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An introduction to insect physiology, genetics and behaviour which looks at the interaction between humans and insects, and explores both the positive and negative aspects of the relationship.

Dedicated with all my heart to
Hannah Leskosky

Preface

THE VAST MAJORITY of people consider it a high priority to minimize the extent of their interaction with the insect world. Homes are sealed, sprayed, and kept meticulously clean so as to reduce the probability that they will be invaded by insects; similarly, bodies are bathed, hair is shampooed, and clothing regularly washed in order to eliminate any unwanted contact with six-legged life forms. In the overwhelmingly vast majority of daily conversations, insects are conspicuous in their absence; those rare conversations in which insects feature prominently are generally carried out in guarded tones, often with a touch of embarrassment. After all, no one likes to admit, even to close personal friends, to being stung, bitten, infested, invaded, or otherwise bested by the loathsome insects that manage to get around the safeguards.

It is indeed a laudable goal to try to distance oneself from the insect world, but it is, alas, an impossible one. There is no other life form on the planet whose lives are as inextricably bound up with our own as are members of the class Insecta. For one thing, they intrude by force of sheer number. Of the world's species, almost 80% of them are insects—in other words, four out of every five creatures have six legs at some point during their lifetime. Over 800,000 species of insects are known to science, and there's really no way of telling how many there are altogether; estimates taking into account species yet undescribed and awaiting discovery range from 2 million to upwards of 30 million. As individuals, they collectively outweigh every other form of life on the planet as well. The total number of individual insects on earth at any given moment has been estimated at 10 quintillion (or 10,000,000,000,000,000,000), a number that's not that unreasonable considering that some termite colonies house over a million individuals and locust swarms can contain up to a billion individuals.

In view of these enormous numbers, it's not altogether surprising that insects can be found just about everywhere (and certainly everywhere that humans have staked a claim). There are insects that live in Antarctica, in cracks in the snow, and in hot springs in Yellowstone, in water where temperatures approach the boiling point. Insects live in horse intestines, where the acidity levels, even in a horse without heartburn, are comparable to vinegar; they live in pools of petroleum in oil fields, in jars of formaldehyde in morgues, and in baptismal fonts in churches. They thrive as well at the tops of the highest mountains as they do in mines almost a mile below the earth's surface, and they are equally at home in the driest of deserts and in the most humid of rain forests. About the only place on earth where they are not well represented is in the ocean, but even there a few hardy species have set up residence—ocean skaters that can be found gliding on the water surface a mile or more from shore, or lice that live in the nostrils of sea lions and stay relatively dry as they accompany their hosts on deep dives underwater.

So wherever humans have broken ground, whatever frontiers humans have explored, they have discovered that they are latecomers, following in the six-legged footsteps of insects. Whatever resources humans have wanted to garner as their own, insects have had a prior claim on. Thus it is that they are our chief competitors, exacting their toll in the form of destruction of crops, domesticated animals, stored products, timber, rangeland, and even human life (since many insects view humans as nothing more than a meal and in the course of feeding can transmit an enormous variety of debilitating and even deadly diseases).

But, because insects are in many cases the chief architects of terrestrial ecosystems, they are also our principal partners in making a living on earth. About a third of our diet (and a higher percentage for vegetarians) is the direct result of insect pollination; insect-pollination services in the U.S. amount to more than 9 billion dollars every year. Without insects, there would be no oranges in Florida, no cotton in Mississippi, no cheese in Wisconsin, no peaches in Georgia, and no potatoes in Idaho. By eating dung, carrion, and other ordure spurned by more discriminating beasts, insects keep the earth's surface free and clear of debris. Moreover, aside from economic services, insects contribute economic products in magnitude unequalled by any other group of organisms. Entire economies have revolved around insect products. Aztecs paid tribute in the form of dead bodies of scale insects, which, due to the pigment they produce, were worth more than gold; fortunes, even lives, were made and lost in the silk trade.

Because the interactions are so profound, all-encompassing, far-reaching, most people are completely unaware of the extent to which life and culture are shaped by insects. Insects have been present on every battlefield of every war and have determined the outcome of those battles more often than have bullets or bombs. Alexander the Great, Napoleon, and other brilliant military strategists were more often defeated by arthropods than by their opponents and would have been well-advised to have studied their habits more closely. Were it not for insects, there may never have been certain major social innovations, like the rise of a middle class in Europe, or scientific advances, like the germ theory of disease or the theory of natural selection. There may never have even been a science of genetics or a field of computer science without insects. Moreover, the world would have been a drab and colorless place, literally and figuratively. To remove all references to insects from English literature would be to gut the works of Chaucer, Shakespeare, Tennyson, and Keats, and to expunge all insect images rendered by artists would be to tamper with the genius of Van Gogh and Dali. Like it or not, insects are part of where we have come from, what we are now, and what we will be. It seems to me that's a pretty good reason for getting better acquainted with them.

Chapter 1

CLASSIFICATION AND NOMENCLATURE ("A ROSE-CHAFER BY ANY OTHER NAME..")

History of classification

THROUGHOUT THE WORLD and throughout history, people have had a penchant for naming things. Even in the Bible, one of the first tasks assigned to Adam and Eve was naming all the other creatures in the Garden of Eden. Taxonomy is the science of naming, classifying, and identifying organisms. Several compelling reasons underlie the universal urge to identify, name, and classify things. Assigning a name tends to improve communication; for example, the statement "Hand me that thingamabob over there" isn't quite as clear to a listener as, say, "Hand me that socket wrench over there." Classification is the arrangement of things into groups sharing certain specified similarities. Thus, by knowing how something is classified, you immediately know something about it. If someone comes up to you and asks you (for whatever reason) to describe the flea he's holding clutched in his hand behind his back, you don't have to be a psychic to tell him that it is flattened side-to-side, wingless, and equipped with sucking mouthparts that it will use to imbibe the blood of some hapless warm-blooded vertebrate—because all fleas (members of the order Siphonaptera) exhibit these endearing traits. Finally, by determining the criteria by which groups are to be recognized, taxonomists greatly facilitate the chore of identifying hitherto unknown items. When in 1823 a German zoologist found a strange "turtle-like little animal" crawling in and around ant nests in the stumps of old oak trees, he was baffled by its appearance, equipped as it was with a "footless, naked belly," "fleshy tentacles" and other peculiar appurtenances; for want of a better idea, he suggested that he had discovered a new and "beautiful addition to the snail fauna of his own fatherland" (Berenbaum 1994); three-quarters of a century later, when the strange little beast was reared through to its adult stage, with three body segments, six legs, and two wings, it was instantly recognizable not as a snail but as an insect—specifically, as a fly (Fig. 1.1). No matter how they start out in life, only flies (members of the order Diptera) end up as six-legged creatures with two wings.

So, in order for things to be classified, they must have some importance to those in charge of classifying things. Insects for a long time were not carefully classified, at least in part because until relatively recently they were not regarded as terribly important—annoying, yes, but, with a handful of exceptions, not of any consequence to the smooth and efficient functioning of society. Moreover, without the aid of a microscope, most insects are so small as to be distressingly similar in appearance, so distinguishing among them presented real difficulties.

One of the earliest attempts to classify insects was by Aristotle, in his monumental Historia Animalium. Aristotle classified insects according to whether they were winged or wingless; winged insects were subsequently divided according to the number and type of wings they possessed. This emphasis on wings in particular, and locomotory appendages in general, proved to be remarkably durable; even today the names of major insect orders reflect wing characters. Aristotle's logical system prevailed despite the subsequent popularity of the writings of Pliny the Elder. Approximately four centuries after Aristotle's Historia Animalium, Pliny the Elder offered his interpretation of insect classification in the form of his magnum opus Historia Naturalis. Pliny wasn't so much interested in insects as he was in everything that existed; by the time he died in A.D. 79, he had authored at least thirty-seven volumes in his series.

Pliny the Elder was definitely not a detail man—not surprising in someone whose ambitious goal was to describe Nature in its entirety. Many of his "facts" were completely unsubstantiated (such as, for example, the notion that caterpillars origi-nate from dew on radish leaves). Despite its inaccuracies, Pliny's Historia Naturalis was the authoritative source on natural history for the next 1,400 years. Medieval compilations borrowed heavily from his text and few innovations were made during the Middle Ages. For example, Bartholomaeus Anglicus (name notwithstanding, a Frenchman) compiled nineteen volumes around A.D. 1230 entitled De Proprietatibus Rerum. The work was intended to be a complete description of the universe. Book 12 is a discussion of air and includes an alphabetical list of flying things that inhabit the air, lumping flying insects indiscriminately in with birds. The bee, appearing along with birds as one of the "ornaments of the heavens," is described as "a little short beast with many feet. And though he might be classified among flying creatures, yet he uses his feet so much that he can reasonably be considered among ground going animals." Also considered a bird was the locust, "a worm engendered by a south wind" that "dies in a northwind." Book 18 discusses terrestrial animals, classified by their means of locomotion and their habitat. Thus, "Creeping beasts and worms pass from place to place by stretching of the body and then drawing it together; worms, adders and serpents move in this way. And they have different means of movement; some draw themselves by the mouth, like small worms, some draw themselves forward by the strength of their sides and the flexibility of their bodies as adders and serpents, and so on." Bartholomaeus Anglicus likely owes Aristotle for the idea of using means of motion as the basis for classifying living creatures.

Figure 1.1

Larval stage of Microdon, a fly once thought to be a mollusc (original drawing by C.L. Metcalf).

The word "bug" dates back to this era and refers to a ghost or hobgoblin—something difficult to see and vaguely unpleasant (a term quite apt for most insects medieval people were likely to encounter). The word "insect," on the other hand, entered the English language only in 1601, when Philemon Holland published a translation of Pliny's Historia Naturalis. A year later, Ulysses Aldrovandus, an Italian, introduced a few taxonomic innovations of his own. Insects were divided according to habitat into Terrestria, or land-dwelling species, and Aquatica, the water-dwelling species. Each group was further divided according to the presence or absence of appendages (Pedata and Apoda, accordingly) and then subdivided further according to whether wings were present or absent (Alata and Aptera, respectively). Legs and wings were then tallied for finer taxonomic distinctions.

The introduction of devices that magnify optical images did wonders for the classification of insects. Indeed, insects were among the first objects of inspection once microscopes became readily available. The earliest recorded microscopical investigations, by Federico Cesi and Francesco Stelluti, were studies of a bee and a weevil, in 1625 and 1630, respectively (published, curiously, not in a scientific journal, but in Stelluti's translation of the first century Satires of Persius). With magnification, many of the anatomical features differentiating insect species were clearly visible for the first time and classification schemes based on morphological features, rather than habitat or means of motion, began to appear. The more detailed the observation, the more complex the name became. One species of butterfly, for example, was known as Papilio media alis pronis praefertim interioribus maculis oblongis argenteis perbelle depictis. The disadvantages of such a naming system are abundantly clear—by the time somebody rushed over and told you he'd seen one, it would be long gone.

For convenience, names were often shortened. In 1758, Carl Linné (or as he was called in scientific circles, Carolus Linnaeus—the tendency to Latinize names extended even to people) published a book called Systema Naturæ, in which he used a binomial, or two-name, system, consistently for the first time. The system so impressed people that it was universally adopted; no scientific names published before Linnaeus's time are considered valid and all subsequent names have conformed (and must continue to conform) to the Linnaean system.

Linnaeus was born in a small town in southern Sweden on May 23, 1707. As a young boy, he disliked school intensely, partly because of a series of uninspiring tutors and partly because he preferred puttering around his father's garden to studying. At the age of 19, his teachers decided he was not suited to the priesthood, and he further disappointed his parents by taking up the study of medicine. He went on to study natural history and medicine at the University of Uppsala, where he wrote a thesis on plant sexuality that was to become the basis of his botanical system of classification. At the time, the idea that plants were sexual organisms was vigorously decried by the church, and proponents of the theory were subject to discipline from the Vatican. Linnaeus didn't help matters much by drawing analogies between plant and human sexual practices in his writings (explaining, for example, that poppy and linden flowers were to be placed in the class he called Polyandria, from poly, or many, and andros, or men, because their sexual organs were effectively "twenty males or more in the same bed with the female"). The Catholic Church notwithstanding, Linnaeus was eminently successful not only as a taxonomist but as a physician—his practice included the Queen of Sweden as a patient.

That the binomial system works is evidenced by the fact that Linnaeus described only about 2,000 species of insects and today there are more than 750,000 with Linnaean names. The two-part, or binomial, name of a species consists of the genus (always capitalized) and the species (never capitalized). Because even today scientific names are rendered in Latin (or at least are Latinized), they are always written in italics, as are all foreign words in English text. Despite the general aversion people feel toward scientific names, they are exceedingly useful. For one thing, they're universally understood, so scientific exchanges can be carried out with precision; in contrast, common names, or vernacular names, for any given insect may vary in different parts of the country, and they certainly vary from country to country. Helicoverpa zea (Fig. 1.2), for example, a caterpillar with very eclectic feeding habits, is called the corn earworm in Illinois, the false tobacco budworm in North Carolina, the cotton bollworm in Arizona, and the tomato fruitworm in California. Secondly, the scientific name conveys information about the place of an organism in the hierarchy of things. Helicoverpa zea used to be called Heliothis zea until about thirty years ago, when a man named D.F. Hardwick realized that the species exhibited several anatomical features completely absent in species placed rightfully in the long-established genus Heliothis and accordingly invented a new generic name to convey its distinctiveness. Finally, if you're up on Greek or Latin, a scientific name can tell you a lot about an organism. The "zea" in H. zea, for example, means "corn," one of the caterpillar's favorite food plants.

Figure 1.2

Helicoverpa zea, the corn earworm, living up to its common name.

Scientific names don't have to be off-putting. Some don't even differ dramatically from their classically derived common name equivalent—e.g., Mantis religiosa, the European praying mantis. Some are even shorter than their common name equivalents—Ips pini is positively pithy in comparison with "California five-spined engraver beetle." And some names even provide insights into the taxonomist's hobbies (e.g., Dicrotendipes thanatogratus (Fig. 1.3), a small fly, from thanatos meaning "dead" and gratus meaning "grateful," named by its discoverer J.H. Epler in honor of the venerable rock band the Grateful Dead) or personality (e.g., Heerz lukenatcha, a parasitic wasp described by inveterate punster P.M. Marsh).

Arthropod arrangements

TAXONOMISTS, THAT IS, people who classify things, have set up an organizational hierarchy for cataloging living organisms. The hierarchy runs from the largest category, the kingdom, progressively through the ranks of phylum, class, order, family, and genus, down to the smallest category, the species. Insects belong to the kingdom Animalia, the animal kingdom. Although many people are of the opinion that only warm and furry creatures are animals, the term is actually much broader, biologically speaking. It encompasses organisms that are multicellular (made up of more than one cell) and that eat food to obtain nourishment (rather than, as plants do, manufacture their own from sunlight, carbon dioxide, and water). Although there are insects that are smaller in size than a one-celled protozoan (such as fairyflies, which are tiny wasps that spend their formative days inside the eggs of water beetles and other aquatic insects), even tiny insects are made of thousands, or even millions, of cells. Since insects can and certainly do eat a remarkable variety of things (a fact to which anyone who has shared an apartment with cockroaches can attest), their status as animals is secure.

Figure 1.3

Dicrotendipes thanatogratus, on a t-shirt printed for the 1994 meetings of the North American Benthological Society (drawing by John Epler).

Every kingdom is made up of a number of smaller units, called phyla (singular, phylum). The phylum Arthropoda, to which the insects belong, consists of all jointed-legged animals (arthro is Greek for "jointed" and poda for "foot") covered with an external, or exo-, skeleton. Aside from the jointed legs and the exoskeleton, another feature that characterizes arthropods is that their bodies are segmented. Arthropods are by no means unique in this respect; even humans and other vertebrates are segmented, but in arthropods, the segmentation is readily apparent, whereas in humans only a close look at the spinal column or vertebrate musculature reveals evidence of segmented construction. Needless to say, there's more to being an arthropod than being segmented. While there are other groups of segmented invertebrates (members of the phylum Annelida, the earthworms and leeches, come to mind), arthropods have done more with their segments than virtually any other group of animals.

Slice open an earthworm and it looks pretty much the same no matter where you slice it; segments are basically repeated identical units. In arthropods, segments become specialized for particular functions and often become consolidated in groups. This process of grouping adjacent segments together for particular functions, or tagmosis, allows for more efficient performance of such tasks as eating or detecting environmental stimuli. In bilaterally symmetrical organisms such as arthropods, these tasks are usually handled by appendages attached to a group of segments that collectively make up the head (the head usually being the first part of the body to encounter the environment, although as always with arthropods there are exceptions—witness crabs that walk sideways and the little tropical insects called webspinners that habitually walk backwards away from danger). Locomotion is usually carried out by appendages attached to segments that collectively make up the thorax, and reproduction and digestion, the province of segments comprising the abdomen.

Organizational plans differ with ecological demands, however, and classification of arthropods is usually based on differences in the specialization and organization of body segments. Although most people tend to regard anything with an excess of legs as an insect, in reality insects have plenty of multilegged company in the phylum Arthropoda. In fact, there is an entire subphylum, the Chelicerata, with nary an insect to claim as its own. Members of the subphylum Chelicerata have two major body regions: a cephalothorax (a fused head-and-chest arrangement with appendages for eating and locomotion) and an abdomen. The name "Chelicerata" refers to the presence of a pair of pincerlike appendages, called chelicerae, on the first segment behind the mouth opening. In addition to chelicerae, chelicerates have a pair of leglike appendages called pedipalps, and four pairs of walking legs attached to the cephalothorax.

Of the three classes of chelicerates, two are exclusively marine—the horseshoe crabs and the sea spiders. The third class, primarily a terrestrial one and therefore the most familiar, is the class Arachnida, the arachnids. Members of this class are as different from insects, taxonomically speaking, as humans are from snakes, fish, or birds. Among the arachnids are the scorpions (order Scorpiones), notable for their pincerlike pedipalps and long segmented abdomen tipped with a venomous sting. They're all carnivorous and use the sting to immobilize small, mostly insect, prey (although they are not averse to using the sting to teach blundering humans a painful lesson). Like scorpions, daddy longlegs (order Opiliones) have four pairs of legs but, as the name suggests, these legs are typically very long and slender; also distinctive is their apparent lack of a waistlike constriction between cephalothorax and abdomen. The daddy longlegs are probably scavengers, although there is some controversy as to what they eat when no one is looking. Mites and ticks (order Acari) are the most numerous of arachnids; while they too have no waist to speak of, they are easily distinguished from daddy longlegs and most other arachnids by their small size (the word acari actually means "tiny" in Greek) and apparent lack of segmentation. Ticks are exclusively parasitic on vertebrates, but mites lead a staggering array of lifestyles, ranging from bloodsucking to plant-feeding to scavenging. Finally, spiders (order Araneida) are the arachnids with a conspicuous constriction, or "waist," between the cephalothorax and abdomen. All spiders are predators, mostly on other arthropods, but a few of the larger species can take down small birds and mammals. They have from two to six structures on the abdomen, called spinnerets, used in spinning silk. Silk is spun for a variety of purposes; silken webs or snares entrap prey, silken pouches contain sperm during copulation, and silken blankets swaddle eggs, to cite a few.

Insects are among the members of the subphylum Mandibulata, whose members, in contrast with chelicerates, have a set of appendages called mandibles on the second segment past the mouth opening. Whereas chelicerates are pretty conservative in terms of number of legs, all more or less sticking to four (though on occasion sporting as many as six) pairs, mandibulate arthropods go to extremes in both directions—from none to hundreds. Members of the class Crustacea go in for legs in a big way; there are legs on every segment of head and thorax, which are occasionally fused, and in some crustaceans legs on every abdominal segment. All told, crustaceans can have anywhere from three to seventy pairs of legs. As well, they are unique among arthropods in having not one but two pairs of antennae; chelicerate arthropods have none and all other mandibulate arthropods have only a single pair. Familiar crustacean faces include barnacles, sowbugs, crabs, lobsters, shrimp, and crayfish; not so familiar are the tadpole shrimp, fairy shrimp, water fleas, and ostracods. Crustaceans are sufficiently different from the rest of the mandibulate arthropods (particularly in their predilection for aquatic habitats) that some people actually place them in their own subphylum.

Diplopods and chilopods are often lumped together in a super-class called Myriapoda, myria meaning "many." The name is appropriate since chilopods are otherwise known as centipedes and diplopods as millipedes. Both the scientific and common names convey the same idea—cent means "hundred" in Latin, chilioi means "thousand" in Greek, and milli means "thousand" in Latin. While it's not exactly true that centipedes have precisely 100 legs and millipedes ten times that number, it is true that they have a lot of legs, at least as adults (baby millipedes start life with, like their insect relatives, only three pairs of legs). Both groups have thirteen or more pairs of legs; the major difference between the groups is not in the number of legs but in how the legs are attached to the body. Millipedes have two pairs of legs per visible segment (hence diplo or "two") and centipedes have a single pair of legs per segment. The apparent doubling up of legs on millipede segments is due to the fact that each apparent segment is actually a fused double segment. While millipedes are inoffensive scavengers, centipedes are predaceous and have a set of poison glands in their jaws that they use to

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On Sale: Jun 18, 1996
Page Count: 400 pages
Publisher: Basic Books
ISBN-13: 9780465024452

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May Berenbaum

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May Berenbaum is professor of entomology at the University of Illinois. She's the author of 99 Gnats, Nits and Nibblers and More Maggots, Mites, and Munchers.

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