The spray of bombardier beetles contains p-benzoquinones (10), compounds well known for their irritant properties (11). A single bombardier beetle can discharge upward of 20 times before depleting its glands (6). The discharges are accompanied by audible detonations, and they have been shown to be potently deterrent to a number of predators, including ants (6, 12-15).

The spray of bombardier beetles is ejected at 100°C (13). This is because the quinones are generated explosively at the moment of ejection by the mixture of two sets of chemicals ordinarily stored separately in the glands. Each gland consists of two confluent compartments. The larger of these (storage chamber or reservoir) contains hydroquinones and hydrogen peroxide while the smaller one (reaction chamber) contains special enzymes (catalases and peroxidases). To activate the spray, the beetle mixes the contents of the two compartments, causing oxygen to be liberated from hydrogen peroxide and the hydroquinones to be oxidized by the freed oxygen. The oxygen also acts as the propellant, causing the mixture to "pop" out (16-18). The heat that accompanies the formation of the spray is perceptible (13) and contributes to the defensive effectiveness of the secretion (14, 15). An early explorer, reporting on large bombardier beetles from the neotropics, commented that when these "play off their artillery" they are so hot to the touch "that only few (can) be captured with the naked hand" (19). Although it was known that bombardier beetles can aim their spray by revolving the abdominal tip (6), the degree of precision with which they target their ejections had escaped notice.§

MATERIALS AND METHODS

RESULTS

Targeting of Legs. Pinching of individual legs of a beetle resulted in an accurate targeting of these legs (Fig. 1 A-C). The beetle aims the spray by deflecting the abdominal tip forward beneath the body. The deflection is maximal when the beetle is spraying furthest forward, as toward a foreleg (Fig. 1A) and minimal when it sprays toward a hindleg (Fig. 1C).


Targeting of Leg Segments. Discharges toward a leg are aimed, not broadly in the general direction of that leg, but with considerable precision toward the particular leg segment, or portion of a segment, that is stimulated. Fig. 2 A-C shows three discharges elicited by pinching, respectively, the coxal region of a midleg, the distal portion of the femur of a midleg, and the basal portion of the femur of a hindleg. Note that, in Fig. 2 B and C, the forceps have been visibly wetted with secretion.


Positional Tracking. In aiming toward a given portion of a leg, the beetle takes into account the postural orientation of that leg. In Fig. 1 D and E, the same stimulus (pinching of tarsus of right hindleg) is twice applied to a beetle, once when the leg is in a raised position and then again when it is in a downward position. The beetle sprayed with accuracy in each case, indicating that it is able to keep track of an intended bodily target site as this is taken through its range of motions. Comparable accuracy is revealed in closeup view in Fig. 2 D and E, in which the distal end of a hindleg femur is stimulated, first when the femur is positioned to the side of the beetle and then when it is positioned directly behind the abdominal tip.


Forward Ejections, Over the Back. When stimulating various sites along the back of the beetle, we noted that the insect is able to direct its spray toward such regions as well. Thus, for example, when the pronotum was pinched from above, the beetle responded by spraying forward over the back, in parallel to the back's surface (Fig. 3A). In achieving such directionality, the beetle rotates the abdominal tip upward and forward, but not so far as to bring the tip to point directly ahead. In fact, to achieve dorsal forward ejections, the beetle resorts to the stratagem of bouncing the secretory jet off a pair of cuticular reflectors that it deploys specifically for the purpose. Indeed, closeup views of the abdominal tip during such forward ejections show clearly how the reflectors operate (Fig. 3 B-D). They appear to be used one at a time, the spray in any one "shot" being deflected off the reflector closest to the site stimulated (Fig. 3 C and D). Sideways rotation of the abdominal tip also may contribute to the overall aiming of such dorsal discharges (Fig. 3B).


Deployment of the reflectors would appear to require special action on the part of the beetle. Ordinarily, the reflectors (Fig. 3E; rf in Fig. 3F) are kept unexposed, in such fashion that they form a lower lip beneath the curved slit-like opening (arrows in Fig. 3F) from which the spray emerges. The upper lip of that opening is a membranous fold (lp in Fig. 3F) that presumably can be retracted, in such manner as to expose the reflectors. We envision the beetle having the capacity to retract the upper lip asymmetrically, thereby achieving the unilateral exposure of the reflectors. One can also envision the upper lip being projected outward over the reflectors, to cause the emergent spray to be deflected downward. In Fig. 2E, the upper lip seems to be effecting precisely such deflection.

DISCUSSION

The assault depicted in Fig. 4 must be of routine occurrence in the life of S. insignis. Yet one can imagine the beetle usually surviving such attacks. Its hot quinonoid spray, in itself formidably repellent (6, 12-15), can only be rendered more effective by being aimed. And as indicated by our photos, that aiming is precise. There is virtually no site on the beetle's body where an ant could inflict a bite without entailing the risk of being sprayed in return.

Other bombardier beetles related to Stenaptinus (subfamily Brachininae), such as those of the large genus Brachinus, probably aim their spray in much the same manner as Stenaptinus. Indeed, we found the abdominal tip of Brachinus to resemble that of Stenaptinus in every structural detail.

Aiming is achieved differently in bombardier beetles of another subfamily (Paussinae), which also eject hot quinones (20, 21). Paussines lack the reflective devices of Stenaptinus. Instead, when "shooting" forward, paussines direct their ejections along a pair of lateral elytral grooves that serve as launching guides for anteriorly aimed ejections (22, 23).

Given that so many insects play off chemical "artillery" in defense, there can be no question that diverse spray-aiming mechanisms remain to be discovered among insects. Photography and cinematography could prove helpful in elucidating how these mechanisms operate. Questions remain, however, even about S. insignis itself. Although we know that the males of this species also aim their discharges, they appear to do so with an apparatus that differs somewhat from that of the female. Thus, for instance, for ejecting forward over the back, males make use of a single broad reflective shield, instead of the pair of devices used by the female (data not shown). We are also ignorant about whether S. insignis always discharges from both glands simultaneously or whether it does so from one gland at a time. And, of course, there is the vexing problem of how the beetle, which inevitably drenches itself when discharging, withstands the heat and irritancy of its own spray.

ACKNOWLEDGEMENTS

We dedicate this paper affectionately to the memory of Professor Dr. Max Renner of the University of Munich, exemplary naturalist and friend. We thank Maria Eisner for help with the manuscript, for preparing the illustrations, and for providing the scanning electronmicrograph. Andrés González and Jerrold Meinwald provided helpful comments on the manuscript. This study was supported by National Institutes of Health Grant AI02908.

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To whom reprint requests should be addressed at: Section of Neurobiology and Behavior, W347 Mudd Hall, Cornell University, Ithaca, NY 14853. E-mail: te14@cornell.edu . This paper is no. 162 in the series "Defense Mechanisms in Arthropods." Paper no. 161 is ref. 25.

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