INTERNAL ANATOMY OF INSECTS. 135 



rous volleys of stinking vapour at its assailants before its 

 ammunition is exhausted a . M. Dufour has given a very 

 particular 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 

 vessels. Thejirst, which is the innermost, presents it- 

 self under two different aspects, according as it is con- 

 tracted or dilated : in the former case it is a whitish, 

 irregularly rounded, soft body, apparently glandular, 

 placed under the last abdominal segments ; communica- 

 ting at one end with the reservoir, and terminating con- 

 stantly at the other in a very long and slender filament : 

 in the second case, or when it is dilated, it resembles an 

 oblong, membranous, 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 b : 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 ground-beetles (Eutrechma) in general have a pair 

 of these anal scent-seci etors, which discharge an acrid 

 and caustic fluid, and sometimes a volatile one c . The 

 external organ of the scent-secretors in Gyrinus consists 

 of two minute hairy cylindrical retractile tubes, of a red 

 colour d . Numerous insects of other tribes and genera 



a VOL. II. p. 243. N. Diet. cTHist. Nat. iv. 308. b Ibid. iv. 309. 

 c Ibid. v. 252. d De Geer iv. 358. t. xiii./, 9. m. 



