316 Miscellaneous. 



concave depression. The anterior sac corresponds to the ovoid 

 reservoir in the other genera ; the posterior sac is the horaologue of 

 their cylindrical ejaculatory duct. It opens by two pores at the 

 tip of the pygidium, a little in front of the anus. In Oambus the 

 discharging-pores are about 2 raillim. from the median line and 

 about 1 millim. behind the posterior stigmata. According to 

 M. Bordas the gland would open into the cloaca, and its action 

 would imply a mechanism of an extremely complex character. 



II. PaYSiOLOGT. — The fluid secreted. — This is colourless, limpid, 

 ■with a slight but characteristic odour, and much less acid than the 

 product of Carabus. Its most remarkable property is its very great 

 volatility. It boils, according to our observations, at a temperature 

 of about +9° C, under a pressure of 760 millim. 



Anatomical proofs. — (1) The feebleness of the muscular wall of the 

 reservoir in comparison with the mechanical effects produced ; (2) the 

 existence in the reservoir of transverse stays, destined without 

 doubt to counterbalance the internal pressures ; (3) the enlarge- 

 ment of the ejaculatory duct near the external aperture of the 

 gland, in view of the abrupt expulsion of the glandular fluid ; 

 (4) the presence- in the discharging-pores of chitinous pieces bristling 

 with hairs, and capable of performing the function of a pulverizer. 



Physiological proofs. — (1) The explosive nature of the crepitation ; 



(2) the effervescence, which results at the expense of the glan- 

 dular fluid on dissection under water or on slides as soon as the 

 organ is injured, but only at a temperature higher than +8° C. ; 



(3) the crepitation that takes place upon the dead insect when one 

 opens the discharging-pores. 



III. Mode of Action. — The insect, on being alarmed, protrudes its 

 hind quarters (degaine Varrie re-train) ; if the obturator sphincters 

 then relax, the liquid contents of the reservoir escape under 

 pressure.^ and the jet is divided up by the chitinous combs of the 

 orifice. It is, on a small scale, the action of a bomb charged with 

 liquefied carbonic acid gas. 



In spite of the perfect limpidity of the glandular secretion, the 

 discharge leaves a fixed yellowish residue, already observed by 

 Dufour. This is shown by the microscope to be identical with the 

 contents of the rectum. At the moment of danger the insect bends 

 its abdomen downwards ; this curvature brings the discharging- 

 pores behind the anal sphincter. When, in this attitude, the vola- 

 tile liquid is liberated, while the rectum empties itself, the squib will 

 go off beneath the body, from behind forwards, reducing to powder 

 the ejected excrement — a case-shot of a new kind, well calculated 

 to disconcert the most audacious and best-armed aggressor *. — 

 Comptes llendus, t. cxxviii. no. 10 (March 6, 1899), pp. 622-624. 



* Extracted from an " Etude compar(5e des Glandes pygidiennes chez 

 les Carabides et les Uvtiscides," carried out under tlie direction of 

 Prof. Gilsou at the Carnov Institute at Louvain. 



