A Shot in the Dark

His name was unpronounceable but his personality was hard to beat. Daniel Aneshansley was a first-year graduate student when he appeared in my office in search of a doctoral project. His field was electrical engineering rather than biology, so I didn’t know whether we had any common ground. As soon as we started talking, though, it became clear that we would really hit it off, so finding a problem of mutual interest became a necessity.

Among the many insects that I had studied over the years there always had been one, the bombardier beetle, which I rated as a favorite. Bombardiers have the most extraordinary defense mechanism. When disturbed, as when you grab them in the fingers, they let off an audible "pop," while at the same time ejecting a little cloud of puff from the rear. They have two large glands in the abdomen that affect this discharge. German researchers had worked on the chemistry of the ejected fluid and found it to consist largely of benzoquinones, familiar chemicals long known for their irritating properties.

I had published a paper showing that bombardiers can accurately aim their spray, and that they make good use of this ability when they fend off enemies such as ants and spiders. The German scientists also reported something quite remarkable about the formation of the spray. They discovered that the benzoquinones are not stored as such in the glands, but are formed by interaction of two sets of chemicals ordinarily kept separately in the glands. At the moment of ejection these chemicals are mixed, causing a reaction to occur, in which the quinones are formed and ejected. The reaction occurs explosively, hence the popping sound that accompanies the ejections.

I had long suspected the spray of bombardiers to be hot. There had been published reports saying that some of the large bombardiers from the tropics eject their spray at such high temperature that it hurts to pick them up by hand, and I myself had experienced a distinct burning sensation when I popped a bombardier beetle in my mouth. Why not look into the thermal properties of the bombardier’s spray? It might be fun to try to measure the spray temperature. I proposed the idea to Dan and he perked right up.

Dan is a fabulous gadgeteer and he was quick to come up with an electronic device that did the job, using a tiny thermistor bead as a thermometer. Thermistors are resistors commonly used in electrical circuits. The amount of current they conduct depends on their temperature. This means that if you calibrate a thermistor according to the amount of electricity it conducts at various temperatures, you can use it as a thermometer.

Dan rigged up a thermistor so that we could cause the beetle to spray directly upon it. By measuring the amount of current flowing through the thermistor at the moment it was hit, we could determine the temperature of the spray. Incredibly, the thermistor registered 100 degrees Centigrade, the temperature of boiling water. Friends of ours at the Chemistry Department at Cornell, Joanne and Benjamin Widom, had already predicted on theoretical grounds that the spray should be scalding hot. We were delighted with the find. No other organism was known to perform thermal tricks at such high temperature.

Dan and I next set ourselves the goal of photographing the beetle in the act of spraying. We knew we had to resort to a special technique, since the discharges are very short in duration, lasting on average only a few thousandths of a second. We tried conventional photography but kept missing the discharges. It was simply impossible to release the shutter by hand in synchrony with the brief moment of spray ejection. We would have to design a system whereby the spray itself acts to trigger the photography. In other words, we had to get the beetle to take its own picture.

We found that there was a relatively simple way of doing this. Since the beetle was unwilling to pose for us, we had to find a way to hold him in place in front of the camera. We did this by fastening him to a metal rod, using a small dab of wax placed on his back. He could thus be adjusted to assume a perfectly normal upright stance. The wax didn’t hurt the beetle and came right off after the experiment, so we could release the beetle when we were finished.

We knew we would have to use a brief and bright burst of illumination to take the picture, which meant using an electronic flash unit. The trick was to cause the popping sound of the beetle’s ejection to trigger the unit. To do this we adopted a standard procedure. After focussing on the beetle in the camera’s viewfinder, we dimmed the room lights to an absolute minimum and opened the shutter. Using a pair of forceps we then briefly pinched one of the beetle’s legs, just as an attacking ant might do with its mandibles, causing the beetle to spray. A microphone placed directly above the beetle picked up the accompanying sound, which was relayed instantly by electronic circuitry to the flash unit, causing it to discharge. The moment we saw the flash we closed the shutter again, and got things ready for the next picture. There were some skills we had to master to insure that the procedure would work. For instance, after dimming the lights and opening the shutter, we had to work fast, to prevent the background illumination from giving us a "ghost" image on the film. But with experience we got to the point where pinching the beetle’s leg was something we could accomplish in a second or two.

We took several rolls of film in our very first portrait session with the beetles. Waiting for the pictures to come back from development was sheer agony. But the reward was there. The beetles had taken some fabulous pictures of themselves. The best one, in fact, was the very first picture of the first roll we had shot. We are proud of that picture, which has been reproduced in many magazines and books. But more than anything else we are grateful to our little collaborators for allowing us to look into their lives. In referring to them in the masculine we are trying merely to avoid the use of the impersonal "it." Gender neutrality is implied in our use of "he." Our lady bombardiers took self-portraits every bit as tantalizing as those taken by the males.

Tom Eisner is a biologist at Cornell University.