Innumerable Insects

by Michael S. Engel

  Diagrammatic Segment of Insect Thorax Showing Point of Wing Articulation

  The base of an insect wing sits between the exoskeletal elements forming the top and side of the thorax, with the wing sitting above a dorsal knob in the side of the thorax, which acts as the fulcrum for the wing. Small exoskeletal pieces anterior (basalare, sometimes divided into two components as shown here) and posterior (subalare) to the fulcrum attach to muscles which, when contracted, result in either forward or reverse tilting of the wing. Adapted fron an original diagram from Robert Evans Snodgrass, The Thorax of Insects and the Articulation of the Wings, 1909.

  While the wings of vertebrates can be actively augmented during flight through any number of muscles within the limb, the insect wing is a passive structure. The only muscles that operate the wing are located in the thorax itself, and they do not extend beyond and into the flight foil. At their simplest, insect wings operate somewhat like a long lever. Muscles within the thorax attach from top to bottom and from back to front, and it is the contraction of these that distorts the overall shape of the thoracic exoskeleton itself. The side of the thorax acts like a fulcrum over which the top of the thorax and wing sit. As the muscles extending from top to bottom contract, they pull down on the upper side of the thorax, while the wing, on the end of the fulcrum, is sent upward. Relaxation of these muscles and contraction of the alternate pair bring the wing down. Additional sets of musculature pull on small portions of the integument anterior and posterior to the wing base, which cause the wing to tilt forward or backward, thereby enriching the range of possible motions. In fact, most insect wings do not simply flap up and down, but instead move in a figure-eight fashion, and the aerodynamics of insect flight is far more complicated than it appears.

  Flight in the smallest insects is different altogether, as for these animals air itself is a viscous environment, and as a result their movement is like swimming through a thick fluid. This is the result of a scaling effect in the relationship of inertial and viscous forces acting on a wing moving through a fluid, such as air. At its most simplistic, a larger wing passing quickly through air experiences mostly inertial forces and minimal disruption—ideally none if one is in an airplane—between the individual layers of the air it is displacing as it moves (this is called laminar flow). As a wing scales down in size, the relative role of viscous forces increases, with significant disruption between the layers as the flyer pushes against the air, resulting in what is called turbulent flow. This increased dominance in viscous forces is why the mechanics of the smallest of insects is so different from that of a large dragonfly or locust. The same effect can be achieved by slowing down a large wing, such that at slower speeds turbulence can have a greater impact. Accordingly, there is a complex boundary line between the relative importance of these and other forces, all relating to wing size and shape as well as the corresponding size and form of the insect’s body.

  Although the foil of the wing is a passive structure, it does change shape during flight. Running through the wing are a series of minute tubes that are extensions of the insect’s tracheal breathing system, and these form the pattern of veins we observe. The veins help give shape to the wing, providing support to the thin membrane formed of exoskeleton. The veins have defined points of weakness within them, and these allow areas of the membrane to give way during certain wing movements, permitting certain folds to form and control the dynamics of flight particular to that group of insects. In addition, several insects have veins more closely clustered along the forward edge of the wing, or even thickened into a block toward the wing’s tip. These lend weight and strength during the powerful downstroke of flight, preventing fluttering that would otherwise defeat the forces generated.

  The wings of insects come in a seemingly infinite variety, and they have been coopted by evolution for many purposes well beyond a mere means of getting about. Wings, properly adapted, are used to warm the body by catching the early rays of the sun after the rigor of cold nights, to startle and confuse potential predators, to display and communicate with mates, to cover and protect the body, and even to be shed such that they do not interfere with other functions of life. Simply put, wings are not always just wings.

  The immature stages—prior to sexual maturation—of winged insects do not fly; they either lack wings altogether or the wings are represented merely by undeveloped pads. Fully formed and functional wings appear only during the final molt to adulthood. The wingless bristletails and silverfish molt throughout life, including after reaching sexual maturity, but molting stops at maturity for flying insects. The one exception to this rule is the mayfly, in which functional wings appear in the next-to-last molt, and these insects shed their exoskeleton a final time after becoming capable of flight.

  The great majority of insects have wings, and it is the variety of these appendages that most loudly speaks to the diversity of insect life: mayflies, dragonflies, crickets, grasshoppers, roaches, mantises, termites, aphids, water bugs, and so many more, each with their unique adaptations and myriad life histories. Winged insects do much more than fly, and during their evolution some invaded freshwater streams and lakes, some built elaborate homes and societies, and yet others shed their wings to become parasites. Like the primitively wingless insects (see chapter 3), the winged insects are classified into taxonomic orders, many of which were first recognized by Linnaeus. He gave them names referring to the particular forms of their wings, so that most names for orders (ordinal names) end in ptera, from the Greek pterón, another term for “wing.” This pattern of naming was largely followed by later entomologists, either when new orders were discovered or when scientific advances revealed that Linnaeus had initially cast too wide a net, grouping unrelated insects into one order. These orders come in such variety that it is impossible to summarize them as a whole, and only by proceeding group by group can one appreciate what makes each so unique.

  Primitive winged insects: from top to bottom, a dragonfly with luxuriously patterned wings, Rhyothemis variegata; the mayfly Hexagenia limbata; and the metallic green damselfly, Neurobasis chinensis. From Félix-Edouard Guérin-Méneville, Iconographie du règne animal de G. Cuvier (1829–1844).

  A NEW HOPE FOR ENTOMOLOGY

  In early nineteenth-century London, the favored place to meet and discuss all things entomological was in the residence and personal “museum” of the Rev. Frederick W. Hope (1797–1862) and his wife, Ellen Meredith (1801–1879). Ellen had previously been courted by Benjamin Disraeli (1804–1881), who would later serve two terms as the United Kingdom’s prime minister, but she declined in favor of Hope. Both Hope and Ellen were from families of considerable means, and they put their fortunes to the task of developing a first-class natural history collection from throughout the world, complete with an extensive library and tens of thousands of engravings. Hope had a strong penchant for beetles, but he did not shy away from the other insects, nor did he forget the other branches of natural science. He was a close friend of the young Charles Darwin, the latter referring to Hope as his “Father in Entomology”; they spent June of 1829 together collecting beetles in Wales.

  Hope and his wife were exceedingly generous, opening their collections for study to suitable individuals; the Hopes even went so far as to provide money and materials necessary for others to carry out their studies. For much of his adult life, Hope suffered from poor health. During his late forties, he began retiring from many societal offices and activities in which he was heavily involved— particularly his beloved Entomological Society of London (today known as the Royal Entomological Society), of which he was a founding member in 1833. (In 1835, Ellen was the first woman fellow admitted to the learned society.) Hope notified his alma mater, Oxford University, of his desire to see his collections transferred to their care and for the establishment of a curatorship to oversee his specimens. In 1855, Oxford laid the foundation stone for its new Museum of Natural History, and sizable endowments from Hope ensured that the Hope Entomological Collecti
on—inclusive of his vast holdings of books and ancillary materials—would be well cared for. Hope’s selection for the custodian of his collections was John O. Westwood (1805–1893), for he trusted no one else with the task, and in 1858 Westwood was so appointed.

  The monographs of John O. Westwood greatly benefited from his considerable artistic talents, and his illustrations were greatly sought out by contemporary naturalists. Displayed here are a large Malaysian moon moth (Actias maenas) alongside a large candy-striped hawkmoth (Leucophlebia lineata) from Westwood’s The Cabinet of Oriental Entomology (1848).

  Westwood had originally studied law but detested it. He was more passionate for natural history, archeology, heraldry, and medieval art, and, by 1820, he was vigorously collecting insects and corresponding and exchanging material with fellow entomologists. In March 1824, Westwood met Hope and the two became fast friends. A decade later, Hope appointed the junior man as conservator of his insects, except for the beetles. Eventually Hope established a professorship, the Hope Professor of Zoology (Entomology), of which Westwood took the inaugural chair in 1861 and remained in that capacity until his death.

  Varied species of stag beetles (family Lucanidae) as arranged by Westwood for his Cabinet of Oriental Entomology.

  Westwood was truly an ideal candidate, for his expertise was extensive, and many have considered him the last of the great polyhistors of entomology. He was also a talented artist, making his work all the more valued for the accuracy and subtle beauty of the subjects depicted, and he generously illustrated works for friends and colleagues. Westwood expanded considerably upon Hope’s collection, and with the resources provided was able to purchase important specimens, engravings, paintings, and virtually anything entomological. Westwood wrote extensively, publishing the leading entomological textbooks of the day—for which he received the Royal Society’s gold medal in 1855—as well as monographs and articles on every group of insects then known, all gorgeously illustrated.

  The different sides of the wings of the northern jungle queen butterfly (Stichophthalma camadeva) from Myanmar, Thailand, and northeastern India, as depicted in Westwood’s Cabinet of Oriental Entomology.

  Much like other learned gentlemen of this era, Westwood’s expertise was not confined to one subject, and he was a regular contributor to the journal Archaeologia Cambrensis as well as a published authority on antique ivories and paleography. Never wishing to see entomological information lost, Westwood would reissue older works, expanding and improving upon them, such as Edward Donovan’s (1768–1837) Natural History of the Insects of China and Natural History of the Insects of India (reissued in 1842), or purchase abandoned projects, seeing them through to completion on behalf of their original authors. The man seemed to be electric with energy, and he drew many to the study of insects. He made himself open to all, all that is, except for Darwin, because Westwood remained a staunch anti-evolutionist. Quite ironically, though, many of his discoveries corroborated Darwin’s interpretations.

  The Hope chair continues today, and there have been five successors to Westwood, each building entomological science in their own way. Through Westwood, the collections he developed, and the endowment he provided, Hope inspired and gave considerable life to entomology, a legacy that continues to benefit the science even now.

  The peculiar males of scale insects (family Coccidae) are often difficult to associate with their corresponding females, whose bodies evolutionarily reduced to flattened, soft ovals often covered with wax. From Westwood, Arcana Entomologica; or, Illustrations of New, Rare, and Interesting Insects (1845).

  Adult dragonflies (order Odonata) as illustrated by August Johann Rösel von Rosenhof for his De natuurlyke historie der insecten (1764–1768).

  EPHEMEROPTERA AND ODONATA

  The earliest flyers had outstretched wings but lacked the specializations that permitted them to be folded flat over the back of the abdomen. When at rest, their wings either extend out to the sides or straight up above the body. This form of wing is called paleopterous. During the Paleozoic era, which ended 252 million years ago, insects with these kinds of wings were varied, abundant, and dominant, while today only two lineages of paleopterous insects—a mere fraction of their former glory—persist: the mayflies, order Ephemeroptera, and the order Odonata, consisting of the dragonflies and damselflies. Mayflies, dragonflies, and damselflies are found near ponds and streams, as their immatures live in freshwater, although the Ephemeroptera and Odonata each independently evolved this mode of aquatic life. The aquatic and wingless immatures of mayflies, dragonflies, and damselflies are called naiads. The term naiad is a nod to Greek mythology, where the Naiads were female spirits who presided over bodies of fresh water such as lakes and streams. The naiads must emerge from the water for their molt into adults, at which time their wings unfurl from within the naiad exoskeleton. These wings gradually stiffen and dry, and the insect can then take flight. Since naiads are so dependent on the water in which they live, the health of their populations is usually an excellent indicator of water quality.

  Mayflies are generally slender insects with broad forewings, while the hind wings are reduced or sometimes entirely lacking. Mayfly naiads must be tasty, as they are a major food source for many fish and other aquatic predators, and they are treasured by fishing enthusiasts, who work tirelessly to mimic them as lures. An entire industry on fly tying exists, and troves of books have been written—and will continue to be written—on how to tie the perfect “hatch” (a naiad emerging as an adult) and the subtle ways in which to tug upon the line such that the lure parodies the movement of a particular mayfly species in the water column.

  Adult mayflies are peculiar in that they retain vestigial mouthparts but they do not feed. This means that the adults survive solely on the nourishment stored up during their juvenile stage. Juveniles of some species are carnivores, while others scrape algae. The lives of adult mayflies are accordingly brief, with many living only a few days or even hours, and their singular purpose is to find a mate. This ephemeral nature is reflected in their ordinal name, Ephemeroptera: in Greek, ephémeros means “for the day.” Since their lives are so brief, there is no time to dawdle or waste while trying to locate a suitable mate, and for this reason the emergence of adult males and females is highly synchronized. Adults appear in mass emergences, and on particular spring or summer evenings, it is not uncommon to find a cloud of mayflies swarming about lights, the largest emergences numbering into the tens of millions of individuals. Swarms have been known to be numerous and dense enough to shut down traffic, obscuring drivers’ views and clogging radiators.

  Mayflies, such as these from Central America, are the most primitive of flying insects: colored, at top, Lachlania lucida; below (left to right): Euthyplocia hecuba, Homoeoneuria salviniae, and Hexagenia mexicana. From Biologia Centrali-Americana. Insecta. Neuroptera. Ephemeridae. (1892–1908).

  While these swarms increase the opportunity for a male and female to meet, they also attract hosts of opportunistic predators eager to make a hearty meal of mayflies. Birds, spiders, dragonflies, and many other predators avail themselves of the feast. Mayflies are ancient, and one can imagine a mass emergence of some primitive species of mayflies being dined upon by the earliest birds, mammals, or even smaller dinosaurs. Males and females couple in flight, and the latter then typically drops eggs into the water, though some species may land to insert their abdomen into the water to deposit the eggs. Their tasks complete— mating and laying eggs—the male and female soon die, bringing to quick close to the ephemeral life of the adult mayfly.

  DRAGONFLIES AND DAMSELFLIES ARE popular with amateur entomologists, as they are large, often spectacularly colored, and are usually about during daylight hours, darting over ponds and streams. The six thousand or so species are adept flyers, capable of rapid maneuvers and coming to a swift stop to hover and survey their territory. They are great aerial predators with acute vision and are capable of snatching their prey in flight. Their method of matin
g is unusual in that the male first transfers sperm to a set of organs on the underside of his abdomen. He then uses claspers at the end of his abdomen to grasp the female by the neck and stabilize her. She then bends her abdomen out to receive the sperm from his secondary abdominal organs. The rather contorted formation they assume quite appropriately resembles that of a heart.

  Various colored dragonflies (order Odonata) with their aquatic naiads, including one (upper right) with its fierce labial mask—used to capture prey—extended. From Drury, Illustrations of Exotic Entomology.

  While our jet engines are considerable engineering achievements, dragonflies invented jet propulsion long before any human or primate stumbled onto the scene. Most dragonfly naiads are capable of darting through the water, moved by a jet propulsion system. The animal draws water into the rectum as it breathes, and then expels it with considerable force, shooting the naiad away from a foe or toward its prey, which, in the case of the latter, is then snagged by a fierce “mask.” The mask is an elongate form of the labium—the posterior set of mouthpart appendages—which extends forward to cover the lower face, including the other mouthpart structures. The mask can pull prey—which for larger naiads may include small fish—toward the fierce mandibles lurking underneath.

  Damselflies (order Odonata) as aquatic naiads, and above as mating pairs, with the male grasping the female by the neck and then together forming an inverted shape reminiscent of a heart. From Rösel von Rosenhof, De natuurlyke historie der insecten.