" There is one elegant arrangement connected with the breathing-tubes of an insect specially worthy of admiration; and perhaps in the whole range of animal mechanics it would be difficult to point out an example of more exquisite mechanism, whether we consider the object of the contrivance or the remarkable beauty of the structure

employed. The air-tubes themselves are necessarily extremely thin and delicate; so that on the slightest pressure their sides would inevitably collapse, and thus completely put a stop to the passage of air through them, producing, of course, the speedy suffocation of the insect, thad not some means been adopted to keep them always permeable; and yet, to do so, and at the same time to preserve their softness and perfect flexibility, might seem a problem not easily solved. The plan adopted, however, fully combines both these requisites. Between the two thin layers of membrane which form the walls of every air-tube, a delicate elastic thread (a wire of exquisite tenuity) has been interposed, which, winding round and round in close spirals, forms by its revolutions a cylindrical pipe of sufficient firmness to preserve the air-vessels in a permeable condition, whilst at the same time it does not at all interfere with its flexibility; this fine coil is continued through every division of the trachece, even to their most minute ramifications, a character whereby these vessels are readily distinguishable when examined under the microscope."*

Man has imitated this exquisite contrivance in the spiral wire-spring which lines flexible gas-pipes; but his wire does not pass between two coats of membrane. One of the most interesting points of the contrivance is the way in which the branches are (so to speak) inserted in the trunk, the two wires uniting without leaving a blank. It is difficult to describe how this is done; but by tracing home one of the ramifications you may see that it is performed most accurately; the circumvolutions of the trunk-wire being crowded and bent round above and below the insertion (like the grain of timber .-around a knot), and the lowest turns of the branch-wire *being enlarged so as to fill up the opening entirely. * "Nat. Hist, of Anim./'i. 6.

You must not suppose, however, that the whole of one tube is formed out of a single wire. Just as, in a piece of human wire-work, the structure is made out of a certain number of pieces of limited length, and joinings or interlacings occur where new lengths are introduced, so, strange to say, it seems to be here. It is strange, I say, that it should be so, when there can be no limit to the resources, either of material, or skill to use it ; but so it is, as you may see in this specimen, which has been dissected out of the body of a silkworm. The spiral is much looser here than in the air-tube of the fly, the turns of the wire being wider apart; and hence its structure is much more easily traced. Here you see, in many places, the introduction of a new wire, always beginning with the most fine-drawn point, but presently taking its place with the rest so as to be undistinguishable from them. In some cases certainly (perhaps this may be the explanation of the phenomenon in all) the wire so introduced may be found to terminate with the; like attenuation before it has made a single turn, and seems to be inserted when the permanent curvature of the pipe would leave the wires on the outer side of the curve too far apart, half a turn, or even much less, then being inserted of supernumerary wire.

I told you that the air enters these tubes through certain "trap-doors." This is not the term which the physiologist employs, certainly : he calls them spiracles. In our own bodies the air enters only at one spiracle, a curiously defended orifice opening just in front of the gullet, at the back of the mouth. But in the class of animals we are now considering there are a good many such breathing orifices. You may see them to great advantage in any large caterpillar, the silkworm for example, where all along the sides of the pearl-grey body you perceive a row of dots, which with a lens you discover to be little oval disks sunken into little pits, of H a black hue with a white centre, through which is a very slender slit. There are nine of these organs on each side, a pair to each segment or division of the body, with the exception of the first, which is the head, and of the third and fourth, which are destined to bear the wings ; these are destitute of spiracles.

Essentially, these organs, under whatever modifications of form and position they may appear, have the same structure. They are narrow orifices, each with two lips capable of being opened at the will of the animal, or accurately closed ; and in many soft-skinned insects, such as the silkworm and most larvae, they are set in a horny ring ; by which means they are prevented from collapsing through the unresisting character of the general integument. The opening and shutting of them is performed by an internal apparatus of muscles, which is sometimes strengthened by being attached to two horny plates, which project inwardly. But the most curious thing to be noted in the structure of these spiracles is the contrivance which induced me to call them trap - doors. Small as are their openings, they are still large enough to admit many floating particles of dust, soot, and other extraneous matters, which would tend to cloa: up the delicate air-passages, and to impede the right performance of their important functions. Hence

SriBACLE OF FLY. they need to be guarded with some sort of sieve, or filter, which, while admitting the air, shall exclude the dust-

Various and beautiful are the modes in which this common purpose is effected, but I can show you only two or three. This is one of the breathing orifices of the common House-fly, in which, as you see, minute processes grow from the margin all round, which extend partly across the open area, branching and ramifying again and again, and spreading and interlacing with those of the opposite side, so as to form a perfect sieve, which the linest atoms of dust cannot penetrate.

The same end is attained, in another way, in the dirty cylindrical grub, which is found so abundantly at the roots of grass in pasture lands, and which country folk call, from the toughness of its skin, "leather-coat." It is the larva of the Crane-fly (Tipula oleracea), so familiar to us under the sobriquet of Daddy Long-legs. I can easily procure one of these, for, unfortunately, they are but too common. Here is one, who shall have the honour of being martyred for the benefit of science. Before we assassinate him, however, just look here at the hinder extremity of his body, where there is a space, surrounded and protected by several points, and in this space, two black spots.

With the dissecting-scissors I have carefully cut out one of these specks, and now I put it for illumination on the stage of the microscope. There is, first of all, a dark horny ring of an oval figure, a little way within which there is an opaque dark plate of the same SPIRACLE OF LEATHEU-COAT. figure, but smaller, occupying the central portion of the area, The space between the margin of the plate and the bounding ring is occupied by a series of slender filaments, placed side by side, ii 2

proceeding from one to the other, through the interstices of which the air is filtered. The central plate seems to be quite imperforate.

The fat, thick-bodied grubs of those beetles called chafers exhibit, in their spiracles, a modification of this structure, rendered still more elaborate. In the case of the larva of the common Cockchafer (Melolontlta vulgaris), for example, the central plate is a projection from one side of the margin of the spiracle ; or, to use a geographical simile, we may say that, instead of being an island in the midst of a lake, it is a promontory. Thus the breathing space is a crescent-shaped band,

SPIBACLE OF COCKCHAFER-GBUB. which is crossed in every part by bars passing from the margin to the projecting plate. But, as if the interstices left by these bars would be too coarse for the purpose, they are made still finer by a membrane, which is stretched across them, and which is pierced with a number of exceedingly minute round holes, through which alone the air is admitted.

In many of the two-winged flies, which inhabit the water in their earlier stages, there are some interesting contrivances and modifications connected with the organs of respiration. It is necessary that the orifices of the air-tubes should be brought at intervals to the surface of the water, in order to come into contact with the external air; while, at the same time, it is important that as small a portion as possible of the animal's body be exposed to danger, by being protruded from its sheltering element. An example in point you may see in this vase.

Here is a slender worm, an inch and a half in length, thickest a little behind the head, and tapering gradually to a lengthened tail, the twelve divisions of the body being very conspicuous. It swims up and down, or to and fro, in the clear water, with a not very rapid, wriggling movement, throwing its body alternately from side to side in the form of the letter S.

This is the maggot of a handsome dipterous fly, sometimes called the Chameleon-fly (Stratiomys chamceleon). There is much about it to reward observation and careful examination with a low magnifying power, especially the head, with its pointed snout, and its pair of foot-like palpi, or feelers. These are situated one on each side of the head, are three-jointed, the last joint being studded with short stiff spines, and the second having a thumblike projection. With these organs, the grub roots and burrows among the decaying vegetable matter at the bottom for its food ; and when not so engaged, they are often rapidly vibrated in a singular manner, the sight of which might induce a feeling of fear, as if they were threatening weapons of offence—a pair of poisonous stings, for instance ; they have, however, no such function, the poor grub being perfectly harmless.

What I wish you chiefly to observe, however, is the tail, with its curious organisation. With the naked eye, you can perceive that the last joint is much slenderer and more lengthened than the rest, and that it is tipped with a beautiful crown of feathers, like the diadem of some semi-savage prince. This is best seen when the animal comes to the surface, which it always does tail uppermost ; for, as soon as the tip reaches the air, the plumes instantly open, and form an exquisite cone or funnel, from which every drop of moisture is excluded, though the water stands around at the level of the brim. A few seconds it remains motionless thus, the whole body hanging downwards, suspended from the caudal coronet : then, sudden])', the tips of the plumes curve inward toward each other, inclosing a globule of air; and the animal wriggles away into the depths, carrying its burden, like a pearl, or a glittering bubble of quicksilver, behind it.

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