Innumerable Insects
Page 2
A cabinet of natural wonders from the world of insects, arrayed in their drawers, from Levinus Vincent’s Wondertooneel der nature (1706–1715).
Although often considered insects, spiders, mites, centipedes, and millipedes belong to different lineages of the phylum Arthropoda. Spiders, ticks, mites, and their other eight-legged relatives are arachnids, while the many-legged millipedes and centipedes are myriapods. This print depicts various East African species of spiders, ticks, and millipedes. From Carl Eduard Adolph Gerstaecker, Baron Carl Claus von der Decken’s Reisen in Ost-Afrika (1873).
“It can never be too strongly impressed upon a mind anxious for the acquisition of knowledge, that the commonest things by which we are surrounded are deserving of minute and careful attention.”
—James Rennie
Insect Architecture, 1857
Entomology is the scientific study of insects, the word deriving from the Greek éntomon, meaning “insect,” and lógos, meaning “subject of study.” Like many branches of biology, entomology, in one form or another, is ancient. Since before our civilizations struggled into existence, we have benefited from and been diminished by insects. Early on we focused our attention on only those elements of our world that were either harmful or beneficent, and this too was true with insects. Perhaps not surprisingly, apiculture and sericulture are the most ancient of entomological endeavors. Exploitation of honey bees for honey and wax was already widespread 8,500 years ago, and a vibrant beekeeping industry from 3,000 years ago was discovered in 2007 in Israel, the biblical land “flowing of milk and honey” (Exodus 33:3). At least 8,000 years ago, in the Araña Caves in Valencia, Spain, early painters depicted people climbing ropes to retrieve honey from cliff-face hives, and wall paintings from the Old Kingdom of Egypt attest to beekeeping practices 4,400 years ago. By at least 5,000 years ago, the cocoons of silk moths were being unraveled by people of the Yangshao culture, in today’s northern China, to produce those fine fabrics we so adore even now.
ARTHROPODA
Despite the lengthy history of our involvement with these creatures, there remains today confusion over what is and is not an insect. Insects belong to a larger group of animals called arthropods, formally known as the phylum Arthropoda. Arthropods are truly ancient, dating back to at least the early Cambrian, around 540 million years ago, and were among the initial diversification of major animal lineages. The arthropods encompass a huge swath of animal diversity and include everything from spiders and scorpions to millipedes and centipedes to crabs, shrimp, and lobsters. The most numerous of the arthropod groups are the insects, and sometimes subsets of those aforementioned lineages are grossly lumped with them into the field of entomology. For instance, it is not uncommon for the average individual to believe that entomology covers spiders and their relatives, or even millipedes and pill “bugs” (aka, roly polies, or doodle bugs). In fact, these all belong to other subsets of Arthropoda and have their own fields of inquiry. Spiders, for example, are arachnids and like scorpions, mites, ticks, and their relatives, they are covered by arachnology. Millipedes are encompassed by myriapodology, and pill bugs, despite the misnomer “bugs,” are actually crustaceans and more closely related to crabs and lobsters than they are to insects.
Arthropods are those animals with a chitinous exoskeleton, much like a suit of armor, and as a result have articulated joints wherever movement is required. The word arthropod literally means “jointed foot” (in Greek, árthron, meaning “joint,” and poús, podós, meaning “foot”), in reference to these necessary joints allowing for movement of the chitinous body. Muscles attach within the exoskeleton to provide movement and support, and collectively the muscles and outer skeleton act as a scaffolding for the internal organs. Arthropods are arranged much like an upside-down vertebrate. Where we have a dorsal nerve cord and ventral heart, arthropods have a ventrally positioned nervous system and extended aorta, or open “heart,” along the back of the body. In this regard, the placement of our nerve chord and our heart is an inverted arrangement relative to that in arthropods.
Hierarchical Organization of Arthropoda
Major groups constituting the phylum of arthropods (those animals with a jointed exoskeleton), and the placement of insects among them.
Since the presence of a rigid exoskeleton imposes a limitation on growth, it must therefore be periodically molted. Shedding of the old cuticle in place of a new, larger one permits arthropods to grow throughout their lives, without being hampered by the confines of their protective skeletons. Arthropods experience their world entirely through their exterior skeleton, and there are any number of modifications that permit different forms of perception ranging from vision and hearing to chemical and mechanical receptors. As the most diverse of all arthropods, insects include some of the best examples of arthropod senses. Some of these sensory structures are familiar to us, such as the large compound eyes of a fly or the feathered antennae of a moth, while others do not quite resemble what we might expect or seem misplaced on the body. For example, the “ear” of a cricket, while resembling a small membranous drum like our eardrum, is positioned on the legs rather than the sides of the head. Other insects can have ears in disparate places throughout the body, such as on the abdomen of certain moths, the chest of some mantises, and even on the wings of a subset of lacewings. All insects have small, slender extensions of cuticle that superficially resemble the hair of mammals. In insects, however, these are called setae, and they serve a plethora of purposes. Some setae have minute pores that allow for the intake of specific chemicals in the environment that are distributed either through the air or from some surface, and these permit insects to smell or taste. The “hair” of a fly or the “furry” antennae of a moth are setae.
Today’s arthropod fauna includes four major groups of animals. These are formally classified as the Chelicerata, Crustacea, Myriapoda, and Hexapoda, with insects belonging to the last of these. Spiders, scorpions, mites, ticks, harvestmen, and their relatives are all arachnids, having eight legs and lacking antennae. Together with horseshoe crabs (which are not crabs at all!), they comprise the Chelicerata. Chelicerata are named after their characteristic fangs, called chelicerae. The Crustacea scarcely need any introduction, as the very name brings to mind a whole host of animals that can be found at your average seafood restaurant. Crustaceans include crabs, lobsters, and shrimp as well as the less delectable krill, barnacles, copepods, and roly polies. Millipedes and centipedes, and the lesser known pauropods and symphylans, form the Myriapoda and are best known for the numerous pairs of walking legs running the length of their body. While chelicerates have chelicerae, the Crustacea, Myriapoda, and Hexapoda all have mandibles, the distinctive set of jaws used as their primary means of feeding. Myriapoda and Hexapoda share a common means of breathing, both having a network of fine tubes, called tracheae, formed from the exoskeleton and permitting the passive movement of oxygen through their bodies.
Spiders and their relatives lack the mandibles found in insects and instead feed via impressive fangs, called chelicerae, as seen at bottom right, with the chelicerae extended upward and the face with its eight eyes below. From top right clockwise: underside of the abdomen, top of the carapace with legs removed, chelicerae in facial view, and underside of the head and thorax with legs removed. From August Johann Rösel von Rosenhof, De natuurlyke historie der insecten (1764–1768).
A dazzling array of the most diverse of all hexapods—the insects. From dragonflies and butterflies to wasps and beetles, the body plan of six legs dominates our terrestrial environments throughout the world.From Hoefnagel, Diversae Insectarum.
TRUE INSECTS
As their name indicates, the Hexapoda are those arthropods with three pairs of legs—in Greek hexa means “six” and podos means “foot.” They are also distinguished from other arthropods by the familiar arrangement of the body into three major parts: head, thorax, and abdomen. Each of these units has a primary function: the head is for sensory input and gustation, th
e thorax for locomotion, and the abdomen for the viscera, which covers digestion, excretion, and reproduction. It is perhaps surprising to many that the presence of six legs and the tripartite body plan do not define an insect. While insects are hexapods with these aforementioned traits, there is one other group that shares these same features. Hexapoda include the true insects, formally named Insecta, as well as their closest living relatives, the Entognatha. Entognatha are small, wingless animals in which the mouthparts are tucked into a pocket within the head, which gives them a puckered appearance. Their name references this internalization of the mouthparts, the Greek entos meaning “inside” or “within,” and gnáthos meaning “jaw” (together, “jaws within”).
So what makes an insect an insect? If it is not the number of legs, what sets true insects apart from Entognatha? In brief, it’s their mouthparts, how they lay eggs, and a sensory capability hidden within their antennae. (See the illustration of an insect body plan, right). Insects’ mouthparts are external, unlike the Entognatha but similar to myriapods, crustaceans, and arachnids. Thus, we can usually see with ease the mandibles, followed by two other sets of mouthpart appendages called the maxillae and labium. While mandibles are unjointed, immediately behind them are the maxillae, jointed structures that can easily manipulate a food item. Posterior to this are what on the surface appear to be a second set of maxillae, but along their midline they are fused to form a composite structure, the labium, which acts as a back wall of sorts for the space created by the mouthpart appendages, and again aiding an insect to hold and manipulate an object with its mouth.
Aside from the externally visible mouthparts, true insects also have a structure at the back of the body called an ovipositor. As its name implies, this structure is used to deposit eggs and is therefore present only in females. In most insects the ovipositor resembles a long tube, and its presence is a major feature influencing insect evolution. An ovipositor allows for the careful placement of eggs, including into concealed locations that may lead to improvements in survival, and is perhaps one of those features that has helped lead to the overall success of insects.
The insect body plan, exemplified here by a tiger beetle (family Cicindelidae), consists of three primary elements: head, thorax, and abdomen. From Georg Wolfgang Franz Panzer, Deutschlands Insectenfaune (1795).
Another defining feature of insects, albeit less noticeable, is the presence of a specialized chordotonal organ within the antenna. This structure is formed of a bowl-shaped cluster of sensory cells within the second segment of an insect’s antenna and is highly sensitive to the movement of the remainder of the antenna. It is called the Johnston’s organ, named after its discoverer, Christopher Johnston (1822–1891), a professor of surgery at the University of Maryland. As the antenna moves, the Johnston’s organ is able to detect whether such movement is the result of gravity or deflection by physical or acoustic vibrations. This seemingly trivial feat has broad implications for insects because the organ expands the insect’s general awareness. The subtle detection and discrimination of these movements by the Johnston’s organ is used for an array of functions, from assisting flight stability to detecting nearby pressure-induced vibrations in the air. For example, some flies can detect the wingbeat frequency of a nearby insect using their Johnston’s organ, going so far as to determine whether the vibrations are those of a courting mate. There are other traits that serve to define insects relative to other arthropods, but these are vastly more obscure.
In some insects, such as particular parasitic wasps, the egg-laying ovipositor can be prominent—even longer than the rest of the body. Shown here, parasitic wasps of the families Ichneumonidae and Stephanidae; the ovipositors of the female ichneumonid (Megarhyssa atrata, top left) and stephanid (Megischus annulator, top right), trail behind them from the tip of their abdomens. From Amédée Louis Michel Lepeletier, comte de Saint Fargeau, Histoire naturelle des insects (1836–1846).
ONE OF THE GREATEST challenges in entomology is the seemingly simple task of documenting those species that exist, learning where they may be found, and determining how they are interrelated and what we might discover of their biology. This is no small feat when you consider that the numbers are so great. There may be 4 million species or more still waiting to be recovered from our world’s varied habitats, and each informs us of the greater tapestry of insectan evolution. Indeed, entomology suffers from a scaling problem. While a population of 1,000 ornithologists means each expert need only cover 10 species of birds, 1,000 entomologists would be responsible individually for thousands of species. The sheer number of insect species is often not appreciated unless compared to other animals. Consider, for example, that there are over 60,000 species of weevils, over 20,000 species of bees, and approximately 18,700 species of butterflies worldwide. Meanwhile, there are 30,000 species of fishes, almost 10,000 species of birds, and about 5,400 species of mammals. As presently known, weevils alone are 6 times the diversity of all birds, and unlike birds, new species of weevils are discovered at such a high rate that some entomologists estimate there may be over 200,000 species of this one insect group alone. Termites are one of the smaller insect lineages, and yet at over 3,100 species, they come close to rivaling all mammalian diversity. And this merely touches on the tip of an incomprehensibly large mountain of species among the other insects, and the countless more waiting to be discovered in our forests, deserts, plains, and streams. Inordinate indeed!
Various African beetles (above and below) and two bees—Amegilla circulata, at left, and Pseudapis amoenula, at right—surround a giant spider wasp (Hemipepsis prodigiosa). From Gerstaecker, Reisen in Ost-Afrika.
A panoply of tropical weevils of the family Dryophthoridae. From Biologia Centrali-Americana. Insecta. Coleoptera. (1909–1910).
Detail from Generelle Morphologie der Organismen (1866) by noted German naturalist Ernst Haeckel (1834–1919) (also see page 23).
Frontispiece to Maria Sibylla Merian’s Over de voortteeling wonderbaerlyke veranderingen der Surinaemsche insecten (1719 Dutch edition of Metamorphosis Insectorum Surinamensium, 1705, see pages 94-95). While a woman and cherublike children look over (or even fight over, as two of them are doing) a collection of specimens, the wild nature of Suriname can be seen through a massive window framed in neoclassic architectural elements.
“I have heard it stated upon good authority that 40,000 species of insects are already known, as preserved in collections. How great, then, must be the number existing in this whole globe!”
—William Kirby and William Spence
An Introduction to Entomology, 1826
One of the first things we do in life is classify the world around us. We learn to recognize and label the persons and objects that are most vital to our well-being, and this process of classification continues throughout our lives. Inevitably, our first words are an act of nomenclature and classification—mommy or daddy. In the same way, humankind has, since its infancy, sought to label and arrange the objects in our world, giving each a unique name so that we might communicate effectively with others. Names give meaning to our universe, and it is true to say that classification is fundamental to the human condition. Our classifications can be artificial, of mere convenience, or natural, reflecting historical or physical processes occurring in nature. We group stars into constellations, although these patterns do not exist in nature and instead reflect regional and cultural influences on our perceptions of the night sky. By contrast, we classify galaxies by the physical laws impacting their forms, spiral versus elliptical for example, or arrange elements by their molecular weight and associated properties. Our natural classifications organize and synthesize knowledge, facilitate effective communication, and, in their ultimate form, permit the formulation of testable predictions.
When one peers into the natural world, the rich variety of life can often become overwhelming and seem chaotic. Nonetheless, there is an order to be found. The process of evolution naturally produces a hierarchical arrangement of
biological traits, such that one can distinguish groups nested within groups. Closely related species can be grouped into a genus, all of which stem from a most recent common ancestor. Closely related genera can be grouped into a family, families into an order, orders into a class, classes into a phylum, and phyla into a kingdom. These are the canonical ranks of the famous Linnaean hierarchy put forward by the great father of biological nomenclature, Carl Linnaeus (1707–1778), and which each of us learns, and frequently forgets, in elementary school.
Linnaeus, a Swedish botanist and physician, was not the first to work toward a grand organization of all species, but he was the first to provide a uniform and structured method by which the diversity of traits among species could be arranged into a natural system. He simplified the names of organisms, with each receiving a binomial indicating its genus and species, such as Homo sapiens for ourselves. Together, the application of binomial nomenclature and the arrangement of species into the Linnaean hierarchy provided a standardized means of communicating about the natural world, with each species in its designated place. Where previously it was challenging to know whether two authors were discussing the same species or not, Linnaeus’s system made this transfer of information far more rigorous. This may seem trivial, but when faced with the difference between a poisonous or edible species, one that may cure disease or one that may pose a threat, it can quickly become a matter of life or death. The first step is agreeing on what we are discussing and establishing an agreed-upon name. As famously stated by Linnaeus, “If you know not the names, then the knowledge of things is lost.” Linnaeus relied considerably on entomological observations recorded by his intellectual forbearers, and it took millennia of steady advances, hampered by false starts and reversals along the way, before intellectual evolution arrived at its own paradigm.