Innumerable Insects

by Michael S. Engel

  The more familiar-looking wasps evolved a narrow waist, allowing great movement of the abdomen, and, more importantly, the ovipositor at its end. These wasp-waisted species became parasitic, first on other wood-boring insects and then branching out to attack the full panoply of insects and even some arachnids. Parasitism apparently kicked wasp diversification into high gear well before the origin of flowering plants. The biology of some parasitic wasps is the stuff of nightmares, even partially inspiring the themes of popular movies such as Alien. For example, a female ichneumonid wasp will grasp her victim, such as a caterpillar, arch her slender abdomen up high as a guide for the hypodermic ovipositor, and inject her eggs into the living host. The host then goes about its life, while the wasp larvae consume them from within, eventually killing the host and bursting from its body to form pupal cocoons. One group of these narrow-waisted wasps evolved a further modification of the ovipositor in which it is no longer used to lay eggs but instead injects venom from associated glands, this being the sting we all fear. The sting is used both to subdue prey and as a defensive weapon. Since it is a modified ovipositor, males obviously lack this structure and are incapable of inflicting such harm.

  While some of these stinging wasps remained parasites, most evolved into predators that went on to hunt spiders, caterpillars, planthoppers, and many other insects. The predator species include our most familiar Hymenoptera: hornets, paper wasps, yellow jackets, ants, and bees, most of which are also social, with intricate nests and colony structures. Rather than hunt prey, bees seek out pollen and nectar and are preeminent pollinators. Yet, even among the bees, parasitism remains a driving theme. Most bees are solitary, and while there are slightly less than one thousand social species, about 5 percent of their diversity, there are several thousand species of parasitic bees. Commonly called cuckoo bees, they place their eggs within the nests of others, just like the birds of the same name.

  The forewings of the fairy wasp (Mymar pulchellum), at less than 1 millimeter in length, have an elongate stalk that expands into a paddle at their apex and bears a fringe of long, stiff setae—a characteristic form of wing for truly minute flying insects for whom flight is similar to swimming through a viscous fluid. From Curtis, British Entomology.

  The western cicada killer (Sphecius grandis, at top and far left) is an impressively sized wasp (2 inches [5 centimeters] in length) that hunts cicadas, while the sphecid wasp (Stizoides unicinctus, near left) is a cuckoo in the nests of related wasps. Despite their fearsome appearance and size, neither are aggressive. From Thomas Say, American Entomology (1828).

  THE ART OF CHANGE

  During a time when women were largely excluded from scientific circles, Maria Sibylla Merian artfully demonstrated a fine aptitude for natural history and left to the world one of the most recognizable folios on insect life. Like her contemporary Jan Swammerdam, Merian documented insect metamorphosis, helping to loosen the grip of many Aristotelian and medieval misconceptions.

  Merian was born in Frankfurt in 1647 to an engraver who died when she was only three. A year later, her mother married the floral painter Jacob Marrel (1613–1681), who tutored Merian in art. As a young girl she was drawn to the beauty of insects and began sketching and painting images of caterpillars on their plants, noticing how in time they transformed to moths and butterflies. Marrel was himself a devotee of the works of Caravaggio (1571–1610), a painter famed for his naturalism. Caravaggio did not shy away from depictions of decay, such as his Basket of Fruit (ca. 1599), which—daring for the time—includes a worm-eaten apple and insect-damaged leaves. This appreciation for realism must have passed down to Merian, whose own works would depart from artistic conventions, often showing within one scene the complete cycle of life—including the less-than-gorgeous aspects of death.

  Merian married one of her step-father’s students, Johann Andreas Graff (1636–1701), in 1665. She eventually had two daughters, and, like Marrel, taught painting. In 1675, she published a collection of floral engravings, followed in 1677 and 1680 by two further sets illustrating the metamorphosis of butterflies and other insects. One set even included a painting of parasitic wasps attacking caterpillars. Her marriage was an unhappy one, and she left her husband in 1685 to join a Protestant commune in the Dutch village of Wieuwerd, living at a residence owned by Cornelis van Aerssen van Sommelsdijck (1637–1688), the first governor of Dutch Suriname. There, Merian became intrigued by fantasies of tropical life. In 1691, she and her daughters moved to Amsterdam. The following year, Merian’s husband divorced her, while their eldest daughter married a merchant based in Amsterdam who traded with Suriname. Merian was determined to explore and illustrate the natural wonders of a tropical environment, and by 1699 she began to plan a voyage to Suriname, covering her expenses through the sale of paintings and apparently with some assistance from her merchant son-in-law.

  Portrait of Maria Sibylla Merian by Jacob Joubraken after Georg Gsell, from A Dictionary of Painters by Matthew Pilkington (1805).

  In July of that year, she and her younger daughter set sail, arriving two months later in the colony she had dreamed of seeing for more than a decade. For two years she traveled throughout the colony, keenly observing the plants and animals, principally insects and their metamorphosis, all of which were foreign to her European experiences. Tropical forests are tall and greatly stratified, with much insect life taking place high above in the canopy, well out of a human’s reach and view. Merian did what she could to access the species up above, and even once had men fell a tree so that she might see what wonders lay beyond the capability of climbers. Everywhere she cast an objective and careful eye.

  From early in her career, Merian was fascinated by the life cycles of insects and regularly depicted the various stages—such as the larva, pupal case, and adult—of moths (depicted here from Histoire des insectes de l’Europe) and butterflies.

  In June 1701, Merian fell ill, possibly suffering from malaria, and had to return to Amsterdam. Upon her return, she sold specimens and for four years finalized her engravings and watercolors of the many tropical plants and insects she observed. These artworks were accompanied by text detailing the insects’ life cycles, and—most importantly—for holometabolous species, descriptions of their complete metamorphosis. Published in 1705, her Metamorphosis Insectorum Surinamensium (Transformation of the Suriname Insects) was a tour de force of empirically derived wisdom, and while the book was a success, the finances it generated did not last for the remainder of her life. Merian preferred scientific knowledge over wealth and comfort, and she continued to sell her paintings as a means of support. Despite the power of her opus in intellectual circles, as a woman she was largely barred from discussions, even when they concerned her own findings. In 1715, Merian suffered a stroke that left her partly paralyzed. She never recovered, passing from this world in 1717. Had she painted her own life, it certainly would have mimicked those of her beloved insects, a representation of her own metamorphosis, utterly changing the expectations of a woman at the dawn of the Enlightenment.

  Merian chara­cteri­stica­lly showed both the beautiful as well as the less attractive aspects of life all within the same image, such as the one seen here, from Histoire des insectes de l’Europe, detailing the various life stages of a moth alongside the same stages of their parasitic flies.

  Merian’s sojourn in Suriname brought her together with large tropical butterflies such as this Deidamia morpho (Morpho deidamia), which she shows feeding as a larva on its host plant, Barbados cherry (Malpighia glabra), and as adults, with both the upper and lower wing patterns replicated. From Surinaemsche insecten (1719).

  MECOPTERA AND SIPHONAPTERA

  Another minor group are the scorpionflies, which have the scientific name Mecoptera (mêkos in Greek means “length,” a reference to the long wings of many in the order). Like the lacewings, these insects are today relict, with scarcely more than 750 species. The order consists of rather disparate groups, many of which have slender wings and
elongate heads. The males of those rightly called scorpionflies have bulbous genitalia that arch back over the abdomen, much like the tail of a scorpion, but which are harmless. Yet others are gangly animals, with long legs used to hang from foliage and to grasp their food, which consists of small arthropods. These are referred to as hangingflies and look like crane flies, the latter of which are true flies and unrelated.

  The earwigflies, also called forcepflies, are another peculiar family of Mecoptera, with only three surviving species. The genitalia of the males is modified into massive pincers, looking much like the tail end of an earwig. Meanwhile, the family of snow fleas, or snow scorpionflies, have either vestigial or missing wings, and they disperse over snow to mate in late winter and early spring. One last family of scorpionflies, called Nannochoristidae, are distributed across the Southern Hemisphere and are the only aquatic Mecoptera, preying on the larvae of true flies that live in freshwater. The larvae of nannochoristid scorpionflies have full compound eyes, something not found in the larvae of any other insects. Modern research into their evolutionary relationships suggests they are likely misplaced among Mecoptera and some rightly consider them as their own order, dubbed as the Nannomecoptera.

  In testament to Ulisse Aldrovandi’s accurate observations, his woodcut of a common European scorpionfly (genus Panorpa) can be immediately recognized by the elongate head and bulbous, arching male genitalia resembling a scorpion’s tail. Detail from a 1638 edition of Aldrovandi’s De Animalibus Insectis Libri Septem (1602).

  A flea, likely the house flea (Pulex irritans), as exhibited for the first time in all of its glory and minute detail by Robert Hooke using his microscopic lenses for his magnum opus, Micrographia (1665, this edition 1667). Although fleas lack wings, they are all descended from a common ancestor that was fully capable of flight.

  With over twenty-five hundred species, the fleas are one group of holometabolous insects many would rather live without. Fleas are exclusively blood-feeding parasites that mostly prey on mammals but also feed on several groups of birds. Unlike lice, who live out their entire lives on the host, fleas spend considerable time off their host when not feeding, fleeing quickly should the animal upon which they are feasting die. Accordingly, while some fleas are specific to particular host species, many are not, and they will dine on a wide range of potential victims. During their evolution into a specialized form suitable for their mode of life, fleas lost their wings and the ability to fly. Their heads are compact and have piercing stylets through which blood is drawn, and the ordinal name, Siphonaptera, references this. The Greek derivation síph n means “pipe,” and the prefix a- expresses negation, thusly giving the insects a name that loosely means “siphon without wings.” The hind legs are modified for jumping, giving fleas their characteristic mode of escape as well as a ready means of landing on their next host. Fleas, like lice, have had a profound influence on human civilization, acting as the carriers of bubonic plague, a bacterium responsible for pandemics such as the Black Death of the fourteenth century.

  Once believed to all be related, the spoon-winged lacewing at top (Lertha extensa), belongs to the order Neuroptera, while the snowflea (Boreus hyemalis, middle), with its vestigial wings, and hangingfly (Bittacus italicus, at bottom) are both related within the order of scorpionflies (Mecoptera). From Guérin-Méneville, Iconographie du règne animal de G. Cuvier.

  DIPTERA

  The order Diptera includes the true flies as well as the mosquitos and midges. The name Diptera means “two wings” (dís is Greek for “twice”), and unlike other lineages, the hind wings are reduced to small, stalked structures called halteres. The halteres assist with flight stabilization, and although flies operate with only two functional wings, they are excellent fliers. Flies are incredibly successful, with over 155,000 species discovered, but this may be only a quarter or less of their global diversity. When we think of flies, most people bring to mind the common house fly, Musca domestica. Flies, however, are more ecologically varied than any other group of insects, making it challenging to succinctly sum them up. Flies may be found specialized for every imaginable habitat, are global in distribution, and although solitary, can exhibit behaviors as intricate and dazzling as any other group of arthropods. Flies can be quite sensational when magnified, with resplendent colors that rival any of those among the more splashy beetles. In fact, observing the mating displays of those with patterned wings is like watching small sailors signaling with semaphore flags. Some have evolved to be wingless, living as parasites on bees and bats, while others have exaggerated head shapes, with the eyes of males widely set on long stalks and used to show off for their mates. Blood-feeding species such as many mosquitos, tsetse flies, and others are extremely hazardous to human health because they can transmit microorganisms that cause devastating diseases such as malaria, yellow fever, leishmaniasis, sleeping sickness, and encephalitis. Yet others, such as screwworm flies, are pests of livestock, although their larvae are also an aid to forensics, permitting investigators to pinpoint the time and even location of death of murder victims.

  Flies are among the most ecologically varied of all insects, and some—such as the stalk-eyed flies (family Diopsidae), figured here among their relatives—are textbook examples of exaggerated male ornamentation resulting from preferences of females during mate selection. From Westwood, Cabinet of Oriental Entomology.

  A range of flies (order Diptera) from E. F. Staveley’s British Insects (1871): from top, house mosquito (Culex pipiens), soldier fly (Stratiomys chameleon), another soldier fly (Sargus cuprarius), marsh horse fly (Tabanus autumnalis), hornet robber fly (Asilus crabroniformis), and down-looker snipe fly (Rhagio scolopaceus).

  Caddisflies (order Trichoptera) resemble small moths and are so named for the distinctive cases built by their aquatic larvae from sand, small pebbles, or plant debris. From Rösel von Rosenhof, De natuurlyke historie der insecten.

  Nonetheless, the truth is that flies are largely beneficial, and the actions of a few have left the majority wrongly maligned. The most deeply researched species—apart from our own—is from the Diptera order: the laboratory fruit fly, or Drosophila melanogaster. The fruit fly has been a model for unlocking the fundamental principles of genetics and development, directly guiding many advances in human health over the last century. The laboratory fruit fly shares with us 75 percent of those genes that can cause disease within humans, and this close genetic relationship and the ease with which the flies can be manipulated makes them ideal tools in medicinal research.

  Flies are also the second-most-critical lineage of pollinating insects, right alongside butterflies, moths, beetles, and bees. Most flies are small, but one mydas fly from Brazil, Gauromydas heros, can have a wingspan of up to 4 inches (10 centimeters) across. The larvae of most flies are informally referred to as maggots, and they are as variable as are their adults. Fly larvae are important recyclers, consuming decomposing plants, fungi, and even animals. While the term maggot brings on images of putrefaction and disease, the larvae of one species, the green bottle fly, Lucilla sericata, are used therapeutically to clean necrotic wounds, as the flies consume the dead tissue, which promotes healing.

  TRICHOPTERA AND LEPIDOPTERA

  The last two lineages are the closely related caddisflies, order Trichoptera, and the butterflies and moths, or Lepidoptera.

  The caddisflies, with about fourteen thousand species, are another of those groups in which the immatures are aquatic, the larvae living in a variety of watery habitats and constructing small cases or retreats. Among the caddisflies are one of only two true marine insect lineages, the other being species of midge in the genus Clunio. The marine caddisflies, belonging to the family Chathamiidae, live amid intertidal algae, although one species, Philanisus plebeius, will oviposit its eggs into the pores of the starfish Patiriella exigua. There, the eggs are protected until they hatch as larvae and exit the starfish’s body.

  Caddisfly retreats are spun from silk secreted by the larva, and some
species will spin small nets to filter food from the water or to snag prey. Others are active predators in moving water and will spin silken “safety lines,” anchoring themselves to rocks while they swim about to catch small arthropods. The most impressive caddisfly constructions are cases built of a variety of materials, which, depending on the species, can range from small slivers of wood to minute stones. While some cases are affixed to the substrate, others are not, allowing the larva to relocate by sticking its short legs out of the front of the case and dragging it behind. Like the larvae, pupae will also spin small silken shelters in the water, eventually emerging as delicate adults with wings covered with fine setae, giving them their hairy appearance. The name Trichoptera literally means “hairy wings” in Greek (trichos, “hair”). As adults, they super-ficially resemble slender moths and survive on land on scarcely anything other than nectar, having generally vestigial mouthparts. The adults live only long enough to find a mate and breed. Females will deposit their eggs on the surface of the water or overhanging vegetation so that the newly hatched larvae can dive in and begin the process of weaving their silken masterpieces.

  Butterflies and moths (order Lepidoptera), perhaps the most familiar of flower-loving insects, have a fine covering of scales over their wings, often beset with spectacular patterns or washes of color, such as the Ceylon tree nymph butterfly (Idea iasonia, top left), or the bee robber hawk moths (Acherontia lachesis, right) and Acherontia styx, below left), all painted around the Javanese orchid (Acanthephippium javanicum). The species of Acherontia are also known as death’s head hawk moths owing to the skull-like patterns of the thorax. They attack honey bee hives by mimicking the bees’ scent. From Westwood, Cabinet of Oriental Entomology.