974 INSECTS AND AKACHNIDA 



in an aquarium, 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 slides in con- 

 sequence 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. Thin sections of insects, cater- 

 pillars, ifcc., which bring the internal parts into view in their normal 

 relations, may be cut with the microtome by first soaking the body 

 (as suggested by Dr. Halifax) in thick gum-mucilage, which passes 

 into its substance, and gives support to its tissues, and then inclos- 

 ing it in a casing of melted paraffin made to fit the cavity of the 

 section-instrument. 



Structure of the Integument. In treating of these separate parts 

 of the organisation of insects which furnish the most interesting- 

 objects of microscopic study we may most appropriately commence 

 with their integument and its appendages (scales, hairs, etc.). 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, how- 

 ever, at the joints. This skin is usually more or less horny in its 

 texture, and is consolidated by the animal substance termed chitine, 

 as well as in some cases by a small quantity of mineral matter. It 

 is in the Coleoptera 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 power- 

 ful muscles which are attached to it. The outer layer of this dermo- 

 skeleton is continuous, the cells which secrete it lying beneath the 

 parallel lamin.se of which it is made up ; on the surface the chitinous 

 cuticle may be seen to be marked out into a number of polygonal 

 (usually hexagonal) areas which correspond to the subjacent secret- 

 ing cells. Of this we have a very good example in the superficial 

 layers (fig. 737, B) of the thin horny lamella? or blades which 

 constitute the terminal portion of the antenna of the cockchafer, 

 this layer being easily distinguished from the intermediate portion 

 (A) of the lamina by careful focussing. In many beetles the hexa- 

 gonal areolation of the surface is distinguishable when the light is 

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

 transparent sections. The integument of the common red mil 

 exhibits the hexagonal cellular arrangement very distinctly through 

 out; and the- broad flat expansion of the leg of the drahrn ('sand 

 wasp') affords another beautiful example of a distinctly cellular 

 arrangciiH'iil of the outer la yer 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 anv distinct organi- 

 sation, though it may l>e usually separated into several lamella\ 

 which are sometimes traversed by tubes that pass into them from 

 the inner surface, and extend towards the outer without reach 

 ing it. 



Tegumentary Appendages- The surface of the insects is often 

 beset, and is sometimes completely covered, with appendages having 



