winding of a continuous spiral fibre through their interior, the fibre is broken into rings, and these rings do not surround the whole tube, but are terminated by a set of arches that pass from one to another (Fig. 287, A). When a portion of one of the great trunks with some of the principal branches of the tracheal system has been dissected out, and so pressed in mounting that the sides of the tubes are flattened against each other (as has happened in the specimen represented in Fig. 291), the spire forms two layers which are brought into close apposition; and a very beautiful appearance, resembling that of "watered silk," is produced by the crossing of the two sets of fibres, of which one overlies the other. That this appearance, however, is altogether an optical illusion, may be easily demonstrated by carefully following the course of any one of the fibres, which will be found to be perfectly regular.

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392. The stigmata" or "spiracles" through which the air enters the tracheal system, are generally visible on the exterior of the body of the Insect (especially on the abdominal segments) as a series of pores along each margin of the under surface. In most larvæ, nearly every segment is provided with a pair; but in the perfect insect, several of them remain closed, especially in the thoracic region, so that their number is often considerably reduced. The structure of the spiracles varies greatly in regard to complexity in different Insects; and even where the general plan is the same, the details of conformation are peculiar, so that perhaps in scarcely any two species are they alike. Generally speaking, they are furnished with some kind of sieve at their entrance, by which particles of dust, soot, &c., which would otherwise enter the air-passages, are filtered out; and this sieve may be formed by the interlacement of the branches of minute arborescent growths from the borders of the spiracle, as in the common Fly (Fig. 292), or in the Dytiscus; or it may be a membrane perforated with minute holes, and supported upon a frame

work of bars that is prolonged in like manner from the thickened margin of the aperture (Fig. 293), as in the larva of the Melo

lontha (cockchafer). Not unfrequently, the centre of the aperture is occupied by an impervious disk, from which radii proceed to its margin, as is well seen in the spiracle of Tipula (crane-fly). In those aquatic larvæ which breathe air, we often find one of the spiracles of the last segment of the abdomen prolonged into a tube, the mouth of which remains at the surface, while the body is immersed; the larvæ of the Gnat tribe may frequently be observed in this position.

393. There are many aquatic Larve, however, which have an entirely different provision for respiration; being furnished with external leaf-like or brush-like appendages, into which the trachea are prolonged, so that, by absorbing air from the water that bathes them, they may convey this into the interior of the body. We cannot have a better example of this than is afforded by the larva of the common Ephemera (day fly), the body of which is furnished with a set of branchial appendages resembling the "finfeet" of Branchiopods (§ 368), whilst the three-pronged tail also is fringed with clusters of delicate hairs which appear to minister to the same function. In the larva of the Libellula (dragon-fly), the extension of the surface for aquatic respiration takes place within the termination of the intestine; the lining membrane of which is folded into an immense number of plaits, each containing a minutely ramified system of trachea; the water, slowly drawn in through the anus for bathing this surface, is ejected with such violence that the body is impelled in the opposite direction; and the air taken up by its trachea is carried, through the system of air-tubes of which they form a part, into the remotest organs. This apparatus is a peculiarly interesting object for the Microscope, on account of the extraordinary copiousness of the distribution of the trachea in the intestinal folds.

394. The main trunks of the Tracheal system, with their principal ramifications, may generally be got out with little difficulty, by laying open the body of an insect or larva, under water, in a dissecting-trough (§ 104), and removing the whole visceral mass, taking care to leave as many as possible of the branches which will be seen proceeding to this from the two great longitudinal trachea, to whose position these branches will serve as a guide. Mr. Quekett recommends the following as the most simple method of obtaining a perfect system of tracheal tubes from a larva: a small opening having been made in its body, this is to be placed in strong acetic acid, which will soften or decompose all the viscera; and the traches may then be well washed with the syringe, and removed from the body with the greatest facility, by cutting away the connections of the main tubes with the spiracles by means of fine-pointed scissors. In order to mount them, they should be floated upon the slide, on which they should then be laid out in the position best adapted for displaying them. If they are to be mounted in Canada balsam, they should be

allowed to dry upon the slide, and should then be treated in the usual way; but their natural appearance is best preserved by mounting them in fluid (weak spirit or Goadby's solution), using a shallow cell to prevent pressure. The finer ramifications of the tracheal system may generally be seen particularly well in the membranous wall of the stomach or intestine; and this, having been laid out and dried upon the glass, may be mounted in balsam so as to keep the tracheæ full of air (whereby they are much better displayed), if care be taken to use balsam that has been previously thickened, to drop this on the object without liquefying it more than is absolutely necessary, and to heat the slide and the cover (the heat may be advantageously applied directly to the cover, after it has been put on, by turning over the slide so that its upper face shall look downwards) only to such a degree as to allow the balsam to spread and the cover to be pressed down. The spiracles are easily dissected out by means of a pointed knife or a pair of fine scissors; they should be mounted in fluid, when their texture is soft; and in balsam, when the integument is hard and horny.

395. Wings. These organs are essentially composed of an extension of the external membranous layer of the integument, over a framework formed by prolongations of the inner horny layer; within which prolongations, trachea are nearly always to be found, whilst they also contain channels through which blood circulates during the growth of the wing and for a short time after its completion. This is the simple structure presented to us in the wings of Neuroptera (dragon-flies, &c.), Hymenoptera (bees and wasps), Diptera (two-winged flies), and also of many Homoptera (cicade and aphides); and the principal interest of these wings as microscopic objects, lies in the distribution of their "veins" or "nerves" (for by both names are the ramifications of their skeleton known), and in certain points of accessory structure. The venation of the wings is most beautiful in the smaller Neuroptera; since it is the distinguishing feature of this order, that the veins, after subdividing, reunite again, so as to form a close network; whilst in the Hymenoptera and Diptera such reunions are rare, especially towards the margin of the wings, and the areolæ are much larger. Although the membrane of which these wings are composed, appears perfectly homogeneous when viewed by transmitted light, even with a high magnifying power, yet, when viewed by light reflected obliquely from their surfaces, an appearance of cellular areolation is often discernible; this is well seen in the common Fly, in which each of these areola has a hair in its centre. In order to make this observation, as well as to bring out the very beautiful iridescent hues which the wings of many minute insects (as the Aphides) exhibit when thus viewed, it is convenient to hold the wing in the stage-forceps, for the sake of giving it every variety of inclination; and when that position has been found, which

best displays its most interesting features, it should be set up as nearly as possible in the same. For this purpose it should be mounted on an opaque slide; but instead of being laid down upon its surface, the wing should be raised a little above it, its "stalk" being held in the proper position by a little cone of soft wax, in the apex of which it may be imbedded. The wings of most Hymenoptera are remarkable for the peculiar apparatus by which those of the same side are connected together, so as to constitute in flight but one large wing; this consists of a row of curved hooks on the anterior margin of the posterior wing, which lay hold of the thickened and doubled-down posterior edge of the anterior wing. These hooks are sufficiently apparent in the wings of the common Bee, when examined with even a low magnifying power; but they are seen better in the Wasp, and better still in the Hornet. The peculiar scaly covering of the wings of the Lepidoptera has already been noticed (§ 381); but it may here be added that the entire wings of many of the smaller and commoner insects of this order, such as the Tineide or "clothes' moths," form very beautiful opaque objects for low powers; the most beautiful of all being the divided wings of the Fissipennes or "plumed moths," especially those of the genus Pterophorus.

396. There are many Insects, however, in which the wings are more or less consolidated by the interposition of a layer of horny substance between the two layers of membrane. This plan of structure is most fully carried out in the Coleoptera (beetles), in which the anterior wings are so much thickened and are so little extended, that they are useless in flight, and serve merely as cases or covers for the posterior, which lie folded up beneath them when not in use; hence these are distinguished as elytra. These elytra, when the insect is at rest, meet along the median line of the back, and cover nearly the whole upper surface of the body; and it is upon them that the brilliant hues, by which the integument of many of these insects is distinguished, are most strikingly displayed. In the anterior wings of the Forficulida or earwig tribe (which form the connecting link between this order and the Orthoptera), the cellular structure may often be readily distinguished when they are viewed by transmitted light, especially after having been mounted in Canada balsam. The anterior wings of the Orthoptera (grasshoppers, crickets, &c.) although not by any means so solidified as those of Coleoptera, contain a great deal of horny matter; they are usually rendered sufficiently transparent, however, by Canada balsam, to be viewed with transmitted light; and many of them are so colored as to be very showy objects (as are also the posterior fan-like wings) for the solar or gas-microscope, although their large size, and the absence of any minute structure, prevent them from affording much interest to the ordinary Microscopist. We must not omit to mention, however, the curious sound-producing apparatus which

is possessed by most insects of this order, and especially by the common House-Cricket; this consists of the "tympanum" or drum, which is a space on each of the upper wings, scarcely crossed by veins, but bounded externally by a large dark vein provided with three or four longitudinal ridges, and of the "file" or "bow," which is a transverse horny ridge in front of the tympanum, furnished with numerous teeth; and it is believed that the sound is produced by the rubbing of the two bows across each other, while its intensity is increased by the sounding-board action of the tympanum. The wings of the Fulgorida (lanternflies) have much the same texture with those of the Orthoptera, and possess about the same value as microscopic objects; differing considerably from the purely membranous wings of the Cicade and Aphides, which are associated with them in the order Homoptera. In the order Hemiptera, to which belong various kinds of land and water insects that have a suctorial mouth resembling that of the common Bug, the wings of the anterior pair are usually of parchmenty consistence, though membranous near their tips, and are often so richly colored as to become very beautiful objects, when mounted in balsam and viewed by transmitted light; this is the case especially with the terrestrial vegetablefeeding kinds, such as the Pentatoma and its allies, some of the tropical forms of which rival the most brilliant of the Beetles. The British species are by no means so interesting; and the aquatic kinds, which, next to the bed-bugs, are the most common, always have a dull brown or almost black hue; even among these last, however,-of which the Notonecta (water-boatman) and the Nepa (water-scorpion) are well-known forms,-the wings are beautifully variegated by differences in the depth of that hue.

397. Feet.-Although the feet of Insects are formed pretty much on one general plan, yet that plan is subject to considerable modifications, in accordance with the habits of life of different species. The entire limb usually consists of five divisions, namely, the coxa or hip, the trochanter, the femur or thigh, the tibia or shank, and the tarsus or foot; and this last portion is made up of several successive joints. The typical number of these joints seems to be five; but that number is subject to reduction; and the vast order Coleoptera is subdivided into primary groups, according as the tarsus consists of five, four, or three segments. The last joint of the tarsus is usually furnished with a pair of strong hooks or claws (Figs. 294, 295); and these are often serrated (that is, furnished with saw-like teeth), especially near the base. The under surface of the other joints is frequently beset with tufts of hairs, which are arranged in various modes, sometimes forming a complete "sole;" this is especially the case in the family Curculionida; so that a pair of the feet of the "diamond-beetle," mounted so that one shows the upper surface made resplendent by its jewel-like scales, and the other the hairy cushion beneath, is a very interesting object. In many Insects,