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 by which this is produced, consists in the 
addition of a second reservatory bladder, placed be- 
hind the first, in which the change adapting the liquid 
for explosion is produced.* (PI. 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 larvae, 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 Brachiuus , 
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 Ozarna, 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 may be said scarcely to know its use, for few have 
witnessed its explosions. 
