an epidermis ; and that the cells are 

 straight-sided and closely fitted together, so as to be polygonal 

 (usually hexagonal) in form. Of this we have a very good ex- 

 ample in the superficial layers (Fig. 286, b) of the thin horny 

 lamellae or blades, which constitute the terminal portion of the 

 antenna of the Cockchaffer (Fig. 285) ; this layer being easily 

 distinguished from the intermediate portion of the lamina (a), 

 by careful focussing. In many beetles, the hexagonal areolation 

 of the surface is often distinguishable when the light is reflected 

 from it at a particular angle, even when not discernible in trans- 

 parent sections. The integument of the common Red Ant ex- 

 hibits the hexagonal cellular arrangement very distinctly through- 

 out; and the broad flat expansion on the leg of the Grabro (sand- 

 wasp) affords another beautiful example of a distinctly cellular 

 structure in the outer layer of the integument. The inner layer, 

 however, which constitutes the principal part of the thickness of 

 the horny casing of the Beetle tribe, seldom exhibits any distinct 

 organization ; though it may be usually separated into several 

 laminae, which are sometimes traversed by tubes that pass into 



