network round the viscera, which retains them in their places. One set pass half way across the abdomen, and join with corresponding branches from the opposite side, to form a secondary trunk which runs up the middle of the abdomen. This central trunk, curiously, seems to begin in the egg tubes, which are gradually transformed into tracheæ. As this central air trunk passes forwards it becomes larger, but it soon shrinks again; and it is finally lost behind the dilated end of the oesophagus, at the entrance to the pedicle.

The general arrangement of the respiratory organs, such as has been described above, can be followed by the eye, without any assistance from the microscope. But without this instrument we cannot form any adequate idea of the extent and minuteness of the ultimate ramifications of the air tubes. They are found everywhere, spreading out just like the blood-vessels of warm-blooded animals, and with a like due proportion to the organic activity of each part. In the neighbourhood of the uterus one could almost forget that one is looking at an insect structure permeated by air tubes, and not at the system of dilated and tortuous veins which is visible in the corresponding situation in one of the Vertebrata.

The air tubes have an outer and an inner coat, between which, to preserve their form, an elastic spiral thread is coiled. When the tube is torn across, the spiral thread may be uncoiled and drawn out, as from the trachea of plants, only not to such a length. The large air vesicles are also kept in shape in the same way by an elastic thread, though their form does not allow of the spiral coils being laid down as

regularly as in the tubes. When the vesicles are torn, as often happens by an unlucky scratch in dissecting, the fibres spring back and carry the tear, transversely to their direction, across the whole sac, leaving, as the result of our labours, nothing more than a membrane spread out, like a drop of wax, on the surface of the water. Such a membrane, however, displays well the structure of the air vesicles. As the trachea approach the vesicles their spiral fibre becomes broader and less defined, and is arranged with a little less scrupulous accuracy. In the walls of the air vesicle the threads are still more distant and less regular, and they have a woolly, even a wavy outline. On the coats of the air vesicles, and of the large air tubes in their immediate neighbourhood, many little specks are occasionally to be seen, the nature of which is a matter of uncertainty. I must ask pardon for venturing to express an opinion on a point which has been left unsettled by competent authorities, but these little specks appear to me to be oil-globules. The irregular mode of their arrangement, their occurrence in a part distorted from its original form, and above all, the fact that they may be removed and examined separately from the membrane, strongly confirms the view which their appearance suggests, that they are oil globules.

Insects breathe, as we have seen, and many of them audibly express their feelings, by means of the air; much as the higher animals do. But their respiratory system is not concentrated in one place, it is made up of many smaller similar systems, repeated over and over again, with a longitudinal as well as bi-lateral symmetry which is only one stage removed

from the radiate type of development. Other than respiratory purposes are answered by the peculiar form and mode of arrangement of these organs, and particularly of the large air cavities. One of these is the forcible distension of various parts of the body, so as to obtain a better fulcrum for, or change the direction of, the action of the muscles. Another, to which the large abdominal air vesicles mainly subserve, is that of altering the external figure, and shifting the position of the weight inside. The specific gravity of the insect is thus also altered, but this,* though very important to animals moving in such a heavy fluid as water, is of comparatively little consequence to those which move in the air. Flight is a question of well directed strength, not of weight, and the advantage of the diminution of specific gravity is more than compensated by the attendant increase of bulk. But the power of trimming the body by pushing the heavy stomach to one or other side, and of poising the whole body on the wings as a centre by lengthening the abdomen, both which can be effected by the expansion of the abdominal air vesicles, is very important. These reservoirs of air act also as springs to equalize the pressure of the intermittent respiratory movements, like the air chamber in a fire-engine. And they assist in those functions of the abdominal viscera which in the higher animals require the aid of the diaphragm. Further, they have their use in all those physiological processes which are accompanied in the higher animals by vascular turgescence, but in the insect economy by increased afflux of air. All these func

tions necessarily imply the existence of voluntary control over the admission and expulsion of air, and we have seen how it is provided for in the structure of the spiracles.

The organs of the circulation are altogether much more difficult to trace than any parts which have yet come under notice. None but a practised insect anatomist can hope to do more than obtain a view of some portions of the main arterial trunk, and not even this much can be effected without great pains and patience.

The movements of the wasp's circulation may be most easily watched through the transparent skin of the larva, by placing it in the live-box, with just pressure enough to steady it and to bring its skin into contact with the glass over a sufficiently large surface. The motions languish and cease if the larva is immersed in water. The actions thus observed, however, convey the impression of a very imperfect circulation, of mere motion rather than of motion in one uniform direction. For there seems to be nothing more than an imperfect peristaltic closure of a membranous tube.

We might naturally have expected to find the mechanical arrangements more complete in the perfect insect than in the larva. But Carus* assures us that it is not so. The heart, the central pulsating vessel, remains almost unchanged through all the metamorphoses; and the smaller ramifications of the blood-vessels, instead of being multiplied and de

* Carus. Traité Elémentaire d'Anatomie Comparée.' Traduit par Jourdan. Tome II, p. 323.

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veloped, fade away with the greater development of the respiratory system in the perfect insect. For now, he says, the air comes to the blood, instead of the blood having to go in search of the air. To obtain a view of the central trunk we must spread out the dorsal scales in one piece, previously cleared of all the abdominal viscera, and having only the lining of fatty tissue lying on them. If the specimen thus prepared and pinned out under water be examined in a good light, we shall observe a transparent line running down the middle, clearer and more transparent as the specimen is more recent. This is the heart, the main trunk of the circulation. Now, with a bent needle carefully scrape off this fatty lining of the scales, gathering it all up, heart and all, into a flake. By drawing on this, a long tube may often be extracted from the thorax and head, even up to the bifurcation of the vessel. By careful manipulation, alternately teasing and pressing this mass under water, much of the fatty matter may be removed, and the parts may be arranged for examination in glycerine, as a transparent object, under the microscope. The abdominal portion will now be seen to consist of a central tube, interrupted at regular intervals by a series of valves which are formed by a small piece of the tube being drawn in,intus-suscepted, as anatomists call it-like the foot of a stocking. On either side of each of these internodes is a triangular strap of muscle tapering off to a point which is inserted into the corresponding dorsal scale. At the point of the strap the fibres are undistinguishable from those of ordinary voluntary muscle, but as the band spreads out to cover the vessel they