For Love of Insects

By Thomas Eisner

Belknap Press

Copyright © 2005 Thomas Eisner
All right reserved.

ISBN: 9780674018273

Chapter One


Most naturalists keep good diaries. I don't. Therefore I will never be able to pinpoint the day I came upon my first bombardier beetle (Brachinus species; see facing page). It was in Lexington, Massachusetts, and I remember the meadow well, except that it probably doesn't exist as a meadow any more. But that is another story.

It must have been in the summer of 1955, the summer I was writing my doctoral thesis, and probably in early June. I was on my knees, uncovering rocks, and ready for any find, particularly if it involved an unfamiliar insect with unusual chemical talents. My thesis had dealt with the anatomy of ants, but I was on the lookout for something new. All my life I had been passionately interested in insects so there was no question that I'd stick it out with bugs. But I also had a very genuine interest in chemistry, and somewhere in the back of my mind was the thought that these two interests could be combined. I didn't realize it at once, but in stumbling upon those little beetles I had struck gold. Bombardier beetles were precisely the sort of champion chemists I was looking for. My having encountered them when I did was the luckiest of breaks.

At that time, while I was a graduate student, there had been some fascinating developments in the interface of entomology and chemistry. Although the term pheromone to designate a chemical signal had not yet been coined, it was becoming increasingly clear that insects flirt by means of chemicals. Female moths, when ripe and ready, announce that fact by emitting a volatile secretion that attracts the male. Word had it that German chemists were hot on the trail of one such attractant, with the intent of identifying the molecule or molecules involved. And in addition there was the exciting research on insect hormones, those remarkable internal chemical messengers that, operating at infinitesimal concentrations, control growth and the transformations in body form known as metamorphosis. An insect hormone, ecdysone, had just been isolated in pure form, and I remember being tremendously taken by a seminar given by Peter Karlson, the German investigator who had been responsible for the isolation. It was now within the realm of possibility, I thought, to decipher the chemical language of insects. Secretly, I wanted to become one of the cryptographers.

Being at the Biological Laboratories at Harvard at that time was inspiring. Two flights down from my room was the laboratory of Carrol M. Williams, one of the great pioneers of insect endocrinology. Carrol, who was to become a close friend later in life, had been my undergraduate adviser at Harvard and teacher in comparative physiology. I learned about bioassays in his course, about ways for testing quantitatively for the biological activity of a chemical substance. And then there was Ed Wilson, fellow graduate student and most inspiring of friends, with whom I had published my first technical paper and shared countless interests.

Ed was himself becoming interested in chemical communication, and he had begun to explore the role of pheromones in ants. He had discovered the gland responsible for producing the trail substance of certain ants, the substance by which foraging ants lay out linear paths to guide nestmates to newly found food sources. He also had devised some clever experiments that showed that individual ants, when assaulted, emit chemicals that alert nestmates nearby to the disturbance and enlist them to come help with the problem. He had shown further that in ant colonies corpses are recognized by certain fatty acids they contain. By tagging various inert objects with such fatty acids he could induce the ants to transport the objects to the graveyards of the colony, as they typically do with corpses. It was an impressive demonstration of the power of the bioassay.

I myself had had experiences that predisposed me for the study of insect chemistry. For one thing, my father was a chemist. He was one of the last graduate students of Fritz Haber, the Nobel laureate who first synthesized ammonia from its constitutive elements. My father would have loved it if I became a chemist, but he was reconciled to the notion that I was destined to study bugs. Yet there was a subtle influence my father was to have on me, and it relates to his having been an amateur perfumer. Wherever we lived before settling in the United States, whether in Spain or Uruguay, my father always had a basement lab in which he concocted, for friends and relatives, perfumes, skin lotions, sun tan oils, and colognes. The house was often mysteriously redolent as a result, which as a little boy I found wonderful. As I grew older, though, I became interested in the odors themselves and in the reasons why they might exist in nature. At the age of 13 in Uruguay, I hadn't read Darwin yet. In fact, evolution hadn't even been mentioned in the biology course I had taken. But quite instinctively I had begun to think in adaptationist terms. What does the fragrance of lavender do for the lavender plant itself? I don't know when the idea occurred to me that plant odors might be defensive, but I know I didn't get the idea from books.

In our summer house in Uruguay we had an icebox that was periodically invaded by ants. Following local custom, we immersed the legs of the icebox in tin cans filled to the height of a centimeter or so with kerosene or turpentine. Either kept the ants out, but turpentine worked better. So why should turpentine be a good insect repellent? I remember putting two and two together when I learned that turpentine is derived from pine resin. The resin must be the pine's defensive juice! I think I must have been 14 or 15 when I did an actual experiment in which I showed that a dab of pine resin placed in the path of ants would cause them to shy away.

I am sure that I owe it to my father that I became so conscious of odors. But I was apparently "nasal" right from the start. My parents recalled that when I was little I could tell from the scent that lingered in the coat closet in the morning that my grandmother had visited the night before. But now, as a teenager, I was coming to realize that I could really learn things from my nose. I was already collecting insects by then, but what had begun as a hobby at the age of 8, and had been directed almost exclusively to the capture of butterflies, was now developing into an interest in live insects, irrespective of kind. Quite casually at first, but with ever increasing fascination, I noted that insects, ever so often, have odors. Some were faintly scented all of the time. Others gave off odors when you handled them, from fluids they emitted when disturbed. In the latter case the odors were often pungent, and I learned to sniff insects carefully lest I end up sneezing and coughing. I also came to realize that I had a good memory for smells. Insect odors seemed to come in categories. Many ants, for instance, had the same acidic odor. I did not know then that this was well documented, and that as early as 1670 a British naturalist by the name of John Wray had published a paper on the acid "juyce" of ants.

I was struck also by one particular odor, very noxious, that I came to associate with millipedes, and with one particular arachnid, a daddy-longlegs, that I had collected in numbers in Atlantida, the seaside resort near Montevideo where we had our summer house. That odor was like none other that I had encountered and it came from juices that the millipedes discharged from pores along the sides of the body and the daddy-long-legs emitted from the edge of its carapace. There was something peculiar about these fluids. They stained the fingers brown, like iodine. The effect was not immediate, but it was invariable: handle any number of those Uruguayan millipedes or daddy-long-legs and within minutes you would end up with stained fingertips. I did not give much thought to these observations at the time, but filed them away in the memory bank. The bombardier beetle was to bring them back to mind.

* I KNEW THE MOMENT I turned over that rock and caught sight of those beetles that they were members of the family Carabidae, the so-called ground beetles. I had picked up many a carabid in my time, so I knew a bit about them. They are quick on their feet, but like most beetles not quick to take flight. They tend to scurry for cover, so if you want to catch them you have to be quick yourself. And I knew that as carabids they belong to that category of insects that give off odors when disturbed.

The ones under that rock were unlike any carabids I knew. With a reddish-brown body and blue iridescent wing covers, they were a pretty sight. And there were several beneath that stone, huddling close together. I had a vial in hand and made my move at once, but they dispersed in all directions and I managed to catch only one. I grasped it in my fingers and was about to put it in the vial when it emitted a series of distinctly audible pops that so startled me I nearly let go of it. I held it closer for a better look, and found that by giving it a squeeze I could cause it to pop again. I also noted that every time it popped it discharged a visible cloud from the rear, at the very moment that I felt a hot sensation in my fingers. I took a sniff and thought I recognized the familiar unpleasant odor of the millipedes and daddy-long-legs from Uruguay. Sure enough, when I checked my fingers after putting the beetle in the vial, there were brown spots on them. I decided then and there that this was a beetle I'd get to know.

I spent another hour or two in the meadow and managed to capture upward of a dozen of the beetles. I took them back to the Bio Labs, where I found I could maintain them in small plastic containers filled with soil, on a diet of freshly cut-up insect larvae supplemented with water. When I showed them to my friend and eventual Cornell colleague, William L. Brown Jr., who often set me straight on matters entomological, he said, "Oh, you've got yourself some bombardier beetles. They go pop when you pick them up and shoot out some real nasty stuff." Shoot they did indeed, and as I was to find out in the months ahead, they even aimed their discharges. The irony that it should have been in Lexington, Massachusetts, of all places, that I first heard those shots didn't strike me until later.

At about that time I had a visit from a young Uruguayan scientist who was working in the Chemistry Laboratories at Harvard only yards away from the Bio Labs. Marma Isabel Ardao had known my father in Uruguay and having heard that I was at Harvard stopped by to say hello. She had a fellowship and was working in the laboratories of Louis Fieser, the eminent organic chemist. It seemed she was studying an arachnid, an Uruguayan species that apparently produced an antibiotic. As she described the animal it became clear that she had been working on the very daddy-long-legs I remembered from Atlantida. I perked up because it was evidently the "juice" from that animal she and Fieser were trying to identify. When I asked whether they had succeeded she said that yes indeed, they had isolated two compounds, and that these turned out to be benzoquinones. Nasty stuff, she said. The work was being published, and they were calling the chemical mixture gonyleptidine, after the generic name of the animal.

So it was benzoquinones. Finally I had an idea of what that pungent odor was all about. My Uruguayan millipedes and my newly found bombardier beetles probably produced benzoquinones as well.

A second person I met coincidentally at the time was Louis M. Roth, an expert on cockroaches, who was working at the Army Quartermaster Research and Engineering Center outside Boston. He and Barbara Stay, a friend of mine who had recently obtained her Ph.D. at Harvard, had identified benzoquinones from the glands of a cockroach, Diploptera punctata. They thought the glands served a defensive purpose, but were not sure. Lou himself had earlier worked on some tiny beetles that also produced benzoquinones. They let me sniff Diploptera and the beetles, and there could be no doubt. It was the familiar stink again. I decided there had to be something very special about benzoquinones if so many insects and millipedes were making use of them.

I told Lou that I'd like to work on Diploptera and he gave me a cage full of them. Back at Harvard the first thing I did was to obtain some crystalline benzoquinones, which were available commercially. I found that there were all kinds of warnings on the labels, so I decided to be careful. "Toxic," one label said, "harmful by inhalation or contact with skin." Too late for that, I thought, given the dousing I had been getting from all those benzoquinone producers.

There were two things I wanted to do. First I wanted to see whether Diploptera ejected its secretion in response to provocation, and second I wanted to find out whether the secretion was repellent to its enemies.

* THE GLANDS of Diploptera were two small saclike structures opening about midway along the sides of the body. They connected to respiratory tubes in such a fashion that one could imagine the animal ejecting its benzoquinones by forcing air through the sacs. I thought I could smell benzoquinones when I handled the roaches but I had no visible evidence of emissions. Whatever Diploptera was ejecting, it was in too small a quantity or in too dispersed a form to see.

I decided I would develop a bioassay. I had obtained some cultures of protozoans, aquatic one-celled organisms, and found that the benzoquinones were hazardous to them as well. Addition of benzoquinone crystals to droplets of culture medium quickly killed the protozoans within. Crystals placed near a droplet caused the protozoans to shun the droplet surface. I thought that if I placed a microaquarium with a protozoan in a confined space with a Diploptera and monitored the behavior of the protozoan when I subjected the cockroach to a simulated attack, I might have devised an indirect way of telling whether benzoquinone discharge took place.

I built the necessary apparatus and the assay worked. I had rigged things so I could stimulate the roach with a warm probe while at the same time observing the microaquarium with a microscope. I had chosen a large protozoan, Spirostomum, as the target organism, and used only one per microaquarium. It became clear that for as long as the cockroach remained undisturbed, Spirostomum swam about actively in its "pool." But no sooner had the cockroach been prodded than the protozoan began showing surface avoidance. It gradually moved to increasingly greater depths within its confines, until the diffusing benzoquinones forced it to the very bottom and to its demise. The evidence was compelling.


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