INTERNAL ANATOMY OF INSECTS. 129 
(Brachinus crepitans), which can fire numerous volleys 
of stinking vapour at its assailants before its ammuni- 
tion is exhausted*. M. Dufour has given a very parti- 
cular account of the organ that secretes this vapour ;—it 
consists of a double apparatus, one on each side, in the 
cavity of the abdomen, both formed of two distinct ves- 
sels. The frst, which is the innermost, presents itself 
under two different aspects, according as it is contracted 
or dilated : in the former case it is a whitish, irregularly 
rounded, soft body, apparently glandular, placed under 
the last abdominal segments ; communicating at one end 
with the reservoir, and terminating constantly at the 
other in a very long and slender filament: in the second 
case, or when it is dilated, it resembles an oblong, mem- 
branous, diaphanous sac, filled with air, then occupying 
the whole length of the abdomen, and appearing free 
except where it communicates with the reservoir. The 
second vessel or reservoir is a small, spherical, brown or 
reddish body, constant in its form, internally hollow, 
placed under the last dorsal segment, precisely above the 
rectum, and opening by a small pore into the anus’: so 
that the tail of this little beetle may be regarded as a 
battery mounted with two pieces of cannon, which our 
alert bombardier fires alternately without intermission 
till all his ammunition is expended. The Caradi L. in 
general have a pair of these dnal scent-secretors, which 
discharge an acrid and caustic fluid, and sometimes a vo- 
latile one®. ‘The external organ of the scent-secretors in 
Gyrinus consists of two minute hairy cylindrical retractile 
tubes, of a red colour’. Numerous insects of other tribes 
4 Vor. I. p. 246. N. Dict. d’ Hist. Nat. iv. 308. » Ibid. iv, 309. 
ST Tbid. vin 2oe. ed De Geer iv, 358.7) xin 7. 9. 77. 
VOL. IV. K 
