Innumerable Insects
Page 5
Some springtails don’t just live near freshwater, they actually live on its surface and are therefore effectively fairly aquatic. In general, Collembola have fairly hydrophobic (water-repellent) exoskeletons, which make it easier for them to come into contact with water without being engulfed and drowned. The semi-aquatic species of Collembola have further specializations among their setae as well as structures on their feet that collectively permit them to avoid breaking the surface tension of the water. Indeed, other modifications allow them to control their movement over the water, and the males of these species produce peculiar spermatophores that rest on the water for retrieval by receptive females. Not only are Collembola the most diverse of entognathans in numbers of species, but they are also the most abundant. Often, when conditions are ripe, they form aggregations with several million individuals clustered together. The purpose of these swarms is not entirely understood but appears to be the result of particularly suitable reproductive circumstances and episodes of dispersal to new localities.
The name springtail references the order’s singular mechanism of rapid movement. Aside from their three pairs of legs, springtails have a true spring on the underside of their abdomen which, when released, literally hurls the animal through the air, often to considerable distance. Some springtails can fling themselves up to eighty times their body length. Today, the world’s record for a running long jump by a human is slightly over 29.3 feet (8.95 meters), or about five times an average man’s height. Imagine leaping nearly 465 feet (142 meters), and without a running start—that approximates the achievement of these springtails!
The globular springtail Dicyrtoma fusca viewed from above, below, and the side. The paired arms of the spring, which permits these animals to leap, can be seen folded beneath. From Lubbock’s Monograph.
On the underside of the abdomen toward the back, a springtail has a structure formed from the fusion of paired abdominal appendages that is the actual moving component of the spring; this is called the furculum. The furculum is folded forward and held tightly in place by a small lock, or retinaculum, positioned more anteriorly on the abdomen’s center. The furculum is held in place with considerable potential energy until the animal is disturbed, at which point it is quickly released and the force propels the springtail aloft. This method of dispersion is not flight, and they cannot be considered flying animals. Springtails have no control over their movement in the air, nor can they glide or direct their descent back to earth, so they can just as easily end up in a worse place whence they started. However, the jumps are sufficiently quick and lengthy, particularly the combination of repeated leaps, to remove the animal from any precarious situation or to aid dispersal to new habitats. Since springtails are tiny and of minimal weight, they are often captured in air currents during their leaps, and by riding the winds they may be deposited on distant islands, mountain peaks, or elsewhere. By becoming “aerial plankton,” springtails have managed to colonize the world as diminutive and often unseen conquerors.
the elongate-bodied springtail Orchesella villosa does not appear capable of rapid escape. Peering below the Orchesella villosa.
the “spring” mechanism is cocked and ready to propel the animal far from danger. From Lubbock’s Monograph.
This means of spring-loaded movement is truly ancient, and the remains of a species of springtail are among the earliest of fossil evidence for hexapods. Rhyniella praecursor was a tiny springtail from the early Devonian, approximately 410 million years ago, and preserved complete with its spring and lock. One can imagine these early hexapods leaping about the seemingly alien world of long ago—a period in which forests had not yet evolved across our world, where the tallest plants were comparatively simple, leafless, vascular organisms living near water, and in which animals had only recently colonized land.
A splendid array of colorful springtails (order Collembola). From Walckenaer, Histoire naturelle des insectes. Aptères.
THE FIRST TRUE INSECTS: ARCHAEOGNATHA AND ZYGENTOMA
All other hexapods belong to the true insects, or class Insecta. The earliest divergence among insects, which predates the origin of wings and flight, are the bristletails and silverfish, each recognized as their own taxonomic order. The Archaeognatha include the bristletails, sometimes called jumping bristletails, while the Zygentoma are widely known as silverfish or firebrats. The name firebrat is usually given to those species preferring high temperatures, with their presence near furnaces or ovens in homes leading to this nom de plume. Like the entognathans, these wingless insects are widespread and typically ignored, even by the majority of entomologists. Neither are especially diverse today, with only around five hundred species of each.
Both groups consist of rather slender animals with long antennae and three elongate filaments at the tail end—two of which are the cerci, like those of diplurans, and at the middle there is a similar prolongation of the last abdominal segment. The bodies are rather low to the ground and frequently there may be scales scattered about. As in the entognathans, bristletails and silverfish do not copulate and instead mate via a spermatophore, produced by the male and then retrieved by the female. As true insects, females have an ovipositor, which guides the deposition of eggs and permits her to place them in secluded crevices or in holes that she prepares for their protection. By the time they hatch, the mother is gone and the young must fend for themselves.
Archaeognatha are usually nocturnal, spending the daylight hours under stones or within the crevices of bark. During the night they emerge to feed and mate. Their diet consists of lichens and algae, although they will also scavenge exoskeletal fragments of other arthropods. Bristletails have large compound eyes as well as three smaller, simple eyes called ocelli, at the top of the head. Ocelli are found in a variety of insect groups; while ocelli do not perceive images, they are sensitive to variations in light levels and may be used in navigation and orientation, particularly at night. The thorax is hunched, and by flexing considerable muscles in the abdomen, the bristletail can jump as a means of evading a predator.
Jumping bristletails (order Archaeognatha) such as Machilis maritima are the most primitive of living insects, scraping lichens and algae with their simple mandibles. From Lubbock, Monograph.
Although resembling each other superficially, bristletails and silverfish— the primitively wingless insects—are not all closely related. From Walckenaer, Histoire naturelle des insectes. Aptères.
The biology of silverfish is largely the same, although they are more generally omnivorous, squatter, and cannot jump. They also have smaller compound eyes and lack ocelli, with the exceptions of a peculiar relict species surviving in northern California and an extinct species that was found in Baltic amber from forty-five million years ago. Although they cannot jump, silverfish are quite agile and have no difficulty darting away when danger looms
Superficially, these two groups seem quite similar, and yet from an evolutionary perspective they are utterly distinct. The most fascinating aspect of their story is hidden within seemingly inconsequential details of their anatomy. Bristletails have mandibles that are attached to the head via a single ball-in-socket joint and that can rotate like an auger. They use these to scrape at surfaces to remove the lichens and algae upon which they feed. This kind of original joint is also present among the entognathans, where it consists of a thumblike knob (called a condyle) on the mandible that inserts into a cuplike impression (the socket, or acetabulum) on the head capsule. The scientific name of bristletails, Archaeognatha—archaîos, “ancient” or “primeval,” and gnáthos, “jaw”—references this primitive form of mandible with its single attachment.
Silverfish (order Zygentoma) such as this Lepisma saccharina, while superficially resembling bristletails, are more closely related to winged insects. In the insect family tree, silverfish and winged insects are sisters, while the bristletails are their distant aunt. From Lubbock, Monograph.
The mandibles of silverfish, however, are articulated to the
head at two points, rather than merely one. The first point of attachment is like that of bristletails, in that the mandible has a thumblike knob in a socket on the head capsule. This attachment is at the tail end of the silverfish mandible. The second point of attachment, at the forward end of the mandible, evolved separately. It also consists of a knob in a socket, but this is reversed, with the knob on the head and the socket on the mandible. This two-jointed, or dicondylic (two condyles), mandible is found in silverfish, and most notably, all other insects. These mandibles cannot rotate but instead move in scissorlike fashion, enabling greater force. Silverfish also share with all other insects, to the exclusion of the bristletails, a unique modification at the base of the ovipositor that gives it greater control. These traits, among others, and the sequence of DNA in their genomes, demonstrate the remarkable reality that silverfish are the closest living relatives of the winged insects—despite their primitive resemblance to bristletails. In fact, the ordinal name for silverfish references this genealogical relationship: Zygentoma combines the Ancient Greek zygón, meaning “yoke,” which is a wooden harness used for joining together two oxen, and éntoma, for “insect.” The Zygentoma are therefore the “yoke” or harness that joins the primitively wingless with the winged insects.
The lowly silverfish therefore holds a distinguished position as sister to life’s most spectacular diversification: the winged insects that comprise over 99 percent of all insectan diversity, and over half of all life we know today. You might consider this astounding fact next time you watch a silverfish scurry out from under your refrigerator or oven during a late-night foray into the kitchen.
Despite the reduced attention received by silverfish relative to their winged relatives, a common silverfish was among the select few observed by Robert Hooke with his newly developed microscope, alongside two arachnids (a mite above, a pseudoscorpion below). From Hooke’s Micrographia (1667 edition).
Detail of a giant cave cockroach from Jean Victor Audouin, Histoire naturelle des insects (1834) (also see page 70).
“A power of Butterfly must be—
The Aptitude to fly
Meadows of majesty concedes
And easy Sweeps of Sky.”
—Emily Dickinson Complete Poems, 1924
When we marvel at insects, it is usually their wings that pique our interest. We are dazzled by the variety of patterns on the wings of butterflies and moths, by the metallic sheen or spots on those of beetles, and the fine veins running through the wings of dragonflies and lacewings. Most insects fly, using a pair of wings to generate the lift and thrust necessary to sustain and control movement through the air. So numerous and pervasive are flying insects that if one is asked to quickly picture an entomologist in their mind’s eye, inevitably what is envisaged is that of a person on the hunt with a net in hand, ready to swipe their catch from the air. Any number of Gary Larson’s Far Side cartoons may come to mind with just such a depiction!
Aside from the aforementioned orders in the preceding chapter, all living insects belong to a hefty subclass called the Pterygota, obviously so named owing to the presence of wings—in Greek, pteryx means “wing.” It is precisely this ability to fly that makes the word fly so ubiquitous in many insect common names, including mayfly, dragonfly, stonefly, owlfly, black fly, and butterfly, to name a few. For the keen–sighted, one sobriquet stands out among this sampling of common monikers. Among all of those named, only the black fly is a true fly; that is, a species of the order Diptera, the taxonomic group comprising those insects we call flies and which have but a single pair of wings (versus the two pairs in others orders, see pages 98-99). The remainder of those insects listed above, despite having fly in their common name, belong to different orders that are as distinct from and distantly related to the true flies as the latter are from and to beetles or roaches. Long ago, when names of convenience were initially formed and our optics on these diminutive animals were limited, any minute arthropod flying about was simply dubbed a fly. Only centuries later did we come to understand that these were disparate lineages of insects. So, how does one keep it all straight when the word fly is appended to the common name of so many unrelated kinds of insects? In entomology there is a general rule of thumb, best articulated by the distinguished entomological anatomist Robert E. Snodgrass (1875–1962): “If the insect is what the name implies, write the two words separately; otherwise run them together.” A silverfish is not a fish, and neither a dragonfly nor a butterfly are flies (for that matter, nor are they dragons or made of dairy products). Flies are those insects in the order Diptera, or, the true flies. A horse fly bites horses and is decidedly a fly, and this is reflected in its appellation. And, of course, it can certainly fly.
The hind wings of the southern African common milkweed locust (Phymateus morbillosus) are spectacularly crimson and contrast dramatically with the bluish forewings and yellow abdomen. From Edward Donovan, Natural History of the Insects of China (1838).
We tend not to think much about the remarkable nature of flight in insects. Mosquitoes buzz in our ears, dragonflies dart over ponds, and butterflies flit through our gardens, and we scarcely give them notice—the mosquito being perhaps an exception. This flight is not a mere matter of gliding safely from a precipice, as is done by many animals, but it is instead an active process allowing for takeoff from any location and control over the speed and direction in which the individual wishes to move. Flight permits new avenues in which an organism may experience life—invading new habitats as they disperse, allowing quick escape from danger, and providing a novel means for locating shelter, food, or a mate. To actively defy the grip of gravity, to slip “the surly bonds of Earth” as said by the Anglo-American poet John G. Magee (1922–1941), is an incredible achievement, and not easily accomplished. The British satirist Douglas Adams (1952–2001) summed up the evolution of flight most succinctly in his book Life, the Universe and Everything (1982), writing that there, “is an art to flying, or rather a knack. The knack lies in learning how to throw yourself at the ground and miss.” Today, perhaps as many as 5 million species casually “miss” the ground on a regular basis, and maybe 100 million more did so as well during the intervening 410 million years since the progenitor of them all first hurled itself at the ground and got the knack of it.
The wings of the tropical American lantern bug, or peanut bug (Fulgora laternaria), not only permit flight but serve in self-defense, as the insect will spread them to reveal characteristic eyelike patterns meant to startle potential predators. From Georges Cuvier, Le règne animal distribué d’après son organisation (1836–1849).
Insect wings have evolved into myriad forms such as those of these African grasshoppers: the gaudy grasshopper of west tropical Africa (Rutidoderes squarrosus), above; and the bird grasshopper of central and southern Africa (Cyrtacanthacrus tatarica), below. From Dru Drury, Illustrations of Exotic Entomology (1837).
Only four lineages of animals have successfully invaded our Earth’s skies in this way: insects, birds, bats, and the long-extinct flying reptiles known as pterosaurs. Among all of these, insects were nature’s first flyers. Insects were the first of all life to take to the skies with powered flight, and today there are over a million known species among the winged insects. In fact, insects were the only flying animals for perhaps 170 million years before they were eventually joined by pterosaurs, and much later by birds and eventually bats. By the time any other animals soared into the air, insects had been perfecting their aerial acrobatics for an expanse of time two and half times greater than that since dinosaurs died out over 65 million years ago. When one looks at the ease with which a bee hovers gently before a flower, it is the summation of evolution’s refinement of insect flight over a period of at least 410 million years. Insects gave the world flight, and flight gave insects the world.
WING EVOLUTION AND MECHANICS
In birds, bats, and pterosaurs, the evolution of the wing is straightforward. In each of these, the wing is a modified form of
the foreleg, with the same arrangement of bones as observed in their relatives, but augmented for flight. By contrast, the origin of the insect wing has been one of the more abominable riddles in evolutionary biology. An insect wing is not merely a modified leg; all flying insects retain their original six legs and still bear two pairs of prominent wings. Wings are not legs. So, what then is a wing? This pernicious problem has vexed the brightest entomologists for generations. Over the last 150 years, hypotheses have abounded, and only recently have comparative anatomy and modern developmental genetics come together to provide a unified answer. The bulk of a wing is formed of a thin extension from the upper wall of the thoracic exoskeleton, which is then hinged at its base. The genetic architecture for forming a hinged joint already exists in the development of the articulated legs, and it was this suite of genes that were copied and coopted to permit movement at the base of the wing.