Dec. 28, 1888.] 



SCIENTIFIC NEWS. 



655 



beetle with a red thorax and red legs, evidently in a vio- 

 lent hurry, and makes for the nearest place of conceal- 

 ment ; and as it hurries along a puff of bluish-white smoke 

 proceeds from its body, probably succeeded by another, 

 and that again, perhaps, by a third ; and each is followed 

 by a slight but perfectly audible report. If the insect be 

 interfered with, the process is repeated, it may be, ten or 

 a dozen times in rapid succession, and one is irresisti- 

 bly reminded of a war-vessel in full retreat, whose stern 

 guns nevertheless keep up a constant fire upon her 

 pursuer. 



This curious little beetle is popularly known as the 

 Bombardier, and scientifically as Brachinus crepitans, and 

 may be considered as a typical example of the tolerably 

 large genus to which it belongs. It is plentiful enough 

 in some places, but correspondingly rare in others, and is 

 usually found, if at all, in the neighbourhood of water, 

 and particularly on or near the banks of running streams. 

 And it affords us an excellent example of one of the very 

 strangest methods of defence to be met with in the whole 

 of the animal kingdom. Its discharges are, without 

 hyperbole, those of insect artillery. They are due to a 

 highly volatile liquid, secreted for its special function by 

 special glands lying in the region of the tail, which passes 

 into vapour immediately upon coming into contact with 

 the air. They are intended for defensive purposes in 

 combating various foes, and more especially in warding 

 off the attacks of the larger Geodephaga. And they have 

 even something of the missile nature, inasmuch as the 

 explosive fluid itself is highly irritant in character, and 

 will even leave a dark stain upon the human skin, which 

 cannot be removed even by frequent ablutions. 



For our knowledge concerning both this fluid and the 

 organs which distil and discharge it we are chiefly 

 indebted to the researches of M. Leon Dufour, who de- 

 voted special care and attention to their examination. 

 And from him we learn that the apparatus in question is 

 double, one division lying along either side of the abdo- 

 men, and each consisting of two parts, the one secretory, 

 and the other conservatory. The first of these assumes 

 the form of a delicate thread, and is, in fact, a gland in 

 what may be termed its primitive or lowest condition ; 

 the other is a kind of reservoir, in which the fluid is stored 

 up until required for use. And the former opens 

 into the latter just as the secreting-glands of the 

 bee and the wasp open into the bag which contains the 

 poison. 



In appearance the secretory portion of this gland varies 

 very considerably, according to its degree of expansion 

 or contraction. When fully distended it is oblong, semi- 

 transparent, and nearly filled with air, and occupies 

 nearly the full length of the abdomen. When contracted, 

 however, it becomes merely a soft, opaque, whitish body 

 lying in the terminal abdominal segments, and scarcely 

 recognisable for the same organ. The reservoir does not 

 change its form in like manner, but is always a globular, 

 reddish-brown object, situated in the anal segment, and 

 provided with special muscles to ensure its due contrac- 

 tion when a portion of the contents is to be expelled. 



By turning theabdomen from side to side, the Brachinus 

 can direct its discharge with some little accuracy, and 

 seldom fails to check a pursuer, provided that it be at 

 tolerably close quarters. In this manner, in its Conti- 

 nental habitats, it frequently contrives to repel the giant 

 Calosoma sycophanta, one of the largest of the predacious 

 " ground " beetles, and a terrible enemy to insects smaller 

 than itself, One well-aimed discharge and the Calasotnq 



relinquishes the chase, evidently suffering very consider- 

 ably from the pungent nature of the volatile fluid which 

 has been employed against it. And in our own country 

 several of the larger Carabi, and such fierce and insati- 

 able creatures as the shore-loving Broscus cephalotes, are 

 frequently repulsed in like manner. 



The exact nature of the explosive fluid seems rather 

 doubtful. It is readily soluble either in water or alcohol, 

 and leaves a slight dust behind it after discharge. And 

 the interior of the reservoir, if that organ be emptied and 

 examined, is seen to be covered with a deposit of the 

 same character. It is strongly irritant, as has already 

 been stated, and, should it enter the eyes, will cause pain 

 and inflammation fully as severe as would an equal 

 quantity of the strongest vinegar. And it will stain any 

 part of the skin upon which it may happen to fall, and 

 that so deeply that its traces will remain uneffaced for 

 several days to follow. 



Still more potent is the same fluid in the larger exotic 

 species, some of which can scarcely be held in the 

 naked hand, so severely do they burn the flesh. If the 

 discharge of our common British example is similar in 

 its effects to strong vinegar, that of its tropical relations 

 approaches very nearly to nitric acid. And both Lacor- 

 daire and Burchell, the African traveller, tell us that they 

 have often been obliged to release specimens which they 

 had captured, merely because the pain caused by their 

 discharges was so intense. 



By slightly compressing the abdomen the explosions 

 can be produced even after death, provided, of course, 

 that sufficient time has not elapsed for the tissues to 

 harden, and the fluid in the reservoirs to dry up. If 

 they be directed upon a sheet of white paper, a reddish 

 spot is left, which afterwards changes to brown. And, 

 after the explosive liquid itself is exhausted, " a black, 

 grainy liquid " is discharged in its place, which does not 

 pass into vapour. In Brachinus displosor a similar 

 liquid, but of a yellow or brownish colour, is emitted 

 after the explosive discharges have ceased. 



The quantity of liquid secreted, or at any rate the 

 amount expelled at each discharge, varies very consider- 

 ably in different species ; and thus we find that, while 

 our own British Bombardier can produce eighteen or 

 twenty explosions in rapid succession — although each is 

 perceptibly more feeble than its predecessor — B. displosor 

 is limited to twelve or so, and other species to fewer 

 still. But it is difficult to ascertain these facts with any 

 degree of accuracy, for the simple reason that it is im- 

 possible to tell whether the beetles selected for experi- 

 ment have not lately made use of their strange capabili- 

 ties, and so partially exhausted the contents of their 

 reservoirs. 



Besides Brachinus, there are one or two other genera 

 of beetles which possess similar powers, and among these 

 may be mentioned Paussus, resident principally in the 

 Moluccas, and certain species of Ceraplerus and Oxana 

 But in none are they so strikingly pronounced as in the 

 true Bombardiers. 



Now what are the analogies of this fluid ? Is it con- 

 nected with, or a modification of, the sexual secretions ? 

 Or is it a special provision, restricted to the compara- 

 tively few insects in which it appears ? 



Scarcely the former, one would think, for it is present 

 both in male and female, and it is scarcely possible that 

 any part of the generative system should be so modified 

 as to produce precisely the same secretion in opposite 

 sexes, Nor, 911 investigation, does any part of the 



