certain neuropterous groups, such as the Hemerobii, and, in many instances, particularly among the Orthoptera and Hymenoptera, they amount to a hundred and fifty. Their length generally bears some proportion to their number, a deficiency the one way being compensated by an increase in the other. The longest surpass the dimensions of the body five or six times. Although commonly of the same diameter throughout their whole extent, they are occasionally attenuated at one or both extremities. Numerous constrictions at regular intervals, in a few instances, give them the appearance of being granular or warty. One of the most notable deviations from their generally simple construction is witnessed in the cockchafer, in which they appear fringed with a double row of thickset projecting processes of equal length, some of which are furcate; (Pl. II. fig. 3, e, e.) their composition, also, is greatly more simple than that of the alimentary canal, the coat consisting of a single membrane of great delicacy. It is likewise transparent, permitting the view of the contained fluids, which are most frequently brown or saffron yellow; the prevalence of the latter induced Swammerdam to call the whole organs saffron-vessels.
The last system of vessels which we shall notice at present in connection with the alimentary tube, is that formed by the urinary vessels. In their general aspect they almost seem to repeat, at the anal extremity of the body, the salivary vessels appended to the head. But their presence seems to be far less general, or at least they are more difficult to detect,
