Innumerable Insects

by Michael S. Engel

Authors prior to the advent of evolutionary thinking struggled over explanations for this observed phenomenon, that nature should appear to be so hierarchically arranged. It took the work of a somewhat reclusive beetle collector to bring it all together, uniting the millennia of observations of biological traits along with population and developmental biology, patterns of geographical distribution, behaviors, and geological history to arrive at the unifying notion that species come from extinct, ancestral species through the isolation of naturally occurring variations, differential survivorship (that is, not all individuals have an equal chance of surviving), and specialization in the face of climatic change, predation, or other influences impacting reproductive success. Those variants that survived the change in the local environment would pass their traits on to the next species, and so on. The remainder would perish, relegated to the annals of extinction. So it was that Charles R. Darwin (1809–1882) articulated a viable mechanism by which evolution would take place. This forever transformed our understanding of the world around us. Darwin helped us realize that natural classifications were those that accurately reflected the underlying relationships established by evolution, and that with this new appreciation there was “grandeur in this view of life.”

  The earliest iconography of genealogical relationships among articulated invertebrates after the Darwinian revolution. The arthropods form the large branch along the left and top of the tree, with the insects nested beyond myriapods in the upper right corner. From Haeckel, Generelle Morphologie der Organismen.

  The common green birdwing butterfly (Ornithoptera priamus), with its dramatically marked wings, is one of the millions of insect species alive today that collectively attest to the considerable success and ancient history of insects. From Robert H. F. Rippon, Icones Ornithopterorum (1898).

  Classifying and understanding the evolutionary relationships among insects is no simple task, partly owing to the aforementioned vastness of their numbers. The clues to their extensive past and interconnected lines is written in their anatomy and underlying genome, and today entomologists bring together these disparate forms of data in order to recover a holistic view of insect history. Today’s evolutionary classification of insects is deeply rooted in the history of their study. While new discoveries refine our understanding, it is phenomenal the degree to which many early researchers, working with quaint tools, were able to correctly discern such important distinctions. In some instances, we have been unable to perfect any further those revelations arrived at a century or more ago.

  When we think of major groups of insects such as roaches, grasshoppers, beetles, wasps, or flies, these are distinctions among what scientists consider taxonomic orders, the Linnaean rank below the class Insecta but above the various families. Examples of families would be the Drosophilidae (fruit flies), Bombyliidae (bee flies), Syrphidae (flower flies), and Muscidae (stable flies), all belonging to the order Diptera (true flies). Long before any of these insects existed, however, their earliest progenitors were among the first animals on land, living more than 410 million years ago and on an Earth so different from our own as to seem alien.

  At the start of the Devonian period, around 420 million years ago, land animals were dominated by arthropods. Originally living in the oceans, particular groups of ancient Arthropoda evolved into terrestrial species, appearing after some early plants had already departed the water and begun colonizing the land. The world had no forests, fields, or meadows. The primitive land plants at this time remained comparatively simple, lacking structures like leaves, which we find so indicative of our present-day flora. Instead, early land plants were short, much like floral beds in home gardens today, lacking roots and never venturing far from water. Vertebrates, specifically amphibians, would not join the insects on land until much later during the period and after terrestrial floras became more developed.

  By the close of the Devonian period, some sixty million years after its start, organically rich soils would appear, supporting forests of giant ancestors of ferns, along with a vibrant insect life—most of which would move about on two pairs of membranous wings. But before all of this, before wings and flight, before arborescence, before our planet’s landscapes greened, there were already insects. The story of the six-legged was already underway, and anyone peering into that ancient Earth would scarcely dream that those animals, of all of life’s variety, would come to rule the world.

  Detail from Charles Athanase Walckenaer, Histoire naturelle des insectes. Aptères (1837) (also see page 39).

  The primitively wingless, six-legged arthropods. Colored specimens from top left down: a globular smithurid springtail, the dipluran Campodea staphylinus, a silverfish of the genus Nicoletia, and the springtail Lepidocyrtus curvicollis. From Walckenaer, Histoire naturelle des insectes. Aptères.

  “ ‘What sort of insects do you rejoice in, where you come from?’ the Gnat inquired. ‘I don’t rejoice in insects at all,’ Alice explained, ‘because I’m rather afraid of them—at least the large kinds. But I can tell you the names of some of them.’ ‘Of course they answer to their names?’ the Gnat remarked carelessly. ‘I never knew them to do it.’

  ‘What’s the use of their having names,’ the Gnat said, ‘if they won’t answer to them?’ ‘No use to them,’ said Alice; ‘but it’s useful to the people who name them, I suppose. If not, why do things have names at all?’ ”

  —Lewis Carroll

  Through the Looking-Glass, 1871

  In Lewis Carroll’s Through the Looking-Glass, Alice finds herself reduced to the size of an average insect and chats about entomological nomenclature with a gentlemanly gnat while sitting beneath a tree. Alice politely names those insects she is most familiar with, such as a butterfly and a dragonfly. Although Alice could rattle off familiar names for some insects, among those most primitive of the six-legged there are species so infrequently encountered by the average person as to have scarcely more than their Linnaean scientific names—names Alice might never have imagined. In fact, all of Alice’s examples were of insects who could fly, but there was once a time in the distant past—before birds, before dinosaurs, before vertebrates left the oceans to walk on land—when insects had not yet evolved their gossamer wings. Few descendants remain today of those earliest wingless hexapods who had already developed their own way of life, confined to the land before other insects evolved an ability to fly. Unfortunately, most of us are unfamiliar with their names or they’ve never been given common names at all.

  Only five groups today represent the descendants of those ancient six-legged animals that could never fly. Three belong to the pucker-faced Entognatha (see page 7); while they have six legs and are sister to true insects, or class Insecta, the Entognatha are not true insects themselves. The three orders comprising the Entognatha are the Collembola, called springtails, and the Diplura and Protura— neither of which has a common name. (Although some people have recently proposed potential names for each, these candidate names are not widely known, even among entomologists). Rounding out the five groups are two belonging to the true insects. These are the bristletails and silverfish, which are orders Archaeognatha and Zygentoma, respectively. While all of these animals can scurry, climb, or even jump, none can, or ever could, fly. While we, like Alice, often recall the names of insects, we are biased toward those with wings.

  There are certainly other flightless insects, such as worker ants, fleas, and myriad more, but in the case of these species, each is the result of the loss of wings from ancestors who were fully flight capable. The entognathans, bristletails, and silverfish, and all of their progenitors, however, never had wings at all, and so can be considered as the truly wingless. By the time other insects took to the air on their membranous wings, the originally wingless subset of hexapods had already been around for generations, and would have looked upon the flying insects as young newcomers.

  Early naturalists, such as Ulisse Aldrovandi (see pages 20-21), often failed to make notice of these Lilliputian hexapods,
or generally grouped them all together as mere vermin of no practical use or harm. Naturalists after Aldrovandi were eager to make sense of all insects, not just those that were obviously of some benefit or nuisance, yet found it challenging to discern the finer details of anatomy that distinguish some types of insect from others. This difficulty was partly the result of their small sizes, most of these wingless hexapods being less than a centimeter in length and often much smaller. Linnaeus, who placed a great emphasis on the various forms of insect wings as a means of defining the orders he categorized in his Systema Naturae (System of Nature) (1758), lumped all wingless insects into a catch-all group he named Aptera, from the Greek for “those without wings.” Unfortunately, in doing this he threw together not only the originally wingless hexapods, but also any other arthropods that lacked wings. The result was a meaningless group of creatures as disparate and clearly unrelated as termites, lice, fleas, and even arachnids and crustaceans, collectively defined not by anything that truly united them but instead by the mere fact that they were not flying insects. Mesmerized by wings, Linnaeus failed to realize the significance of the difference in body plans between the wingless hexapods, arachnids, and crustaceans, and it would remain for later taxonomists to bring to the forefront the evolutionary distinctiveness of these latter groups.

  Originally wingless insects are found throughout the world, although some are more abundant and frequently encountered in temperate or tropical habitats. Most have small structures called eversible vesicles present on the undersurface of the abdomen. These are tiny, fleshy lobes that can be extruded by internal blood pressure and are used to absorb water. Not surprisingly, then, most originally wingless hexapods live in moist habitats, alongside sources of water, and some even live on the water’s surface. Bristletails and silverfish are the best exemplars for what the ancestor of all insects might have looked like, and they retain many traits that are considered primitive relative to the remainder of the class Insecta. For instance, bristletails and silverfish, as well as most entognathans, molt continuously throughout their life, including after sexual maturity. By contrast, all other insects stop molting after reaching maturity, with one notable exception to be highlighted later on. In addition, entognathans, bristletails, and silverfish do not copulate, unlike other true insects, but instead males transfer sperm indirectly via a structure they produce called a spermatophore, which is a packet that contains the sperm within an environment suitable for the sperm’s survival outside of the body. The female then collects the spermatophore and fertilization is completed internally as she pulls the spermatophore within her. The spermatophore frequently contains various nutrients that nourish the female and the eggs.

  THE ENTOGNATHA: DIPLURA, PROTURA, AND COLLEMBOLA

  The Entognatha are so infrequently encountered by anyone other than entomologists that most lack common names. These species generally live on the soil surface or under vegetation or rotting bark, frequently near a source of water such as along rivers or ponds. As the name Entognatha implies—ento, “inside,” and ancient Greek gnáthos, “jaw”—the mouthparts are tucked up within a pocket, known as a gnathal pouch, in the head capsule.

  The small and herbivorous diplurans such as Campodea staphylinus are rarely observed, with scarcely more than two hundred known species. Illustration from John Lubbock’s Monograph of the Collembola and Thysansura (1873).

  LORD OF THE FLIGHTLESS

  Precisely one year prior to her death, Queen Victoria (1819–1901) directed the passage of letters patent raising Sir John Lubbock (1834–1913) to the peerage as 1st Baron, Lord Avebury. The title honored Lubbock’s role in conserving Britain’s greatest Neolithic site, Avebury. In order to prevent the land from being disturbed, Lubbock bought the area outright and then, in his parliamentary role, pushed legislation forward that conserved many prehistoric sites. It was the first law of its kind within the United Kingdom.

  Lubbock was quite a Renaissance man, and although a banker and politician by trade, his real passion was science, particularly archeology and entomology. In fact, his contributions to both archeology and entomology are so considerable that one would scarcely believe he had time to do much else. Our concepts of Paleolithic and Neolithic periods among Stone Age peoples were first articulated in Lubbock’s works, and—quite shockingly for the age in which he wrote—applied the notions of Darwinian evolution to humans and their civilizations in The Origin of Civilisation and the Primitive Condition of Man (1870). Lubbock was even so bold as to mimic the title of Darwin’s landmark 1859 book in one of his, titling it On the Origin and the Metamorphoses of Insects (1872). In fact, Lubbock regularly corresponded with Darwin, who lived not far away, and served as a pallbearer at Darwin’s funeral in Westminster Abbey in 1882.

  Lubbock wrote a particularly delightful book on the biology of ants, bees, and wasps in 1882, among many other subjects. Most remarkable among his many entomological achievements was a beautifully illustrated monograph on the primitive wingless hexapods, which prior to this work were collectively lumped into an artificial grouping known as the Thysanura. The name comes from the Greek thysanos, meaning “tassel” or “fringe” and our for “tail,” and refers to the filament-like tails of silverfish and bristletails (see pages 38-41). Lubbock was the first to clarify the many distinctions among these early hexapods, not a simple task given their minute sizes and the relatively crude scientific optics of the day. In fact, many times researchers relied on candlelight reflected off mirrors to illuminate their microscopes, or they had nothing more than a hand lens through which to peer at the tiny arthropods.

  Sir John Lubbock, 1st Baron, Lord Avebury. Reproduction after a pencil drawing by H. T. Wells (1896).

  Nonetheless, using these simple tools, Lubbock came to the realization that the primitive hexapods were not a natural group, that is to say that they were not all each other’s closest relatives. He formalized two taxonomic groups—the Collembola for the springtails (the entognathan orders Protura and Diplura were not yet known at the time of his work) and a more narrowly circumscribed Thysanura—to include the wingless true insects of the orders, Archaeognatha and Zygentoma. Lubbock created the name Collembola and was the first to acknowledge that they lacked the filament-like tails of silverfish and bristletails.

  In his work Monograph of the Collembola and Thysanura (1871), Lubbock described many new species, discussing their evolution and elaborating as no one before on the morphology and anatomy of these animals in relation to their natural history. While Lubbock prepared the original sketches of anatomical details, the many beautiful illustrations in the book were the work of another. The lithographs were executed, and some painted by hand, by a Mr. A. T. Hollick, who Lubbock remarked as being, “a gentleman who is unfortunately deaf and dumb, but in whom these terrible disadvantages have been overcome by natural genius.” Lubbock thanked Hollick effusively for the “beauty and accuracy of his work.” Whereas many sumptuously illuminated volumes were produced throughout the nineteenth century that reveled in gaudy butterflies and showy beetles, this singular text delighted in the hidden wonder of springtails, silverfish, and their kin. One might truly say that the systematization of the primitively flightless hexapods was founded by Lubbock, making him the first “Lord of the Flightless” in the peerage of entomological evolutionary biology.

  The subtle silvery blue and gray colors of the hunched and elongate springtail Lepidocyrtus curvicollis, as rendered by Hollick for Lubbock’s Monograph.

  Lubbock was one of the few nineteenth-century scholars to appreciate the primitively wingless hexapods for their subtle beauty, such as seen in this miniscule globular springtail (Ptenothrix atra), as well as the intricacies of their biology and anatomy.

  The many magnificent illustrations of springtails (such as this Orchesella cincta, diplurans, silverfish, and bristletails in Lubbock’s Monograph were executed by A. T. Hollick, a deaf and mute artist.

  Among the approximately one thousand known species of Diplura—most o
f which are 2 to 5 millimeters in length (although a minor few can be almost ten times this size)—there are two basic types: herbivores with long, multi-segmented appendages called cerci at the back of the body resembling antennae or paired tails, and predators with cerci shortened into a pair of stout pincers, used for grasping their prey. Dipluran mothers will guard their eggs and the young after they have hatched. Being a dipluran mother, however, can be a dangerous calling, as some hatchlings can turn cannibalistic and devour their parent.

  The pincer diplurans, such as this Japyx solifugus, are the predators of the order Diplura. They use the forceps at the apex of their abdomen to seize prey. Also from Lubbock’s Monograph.

  Owing to their minute size, the often ornate and delightfully patterned colors of many springtails go unnoticed, such as those of Dicyrtomina ornata. From Lubbock’s Monograph.

  The similarly obscure Protura comprise about five hundred species of truly minute (less than 2 millimeters in length) and wholly bizarre animals, which were not discovered until 1907. Nonetheless, they inspired the late Swedish entomologist Søren L. Tuxen (1908–1983) to devote the majority of his career to their study, producing what remains the definitive monograph on them in 1964. Species appear to be specialized herbivores that feed on fungi and are distinctive for the loss of their antennae, and instead use their front legs as sensory organs. Holding their forelegs up and in front of them, proturans walk using only four legs, despite being hexapods.

  Last but certainly not least among the Entognatha are the Collembola, the most diverse of them all, with around nine thousand species and the only entognathans to have a common name, the springtails. Springtails are putatively related to the Protura, although evidence for this has been conflicting. Species of springtails are found throughout the world, even in the seemingly uninhabitable polar regions, on the tops of the highest mountains, and in the deepest of caves. Some individuals can grow to more than 20 millimeters in size, but most are far smaller, even less than a third of a millimeter in length. They are generally scavengers of decomposing matter and fungi, but a few will prey on a variety of micro-organisms such as minute worms or other tiny arthropods. The antennae of springtails are rather reduced and squat, and the bodies are either globular in form or somewhat cylindrical, and are frequently marked with patterns or color. For instance, the largest springtails, which belong to the genus Tetrodontophora, are usually a bright, velvety purple or blue. They are often found in aggregations in caves or along the edges of ponds and streams.