cause trouble farther down the canal, ever getting into the stomach at all. With the close fitting valve at its entrance, and the free power of emptying the crop by aid of the air vesicles, their stomach is pretty safe from having anything passed into it which cannot pass out of it.

Some of the tangled threads which we tear through are air tubes, distinguished, even to their minutest ramifications, by their white colour and the presence of a spiral fibre wound round them. In the yellow threads we recognize another elementary tissue. These are gland tubes; they are opaque and finely granular, and, from their analogous position, have been thought by many to represent the biliary vessels of the higher animals. Referring again to Van der Hoeven's admirable summary* of the latest-as far as I knowdiscoveries in insect anatomy and physiology, we find reasons for hesitating to accept this opinion without qualification. The economy of insects is so very different from that of the Vertebrata, that we are not justified in inferring, from mere analogy of position, as to the functions of any organ. The discovery of uric acid in the secretion of these yellow gland-tubes has led some to infer that they represent the kidneys rather than, or as well as, the liver. But, in truth, the present state of our knowledge scarcely justifies any definite conclusion on this matter.t

* Op. cit., p. 256.

The excretions of insects have been made the subject of special examination by Dr. John Davy. Wasps have come in for their share of attention, and uric acid and urate of ammonia are set down as constituents of the excretions of the perfect insect and the larva respectively. Trans. Entomolog. Society.' Vol. III., N.S. 1854–56, p. 18. I have often failed to find uric acid in these excretions,

Insect glands, with few exceptions, of which the salivary glands of the wasp are perhaps the one most familiarly known, are constructed, or, better perhaps, are arranged, in a tubular form. The glandular element, consisting here, as in the higher animals, of epithelial cells, is placed within the tube, and their secretion is poured out from its open end. In the particular instance before us, no better or simpler means to gain the object in view, under all the circumstances, could be devised. The long capillary tubes float free, by the side of the trachea in the fluid on which they are designed to act, and the products of their operations are poured directly into the alimentary canal. Much of the ordinary glandular system, and the whole lymphatic system are thus dispensed with in the wasp's economy.

By very careful manipulation, with the forceps and dissecting needle, this tangled mass of gland tubes and trachea may be removed, leaving the small intestine quite clear, as a tube of much smaller calibre and much less strength than, though otherwise bearing a general resemblance to, the stomach. It lies deep, at the back of the abdominal cavity, behind the left ovary, carrying the spiral coil which we have been tracing still farther downwards and to the left. It is separated from the next following section, the colon, by a line of demarcation yet more distinct than that which separates it from the stomach. Thus ;-it contracts rapidly to a narrow point, which indents the

though the food happened to be the same as in the instances where I had found it present. It was absent from the contents of the distended colon of Ligurian bees dying of an epidemic disease. It was present in large quantity in the excretions of a leaf-cutter bee.

head of the colon, and appears as a nipple-like projection in its interior, forming a valvular constriction at this point. As the valve at the bottom of the crop was set to prevent substances passing involuntarily onwards into the stomach, so this is set, like the valve at the top of our own colon, to prevent substances regurgitating under pressure, from the large, back again into the small, intestine. It may be shown most plainly under the microscope by black back-ground illumination.

The insect economy is so different from that of the more familiar subjects of physiological observation, that it would be unsafe to draw any more than general inferences from the higher animals as to the nature of the changes in the food which are effected in insects by each separate section of the alimentary canal. The distinct functions, however, of the large and small intestine of the wasp are easy to be recognized in their structure. The glandular element predominates in one, and the muscular element in the other; pointing to an important difference in their operations. The colon has not nearly so abundant a supply of air tubes and glands, or glandular appendages, as the small intestine has. It is larger, and evidently mechanically stronger; the annular folds of the mucous membrane of the small intestines being here replaced by longitudinal folds, in which the greater development of the muscular coat is apparent. But the functions of this part of the bowel are not exclusively mechanical. The fluid, which was necessary to float the refuse of the food and the excretory matter through the narrow intestinal or glandular passages, is here re

absorbed into the system. If the fluid enters faster than it can be strained off, the colon may become greatly distended. And to equalize the pressure, to allow of very considerable expansion without risk of laceration of the bowel, six horny bands are inserted into its walls longitudinally.

These horny slips mark the limits of the colon; below them the canal contracts at once into a strong, straight tube-the rectum. Its functions are merely to expel from the body what can be of no further use to it, namely the substances from which all nutritive matter has been extracted, and the excretory results of the daily wear and tear of insect life. Purely mechanical in its functions, its structure presents the simplest mechanical arrangements. In the higher animals a circular muscle, called a sphincter, surrounds the opening of the bowel and secures a voluntary control over the evacuations. In the wasp the same object is attained by a different contrivance. The open end of the bowel-referring now to the female wasps-is attached to the base of a triangular horny scale, called the vent-cover. This generally stands at right angles to the canal, stopping the passage as effectually as a clip on a bent Indiarubber tube does. But, at the will of the insect, the vent cover can be drawn into a straight line with the canal. There is no resistance from an opposing sphincter to be overcome, nor is any co-ordination of movements required; but, by the most simple arrangement, the same movement which opens the passage expels the contents of the bowel.

The whole length of the canal coiled away in the abdomen of a healthy female wasp may be reckoned

at about thirty lines. But this measure must be accepted only as a very rude approximation, on account of the yielding nature of the tube, which may be stretched to various lengths by different observers. The spiral makes about two complete turns in its passage through the abdomen. Speaking of insects generally, the length of the alimentary canal varies in each family inversely as the digestible nature of their food. Predaceous insects, that is to say those which live on animal food, have, as a rule, shorter alimentary canals than vegetable feeders, because, as a rule, their food is more easily digested. But this rule has numerous exceptions. If the food be easy of assimilation, whatever its nature, animal or vegetable, the canal is short and simple, but if it be more refractory the canal is longer, and its appendages are more complex. The food of the wasp, though mixed, is generally fluid and easy of digestion, and the alimentary canal of the wasp, accordingly, as we have traced it, is short and simple.*

THE REPRODUCTIVE SYSTEM comes next under consideration. There is no great difficulty in the anatomical examination of these parts, none but what patience and practice will surmount. The several organs are large enough to be easily recognized, when we have once learned where to look for them, and how to disentangle them. It is in the morphological interpretation of what we see that the chief difficulty and the great interest of the inquiry lie. I must refer to Burmeister's elaborate

* See this question fully examined and illustrated, Cyclop. Anat. Phys.' Vol. II, pp. 966-973.