OF INSECTS. 141 



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 hy which this is produced, consists in the 

 addition of a second reservatory bladder, placed he- 

 hind 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 com- 

 plex 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 larvse, 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 



* The insects in question constitute the genus Brachi?ius, 

 some species of which are natives of Britain ; but this singular 

 means of defence is best witnessed in foreign Brachini, of 

 larger dimensions, a few discharges from which cause such a 

 burning sensation, that it is necessary to let the animal escape. 

 The American genus Ozcena, Aptinus, and some of the Paussi 

 likewise, possess this remarkable property. It has also been 

 attributed to the very common Anchominus prasinus, but if 

 rightly, it mpy be said scarcely to know its use, for few have 

 witnessed its explosions. 



