rable, in almost all essential particulars, to that of Cyclops (§ 367). In the case of the Lobster, Prawn, and other "macrourous' species, the metamorphosis chiefly consists in the separation of the locomotive and respiratory functions, true legs being developed from the thoracic segments for the former, and true gills (concealed within a special chamber formed by an extension of the carapace beneath the body) for the latter; and the abdominal segments increase in size, and become furnished with appendages (false feet) of their own. In the Crabs, or "brachyourous" species, on the other hand, the alteration is much greater; for besides the change first noticed in the thoracic members and respiratory organs, the thoracic region becomes much more developed at the expense of the abdominal, the latter remaining in an almost rudimentary condition, and being bent under the body; the thoracic limbs are more completely adapted for walking, save the first pair, which are developed into chele or pincers; and the little creature entirely loses the active swimming habits which it originally possessed, and takes on the mode of life peculiar to the adult. We have, in this history, a most characteristic example of Von Bär's great law of "progress from the general to the special" in organic development; for the Entomostracous form is thus seen to be common to the highest and the lowest Crustaceans in the earliest phase of their lives; but whilst the latter remain and go on to completion upon that type, the former entirely diverge from it; and whilst diverging from it, they also become differentiated from each other, the distinctive characters of their families, genera, and species, evolving themselves, as the individuals advance towards their mature forms.

376. In collecting minute Crustacea, whether fresh-water or marine, the use of the ring-net, as for minute Acalephæ or Echinoderm larvæ, will be found the most efficient instrument; and in favorable localities, the same "gathering" will often contain multitudes of various species of Entomostraca, accompanied, perhaps, by the larvae of higher Crustacea, by Echinoderm larvæ, by Annelid larvæ, and by the smaller Meduse. The water containing these should be put into a large glass jar freely exposed to the light; and after a little practice, the eye will become so far habituated to the general appearance and modes of movement of these different forms of Animal life, as to be able to distinguish them, one from the other. In selecting any specimen for microscopic examination, the dipping tube (§ 71) will be found invaluable. The study of the metamorphoses of the larvae will be best prosecuted, by obtaining the fertilized eggs which are carried about by the females, and watching the history of their products.

CHAPTER XVII.

INSECTS AND ARACHNIDA.

THERE is no class in the whole Animal Kingdom which affords to the Microscopist such a wonderful variety of interesting objects, and such facilities for obtaining an almost endless succession of novelties, as that of Insects. For, in the first place, the number of different kinds that may be brought together (at the proper time) with extremely little trouble, far surpasses that which any other group of Animals can supply to the most painstaking collector; then, again, each specimen will afford, to him who knows how to employ his materials, a considerable number of microscopic objects of very different kinds; and, thirdly, although some of these objects require much care and dexterity in their preparation, a large proportion may be got out, examined, and mounted, with very little skill or trouble. Take, for example, the common House-Fly:-its eyes may be easily mounted, one as a transparent, the other as an opaque object (§ 383); its antennæ, although not such beautiful objects as those of many other Diptera, are still well worth examination (§ 385); its tongue or "proboscis" is a peculiarly interesting object (§ 386), though requiring some care in its preparation; its spiracles, which may be easily cut out from the sides of its body, have a very curious structure (§ 392); its alimentary canal affords a very good example of the minute distribution of the "traches" (§ 391); its wings, examined on a living specimen, newly come forth from the pupa state, exhibit the circulation of the blood in the "nervures" (§ 390); the wing of the insect when dead, moreover, exhibits a most beautiful play of iridescent colors, and shows a remarkable areolation of surface, when it is examined by light reflected from its surface at a particular angle ($395); its foot has a very peculiar conformation, which is doubtless connected with its singular power of walking over smooth surfaces in direct opposition to the force of gravity, although the mode in which it serves this purpose is not yet certainly ascertained (§ 397); and the structure and physiology of its sexual apparatus, with the history of its development and metamorphoses, would of itself suffice to occupy the whole time of an

observer who should desire thoroughly to work it out, not only for months but for years. Hence in treating of this department in such a work as the present, the author labors under the embarras des richesses; for to enter into such a description of the parts of the structure of Insects most interesting to the Microscopist, as should be at all comparable in fulness with the accounts which it has been thought desirable to give of other classes, would swell out the volume to an inconvenient bulk; and no course seems open, but to limit the treatment of the subject to a notice of the kinds of objects which are likely to prove most generally interesting, with a few illustrations that may serve to make the descriptions more clear, and with an enumeration of some of the sources whence a variety of specimens of each class may be most readily obtained. And thus limitation is the less to be regretted, since there already exist in our language numerous elementary treatises on Entomology, wherein the general structure of Insects is fully explained, and the conformation of their minute parts as seen with the Microscope is adequately illustrated.

377. A considerable number of the smaller Insects,-especially those belonging to the orders Coleoptera (beetles), Neurop tera (dragon fly, May fly, &c.), Hymenoptera (bee, wasp, &c.), and Diptera (two-winged flies), may be mounted entire as opaque objects for low magnifying powers; care being taken to spread out their legs, wings, &c., so as adequately to display them, which may be accomplished even after they have dried in other positions, by softening them by steeping them in hot water, or, where this is objectionable, by exposing them to steam. Full directions on this point, applicable to small and large Insects alike, will be found in all text-books of Entomology. There are some, however, whose translucency allows them to be viewed as transparent objects; and these are either to be mounted in Canada balsam, or in weak spirit or glycerine, according to the degree in which the horny opacity of their integument requires the assistance of the former to facilitate the transmission of light through it, or the softness and delicacy of their textures renders a preservative liquid more desirable. Thus an ordinary Flea or Bug will best be mounted in the former medium; but the various parasites of the Louse kind, with some or other of which almost every kind of animal is affected, should be set up in the latter. Some of the aquatic larvæ of the Diptera and Neuroptera, which are so transparent that their whole internal organization can be made out without dissection, are very beautiful and interesting objects, when examined in the living state, especially because they allow the circulation of the blood and the action of the dorsal vessel to be discerned (§ 389). Among these, there is none preferable to the larva of the Ephemera marginata (day fly), which is distinguished by the pos session of a number of beautiful appendages on its body and

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 off 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; and by keeping these larvæ 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 separate parts of the organization of Insects, which furnish the most interesting 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 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 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 example in the superficial layers (Fig. 286, B) of the thin horny lamellæ 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 transparent sections. The integument of the common Red Ant exhibits the hexagonal cellular arrangement very distinctly throughout; and the broad flat expansion on the leg of the Crabro (sandwasp) 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 laminæ, which are sometimes traversed by tubes that pass into

them from the inner surface, and extend towards the outer without reaching it. Occasionally, however, even this exhibits very clear indications of cellular structure; of which a good example is afforded by the middle layer of the lamellæ of the antenna of the Cockchaffer (Fig. 286, A), wherein is plainly to be seen an assemblage of rounded cells with large nuclei, lying in the midst of a homogeneous intercellular substance, and thus closely resembling Cartilage (Fig. 324) in structure, though differing from it in chemical composition.

379. Tegumentary Appendages.-The surface of many Insects is beset, and is sometimes completely covered with appendages, having sometimes the form of broad flat scales, sometimes that of hairs more or less approaching the cylindrical shape, and sometimes being intermediate between the two. The scaly investment is most complete among the Lepidoptera (butterfly and moth tribe); the distinguishing character of the insects of this order being derived from the presence of a regular layer of scales, upon each side of their large membranous wings. It is to the peculiar coloration of the scales, that the various hues and figures are due, by which these wings are so commonly dis tinguished; all the scales of one patch (for example) being green, those of another red, and so on; for the subjacent membrane remains perfectly transparent and colorless, when the scales have been brushed off from its surface. Each scale seems to be composed of two superficial colored laminæ, enclosing a central lamina of structureless membrane, the surface of which is highly polished, and which acts as a "foil" to increase their brilliancy by reflecting back the light that passes through them,—an arrangement which may often be discerned in scales that have lost a portion of their superficial layer by some accidental injury (Fig. 281, c). The color of the superficial laminæ seems to be generally inherent in their substance, especially in the Lepidop tera; but it sometimes appears to be (like the prismatic hues of a soap-bubble) a purely optical effect of their extreme thinness, this being especially the case among those beetles, as the Curculio imperialis (diamond beetle), the scales of which have a metallic lustre, and exhibit colors that vary with the mode in which the light glances from them. Each scale is furnished with a sort of handle at one end (Figs. 279-281), by which it is fitted into a minute socket attached to the surface of the insect; and on the wings of Lepidoptera these sockets are so arranged, that the scales lie in very regular rows, each row overlapping a portion of the next, so as to give to their surface, when sufficiently magnified, very much the appearance of being tiled like the roof of a house. Such an arrangement is said to be "imbricated." The forms of these scales are often very curious, and frequently differ a good deal on the several parts of the wings and of the body of the same individual; being usually more expanded on the former, and narrower and more hair-like on the latter. The