is placed in the head, the posterior in the thorax. The excretory duct is of such tenuity, that, if it exists, M. Léon Dufour was unable to detect it. The prevailing number of these organs is two, one on each side; frequently there are four, and Burmeister asserts that no fewer than six are to be found in the genus Nepa, three on each side, all of them opening into the cavity of the mouth.
The peculiar fluid termed bile appears so indispensable to digestion, both in the higher and lower animals, that, in most cases, we find the organs destined to secrete it very conspicuous. In insects, what must essentially be regarded as such are always present, as far as yet known, except in the genera Chermes and Aphis. They assume the shape of filiform tubes, usually very long and flexuose, meandering over the surface of the chylific ventricle and the intestines. The point of the alimentary canal where they are generally inserted is just behind the pylorus; in a few instances they open into the chylific ventricle. In the whole of the order Hemiptera, the point of insertion is the place where the slender intestine meets the cœcum; an arrangement of which few examples are to be met with elsewhere. Sometimes they are inserted only by one extremity, the other being free, at other times they are fixed by both ends. In regard to number they vary extremely; two is the prevailing number among the Coleoptera, Hemiptera, and Diptera, three occur among some coleopterous tribes, such as the Cerambycidæ, four in many Diptera and a few Neuroptera, six in the Lepidoptera, eight in
