Page:Jardine Naturalist's library Entomology.djvu/147

Rh rapid evaporization of the ejected matter, which appears as a white vapour of a very penetrating and powerful odour analogous to that of nitric acid. The mechanism by which this is produced, consists in the addition of a second reservatory bladder, placed behind the first, in which the change adapting the liquid for explosion is produced. (Pl. II. fig. 8, d, accessory bladder; c, principal bladder.)

The structure of these organs is a little more complex than that of the biliary vessels, there being always two distinct membranes present; the interior one soft and delicate, the exterior frequently appearing annular or transversely folded. Such are the principal parts and appendages of the great alimentary tube in which digestion takes place. As the preceding description refers to it chiefly in the perfect insect, it is necessary to add a few particulars in relation to its modifications in the larvæ, and the changes produced upon it by the phenomena of metamorphosis. When the latter is incomplete, not altering materially the external aspect, it is found, as might be expected, that the canal remains pretty uniform in all the different