Innumerable Insects

by Michael S. Engel

  Unlike mantises, which tend to fascinate us, roaches are maligned and looked upon with disgust. There are over forty-five hundred species of roaches, collected under the order Blattaria, one of the few insect ordinal names of Latin origin—blatta being Latin for “light-shunning insect,” while -ria is a suffix used to modify nouns into abstract groups, with the name effectively meaning “the group for roaches.” Most species prefer warm, natural environments but a few species are quite amenable to moving about in urban settings. Sadly, these few have made roaches the poster child of pests and mistakenly synonymous with filth and disease, while the majority of wild species are really quite clean animals and do not spread infection.

  Most roaches are nocturnal and are usually found amid detritus on forest floors. They are generally scavengers, although many exceptions exist. There are species who can emit light as a form of communication, much like a firefly, or that rub portions of their body together to make stridulating calls. Species of the genus Cryptocercus, commonly known as wood roaches, are gregarious; they live within rotting logs and feed on wood, just like termites. In fact, the wood roaches are the closest-known relatives of termites, and, like the latter, have symbiotic microorganisms in their guts that permit them to digest wood.

  Early entomologists and artists often attempted to depict the complete life cycle of a species within a figure. Here, the life history and biology of the European praying mantis (Mantis religiosa) is outlined, from the hardened ootheca encapsulating the eggs, to the emerging nymphs, and eventually to the predatory adult. From Rösel von Rosenhof, De natuurlyke historie der insecten (1893–1909).

  Termites, or Isoptera (from the Greek ísos, meaning “equal” and referring to their largely identical fore and hind wings), were the first insects to become truly social—they evolved their societies over 140 million years ago. All of the nearly 3,150 species of termites are social. Most live in large, perennial colonies, with their societies organized around a system of castes. The queens provide the reproductive output for the colony, continually laying eggs in order to keep up a steady production of new termites, while the workers are sterile and undertake all of the main chores of colony life. Some termites have a third caste, soldiers, who are similarly sterile and modified solely for defense—sometimes so dramatically that they are unable to feed themselves or care for themselves. (See illustration on page 58). Soldier termites vary widely and employ many methods to defend their homes. Typically, soldiers have large heads accommodating powerful muscles; these control fearsome mandibles that are used to bite at and slice through invaders. The heads of soldiers may also be specialized into bulbous nozzles that spray noxious glues meant to entangle invading ants.

  The finely etched lines produced by lithography beautifully capture the intricate details of the lovely wings of the Central American giant cave cockroach (Blaberus giganteus), females of which can reach 4 inches (10 centimeters) in length. From Jean Victor Audouin, Histoire naturelle des insectes (1834).

  Like wood roaches, termites harbor symbiotic microorganisms in their guts that aid the breakdown of the tough cellulose of the plants consumed. This dietary specialization along with their efficient and sometimes large colonies—which can number upward of one million workers—have made them one of the more ubiquitous of all insects—although less than 13 percent of all species are injurious to crops or structures, and less than 4 percent are considered serious pests.

  Roaches (order Blattaria), such as this variety of species from Madagascar, are slightly more diverse than placental mammals and number over 4,500 species worldwide. From Saussure, Histoire physique…de Madagascar, Orthoptères.

  PSOCODEA, THYSANOPTERA, AND HEMIPTERA

  The order of lice, Psocodea (from the Greek psokos, meaning “rubbed” or “gnawed”), comprises two groups that were once placed in related but separate orders: the bark lice (or book lice) and the true lice. Bark lice can be found virtually everywhere—under bark or on foliage, beneath stones or in caves, and even in our very homes. In fact, they are not uncommon in libraries, where they can do considerable damage to books by chewing away on fragile pages—hence their other moniker, “book lice.” In general, they feed on spores, plant tissue, algae, and lichens, but sometimes also on other insects. The nearly fifty-seven hundred species are largely solitary animals, although some can be found in abundant aggregations. Bark lice wings are fairly simple, with a reduced number of veins relative to most other winged insects and usually held over the body like a tent when not in use.

  Perhaps no insects are more detested than lice. Although entirely wingless, true lice are descended from winged ancestors among the Psocodea (bark lice). Here, British canine lice from Henry Denny’s Monographia Anoplurorum Britanniae; or, An Essay on the British Species of Parasitic Insects Belonging to the Order of Anoplura of Leach (1842).

  The more familiar and certainly detested members of this order are the true lice, the very paragons of parasitism. Like their nonparasitic relatives, there are approximately five thousand species of true lice, all of which feed on the blood of bird or mammal hosts. Species usually have a high degree of specialization for feeding upon a given host and cannot survive on alternative hosts, although there are certainly several exceptions to this general rule. All true lice are wingless, having lost their wings during their evolution to parasitism, and while some retain the chewing mandibles of their bark lice relatives, one specialized group, the Anoplura (sucking lice), have a small beak used to pierce their host. Although lice are quite diverse, only three species feed on humans and these are aptly named as the head louse, body louse, and pubic louse, leaving little question as to where each might be found. Lice live out their entire lives on their hosts, cementing their eggs, called nits, to the feathers or hair of the host’s body. In the case of human lice, this gives parents and children ample time to bond while the former carefully comb out the nits after the latter have been sent home early from school.

  Aphids, whiteflies, and scale insects, seemingly outliers among the insect order Hemiptera, are closely related to cicadas, planthoppers, and true bugs. Also included here is a single thrips (dark brown insect, upper right), representing the closely related order Thysanoptera. From Cuvier, Le règne animal distribué d’après son organisation.

  Related to the lice are two orders of largely plant-feeding insects, both of which have stylet-like mouthparts used to pierce and suck fluids. The thrips (a name that is both singular and plural, like sheep), or Thysanoptera, include fifty-eight hundred species of tiny insects specialized to feed on fungi, pollen, and plant tissue. Their wings are slender and fringed with elongate setae, and it is this feature that serves as the basis for their ordinal name, with thysanos meaning “fringe” or “tassel.” Usually less than a millimeter in length, thrips can occur in dense populations, and some are pests of vegetable crops or ornamental flowers. Sometimes it is not so much the damage this type of thrips will do through its own feeding that makes it a pest, but the plant diseases it can unwittingly spread—such as viruses that produce large necrotic areas on tomatoes or other crops. Despite this nuisance, other thrips are important pollinators and are relied upon by flowers such as those of the heath family, Ericaceae. Many thrips induce gall tissues (abnormal growths) in plants during feeding or while injecting their eggs in plants, and in one family these galls serve as the nests for small societies, complete with castes like those of termites—queens, sterile workers, and even soldiers.

  Cicadas, planthoppers, and lantern bugs together form one of the major diversifications of species among the largely herbivorous insect order Hemiptera (see next page). From Guérin-Méneville, Iconographie du règne animal de G. Cuvier.

  Lantern bugs (family Fulgoridae) may at times be mistaken for colorful moths. They are named for the frequent occurrence of hollow projections on their heads, which artists before the nineteenth century erroneously believed emitted light. From Westwood, Cabinet of Oriental Entomology.

  The other piercing-s
ucking order, Hemiptera, is the first of the truly megadiverse lineages, with slightly over one hundred thousand species. These are the aphids, whiteflies, scale insects, planthoppers, cicadas, lantern bugs, and true bugs. While bug is an oft-employed pejorative for any insect, it really refers to a diverse subset of Hemiptera, such as stink bugs, shield bugs, seed bugs, bed bugs, water striders, and many more. The name Hemiptera is derived from the Greek hémisus, meaning “half,” due to the partially hardened forewings of the true bugs, where only the half farthest from the body is membranous. This is a lineage of seemingly disparate animals, but all share a characteristic form of rostrum (snoutlike projection) that has piercing stylets. Many Hemiptera—including the aphids, whiteflies, planthoppers, cicadas, and lantern bugs, many of which are major agricultural pests—use their piercing rostrum to access the nutritious fluids of plants. Conversely, most of the true bugs have become predators, although here and there some, such as palm bugs and the intricately beautiful lace bugs, that have reverted to a plant-feeding lifestyle. The predatory true bugs largely prey on other insects, but some have evolved to feed on bird or mammal blood. These include bed bugs and kissing bugs, the latter of which are notorious for the transmission of the tropical parasitic Chagas disease.

  WINGS WERE, WITHOUT A DOUBT, one of the most significant factors in the success of insects. Flight certainly poised the many lineages to flourish and occupy diverse habitats, which along with feeding and other specializations led to the vast variety of winged insects. Wings alone, however, cannot entirely account for insect success, and it is a popular misconception that one evolutionary novelty is responsible for diversification, although such over­simpl­ifica­tions do make for good soundbites. In reality, it is the coming together of many evolutionary factors—an interaction of several key traits in combination with the wider environment, stochastic events, and the concomitant evolution of other lineages—that breeds diversity. For insect diversity, a significant change subsequent to the origin of wings metamorphosed a large group of flying insects into evolutionary superpowers, with true hegemony over our world.

  True bugs, such as these species of Coreidae, can have a spiky or lacelike appearance owing to expansions of the hardened wings and thorax. Here, golden egg bugs, so named because they carry their orange eggs on their back. From Westwood, Arcana Entomologica.

  Detail of a larva, pupa, and adult moth, from Maria Sibylla Merian’s Histoire des insectes de l’Europe (1730) (also see page 80).

  “Nothing in a single-frame picture of a caterpillar tells you it is going to transform into a butterfly.”

  —R. Buckminster Fuller Cosmography, 1992

  As children, we are essentially miniaturized versions of our later selves. The same is true for many groups of insects, and the immature forms of almost all of the orders discussed in the preceding chapters resemble their corresponding adult forms to a large degree. After hatching from the egg, nymphs become progressively larger at each molt, ultimately reaching sexual maturity and gaining their fully functional wings at the final molt to adulthood. This kind of development is termed hemimetabolous (from the Greek hemi, meaning “half,” and metabolos, or “changeable”); it is sometimes referred to as incomplete metamorphosis, as the changes between molts are slight and the nymphs typically have lives similar to their mature forms. For example, the nymphs of grasshoppers have the same diet and habits as the adults, but they are smaller, have nonfunctional wing pads, and have not yet become reproductively capable. The most diverse groups of winged insects, however, have a fundamentally different mode of development, beginning life as a larva—whether as a caterpillar (moths and butterflies), grub (beetles), or maggot (flies)—and undergoing a more dramatic form of transformation from a larva into a pupa and then ultimately into an adult. By contrast, a hemimetabolous insect, such as the aforementioned grasshopper, has no larval or pupal stage prior to adulthood, only nymphal.

  Complete metamorphosis, properly termed holometabolous (in Greek, hólos means “complete” or “total”), stands in stark contrast to hemimetabolous metamorphosis. Holometabolous insects emerge from the egg as a larva, which grows through a series of molts prior to undergoing a developmental shift into a largely quiescent stage known as the pupa—sometimes within a cocoon, such as those made famous by silkworms—before taking the stage as an adult. The larva differs greatly from the adult, and in most instances these two stages are lived by the one individual as entirely distinct modes of existence. Larvae frequently live in different habitats from adults, feed on different foods, and require different conditions in order to succeed. So different are larvae from their adults that for the greater part of human history, it was mistakenly believed that larval and adult insects had nothing to do with one another and were completely separate animals. For centuries, observers failed to see whence adult holometabolous insects sprang, or if they did connect a larva with its associated adult, then it was supposed that some fantastical transformation had taken place such that a wholly new animal appeared.

  Developmental stages of a lucanid stag beetle: eggs (bottom left), larvae (bottom right and center left), pupae (center right and bottom center), and pupal chamber (top). From August Johann Rösel von Rosenhof, De natuurlyke historie der insecten (1764–1768).

  One of the greatest contributions to understanding insect metamorphosis came from the great illuminated folios produced by Merian during her time living in Suriname. Top to bottom: the longhorn beetle (Macrodontia cervicornis), the large tropical weevil (Rhynchophorus palmarum), and a common orchid bee (Eulaema cingulata). From Merian’s Surinaemsche insecten (1719).

  Before the work of Jan Swammerdam and Maria Sibylla Merian, the larva, pupa, and adult moth (illustrated here) were considered to be distinct animals rather than continuous stages in the life of a single species. From Merian, Histoire des insectes de l’Europe.

  Any number of bizarre hypotheses filtered through the ages. It was believed that most insects did not mate, but generated directly from rotting material, either flesh or vegetation. One famous hypothesis was the notion that bees were born from the rotten carcasses of oxen, a misconception that persisted for over a thousand years before it was dispelled. In his Etymologiae, Isidore of Seville (see pages 16-17) taught that honey bee workers were formed from rotting oxen, while hornets sprang from decomposing horses, drone bees (males) issued from decaying mules, and some wasps emanated from putrid asses. Interestingly, the details of metamorphosis as we understand it today were principally elaborated by the work of two individuals in the late seventeenth century who were both based in Amsterdam: Jan Swammerdam (1637–1680), a Dutch anatomist (see pages 82-83), and Maria Sibylla Merian (1647–1717), a native German illustrator and naturalist (see pages 94-96). Both devoted their talents to the mysteries of insect development, and each published accounts that demonstrated the continuity in life of holometabolous insects—from egg, to larva, to pupa, and finally adult.

  Despite Fuller’s quote at the beginning of this chapter, within developing larvae are islands of tissue that represent the primordia of those structures that will be unique to the adult, such as wings, antennae, and reproductive organs. These clusters of cells, called imaginal discs, were discovered by Swammerdam, who correctly interpreted their role in metamorphic development.

  Holometabolous insects account for the real bulk of insect diversity, with approximately 85 percent of all insects undergoing complete metamorphosis. In fact, the ascendance of insects is partly credited to this reality by which complete metamorphosis allows larvae and adults to experience such different lives. All of the taxonomic orders discussed in the remainder of this chapter belong to this major grouping, and they are collectively known as the Holometabola, a less-than-subtle allusion to their characteristic mode of development. Despite the considerable success of the Holometabola, not all holo-metabolous insect orders are rich in species, at least by insectan standards. Indeed, while four particular groups number over one hundred thousand species each, the oth
er groups include ten thousand species or less in all.

  MEGALOPTERA, RAPHIDIOPTERA, AND NEUROPTERA

  Three groups form a tight set of related orders among insects that undergo complete metamorphosis. Most commonly called dobsonflies, snakeflies, and lacewings—or Megaloptera, Raphidioptera, and Neuroptera—these are minor groups among holometabolous insects, and they are collectively relicts of a lineage going back over 280 million years but that was dwindling by 50 million years ago. Today, these three groups of insects weigh in at about 380, 250, and 5,800 species respectively—meager numbers relative to the likes of beetles, moths, and flies. All three have greatly innervated, membranous wings to one degree or another, and while entomologists have debated relationships among the principal lineages of insects, the evolutionary affinity among these has almost never been in any serious doubt.

  Representative species of the closely related insect orders Megaloptera, Raphidioptera, and Neuroptera. Top to bottom: Megaloptera—the alderfly (Sialis lutaria) and the fishfly (Chauliodes pectinicornis); Raphidioptera—the snakefly (Raphidia ophiopsis); and Neuroptera—the mantis lacewing (Mantispa styriaca). From Georges Cuvier, Le règne animal distribué d’après son organisation (1836–1849).

  The predaceous larvae of giant dobsonflies, such as Corydalus cornutus (adult shown here), are called hellgrammites and are a popular bait among anglers. From Cuvier, Le règne animal…

  Snakeflies (order Raphidioptera) take their common name from their serpentine necks that give them an archaic, almost antediluvian-like, appearance. Detail from M. Olivier, Encyclopédie méthodique. Histoire naturelle. Insectes. (1811).