Mimetic species are known to have existed 150 million years ago, demonstrating the considerable antiquity of their mode of life. With this kind of longevity, they must be doing something right. The masters of mimesis, however, are the stick and leaf insects (see pages 63-66). This group of insects has gone all-in when it comes to deception—a commitment that runs through nearly every life stage. Most stick insects, as their name implies, resemble twigs, branches, or sticks of some kind, having the slender forms and colors necessary to pull off such mimesis. Their legs are typically long and slender, also resembling sticks, but rather than use their legs to stand, the insect often presses itself against the plant, pulling its legs alongside its body, stretching them in front and behind. Broader species resembling leaves often sit or even dangle amid foliage, perfectly disappearing from view among the “other” leaves. Some species change colors during different stages of life, and in each stage they are better colored in order to blend in with their host plant at the time—going so far as to match the stages in which leaves discolor and die.
Stick insects, such as these Bacteria virgea, are paragons of camouflage. Although we are more familiar with the word bacteria referring to single-celled microbes, the Greek baktría refers to a “staff,” “rod,” or “stick,” and the Greek term was first adopted as a scientific name for a genus of the order Phasmatodea. From Westwood, Cabinet of Oriental Entomology.
The stunning leaf insects of Southern Asia and Australia are specialized Phasmatodea who evolved from generally sticklike ancestors; they usually remain still amid foliage while they feed, like their stick insect relatives. At top right, a male nymph of the diminutive Phyllium donovani, alongside a male nymph (top left) and adult female (bottom) of Phyllium siccifolium. From Edward Donovan, Natural History of the Insects of India (1838).
Yet other stick insects correspond perfectly with certain mosses and may be mottled to resemble lichens, all depending on their particular habitat. During the day, these insects do little, often remaining motionless. Nearly all, however, exhibit some form of cryptic behavior, and they will even gently sway side to side, attempting to imitate the action of the breeze on the actual twigs, thereby enhancing their disguise. If startled or detected, most will go cataleptic, freezing and dropping to the ground where, lying motionless, they usually blend in with the other plant litter on the forest floor. Should this fail to deter an attacker, then all stick insects have a repugnant gland at the front of their thorax from which they can exude a noxious chemical that can be quite effective as a last resort. The chemical spray of some species in the genus Anisomorpha can potentially induce blindness should the insect successfully squirt it into the eyes of its foe. There are a few species of stick insects that are not cryptically colored at all, but instead they sport bright colors and patterns. These insects don’t need to hide, because they have these particularly powerful chemical defenses and choose to advertise this fact as a warning through their coloration. Thus, if you can easily see the stick insect, it is best not to bother it.
As if this suite of camouflaging color, behavior, and mimetic form were not enough, mimesis even extends to the eggs of stick and leaf insects. Instead of cream-colored ovate, spherical, or sausage-shaped eggs—the most common color and shapes of insect eggs—the eggs of stick and leaf insects actually resemble the seeds of particular plants. The mimetic form of the eggs is remarkable and so specific that one can usually identify the species of insect simply from the egg alone. Some adult females will lay individual eggs, usually flicking them from the tip of her short ovipositor, scattering her offspring amid the leaf litter on the ground. Others may glue their eggs to leaves or branches. In species of the western North American genus Timema, the female goes so far as to ingest soil that is then coated over the eggs as they are laid. In certain stick insects—such as Macleay’s Spectre, Extatosoma tiaratum, a giant spiny stick insect from Australia that can reach up to 8 inches (20.3 centimeters) in length and is usually heavier than a hamster— the eggs have a knoblike structure at one end called the capitulum. The eggs are tossed to the forest floor by the female, where foraging ants of the genus Leptomyrmex find and collect them, bringing them back to their nests. The ants will consume the capitulum while leaving the rest of the egg unharmed, and in this way the egg is afforded the general protection of the colony, while the ants get nourished—a spectacular mutualism. Upon hatching, the first-stage nymph has a form and coloration that generally resembles that of the ant species. Uncharacteristic for a stick insect, and even departing from what the animal will do during the remainder of its life, the nymph is agile and quick, rapidly exiting the colony before being detected and moving up into the surrounding forest, where upon their next molt they begin to take on more of the stick insect form.
A male (left) and female of the common walkingstick of North America (Diapheromera femorata), painted against twigs to highlight their resemblance with their surroundings. From Thomas Say, American Entomology (1828).
Mimicry can be used by predators just as much as it is by prey. As ambush predators, praying mantises employ numerous forms of camouflage and mimicry in order to steal closer to their victims. From top to bottom: the mantis Empusa pennicornis, the Indian rose mantis (Gongylus gongylodes), and the common walkingstick (Diapheromera femorata); unlike the aforementioned mantises, the stick insect is not a predator. From Dru Drury, Illustrations of Exotic Entomology (1837).
Although stick insects go to extremes with their disguises, many other insects also exhibit mimesis. Thorn bugs (opposite) come in a variety of shapes, giving the viewer the impression that they are nothing more than a prickly part of the plant upon which they peacefully suck fluids. Meanwhile, flower mantises have forms that are mimetic of the flowers upon which they patiently await their prey, snatching floral visitors who fail to notice their presence. One of the most remarkable is Hymenopus coronatus, the orchid mantis. This mantis, from the rain forests of Southeast Asia, has flat extensions on its legs that resemble petals and an overall pink-and-white coloration similar to that of several orchids in the region. The resemblance is so striking and so complete that the mantis need not rest on the orchid in order to attract prey. Orchid-visiting pollinators actually approach the mantis itself, thinking it to be the flower, and are greeted with an alarming surprise upon getting too close.
Butterflies, moths, and katydids also widely exhibit leaf mimesis, usually with their forewings broadened to resemble the shape of a specific leaf, with colors ranging from green to brown so that they may appear fresh or dried, depending on the habitat. Others have mixed colors, with an overall green surface and patches of brown near the tip, suggesting the form of a leaf that has begun the process of decay. In some katydids, the mimetic form is enhanced by a strong fold line running along the length of the wing, resembling the midrib of the leaf (see page 176). In others, the margins of the wing toward the apex may even be scalloped, looking as if some herbivore has recently taken a bite from the leaf!
While most katydids resemble leaves or other plants, there are some who have evolved a different approach and are mimetic of other animals, particularly of models that most predators might choose to avoid. For example, spider wasps are large, robust wasps feared for their powerful stings, which are considered to be among the most painful of any wasp. Many of the spider wasps have an overall jet black color, with contrasting orange wings and sometimes orange tips to the antennae. The pattern of color is a form of warning coloration (called aposematism) and is a well-understood advertisement that these wasps are to be reckoned with. Perhaps not surprisingly then, several other groups have evolved a similar pattern of coloration in the hopes of conveying the same warning signal to any predator that might glance their way. Thus, there are large katydids, similar in size to some of the large spider wasps, that are black with narrow, orange wings. They even go so far as to have the tip halves of their antennae similarly orange, just like the wasps. To a quick glance, they certainly look like a spider wasp at rest.
While the katydid is no real threat to anything other than the plant it will consume, the wasp is definitely venomous. Nonetheless, the katydid gains some degree of protection from vertebrate predators who have learned through unpleasant encounters with the spider wasps to avoid that particular color pattern.
This kind of disguise is considered outright mimicry, and more specifically a strategy known as Batesian mimicry. Batesian mimicry is named for English naturalist Henry Walter Bates (1825–1892), who first described the phenomenon from butterflies he observed during an eleven-year sojourn in Brazil. In Batesian mimicry, the model species is unpalatable to predators, either possessing some toxicity or being capable of aggressive defense, like the aforementioned wasps. The model species advertises its danger through aposematic coloration, usually bright or distinctive patterns that are easily learned by predators such that they avoid individuals with such markings. The mimic, however, is perfectly delectable and lacks its own toxic defenses. By assuming the same pattern as the unpalatable model, though, the mimic dupes the predator, who fails to distinguish the mimic from its toxic archetype. These associations can be large, with many species converging on the same color pattern and widely benefiting from the protection it bestows. Clearwing moths will sometimes evolve color patterns similar to those of stinging social wasps, while the larva of the hawkmoth Hemeroplanes triptolemus is colored so as to perfectly match the head of a small snake, complete with an expanded end that looks like a serpent’s head and blackened patches fringed by white spots to look like dark eyes with areas of reflected light. Add to this a cryptic behavior of short forward thrusts and the mimicry is complete, convincing birds to avoid a possible strike from this “dangerous” serpent.
Thorn bugs (family Membracidae) come in as many shapes and varieties as there are thorns on the plants in which they hide, such as this array of species from Central America. From Biologia Centrali-Americana. Insecta. Rhynchota. Hemiptera-Homoptera. (1881–1909).
Katydids can often have large wings that resemble leaves, such as the sizable Siliquofera grandis of New Guinea (center). From Jules-Sébastien-César Dumont d’Urville, Voyage au pôle Sud et dans l’Océanie sur les corvettes l’Astrolabe et la Zélée (1842–1854).
Sometimes both the mimic and the model, however, have their own chemical defenses and neither is palatable to a predator. Yet, despite each having its own defense, they mimic each other in advertising colors. Such mimicry functions differently from Batesian mimicry and is called Müllerian mimicry, again named for its discoverer, the German biologist Johann F. T. “Fritz” Müller (1821–1897), who, like Bates, spent many years living and observing nature in Brazil. In such mimicry complexes, the convergence on a common warning coloration provides a predator with a single pattern to learn, rather than many. Müller demonstrated this through a detailed study of butterflies in the genus Heliconius. Despite the fame of heliconiine mimicry among evolutionary biologists, perhaps the most familiar species involved in Müllerian mimicry are the monarch and viceroy butterflies, Danaus plexippus and Limenitis archippus respectively. Like Müller’s heliconiine butterflies, the monarch and viceroy have converged on a common warning pattern, with their bird predators learning to avoid both as each butterfly species is noxious. For a long while, entomologists incorrectly believed viceroys were not noxious to birds. In fact, the viceroy and monarch together were mistakenly considered as a textbook example of Batesian mimicry before it was recently discovered that viceroys are themselves unpalatable to birds. Thus, monarchs and viceroys represent a case of Müllerian mimicry. Rather than duping a would-be predator, the viceroy is in fact legitimately advertising itself as an unsuitable meal.
Day-flying moths of the family Noctuidae, shown with unrelated species exhibiting similar patterns of coloration as well as related species with disparate colors. Clockwise from top: Episteme westwoodi, Exsula victrix, Exsula dentatrix, Scrobigera amatrix, and Episteme bellatrix. From Westwood, Cabinet of Oriental Entomology.
WHEN WE PRACTICE TO DECEIVE
Henry Walter Bates was born in Leicester, England, in 1825, and received the usual education of someone from his middle-class background. At thirteen he became an apprentice to a manufacturer of hosiery. During his spare time, however, he explored the forests and collected insects. Bates eventually met fellow nature-lover Alfred Russel Wallace (1823–1913), who was teaching at the nearby Leicester Collegiate School, and together the two men would collect and reflect. Both dreamed of being explorers and read William H. Edwards’s (1822–1909) A Voyage up the River Amazon (1847), which piqued their interest in doing the same.
Intent on making a contribution toward explaining biological diversity, the two devised a plan to explore the Amazon themselves, which involved shipping back specimens to be auctioned off to support their efforts. They even gathered particular wish lists from museums and sponsors. Together they sailed from England in April 1848, reaching the ports of southern Brazil just before June. They set themselves to collecting, at first together, but then each went in a different direction, covering different territories. Wallace headed back in 1852, but the ship caught fire and his invaluable collections were lost. He and the others aboard spent ten days adrift in a small boat before being rescued. Undeterred, Wallace headed next to the Malay Archipelago in 1854, not returning to England until 1862. While on these islands between the Pacific and Indian Oceans, he arrived at the same conclusions as Darwin regarding the origin of species. Wallace wrote to Darwin about his idea, and the two coauthored the first paper on evolution, which they did just prior to the release of the latter’s explosive book.
Portrait of Henry Walter Bates, ca. 1880.
Meanwhile, Bates had greater success in Brazil than his compatriot, shipping back crates of specimens representing nearly fifteen thousand species, more than half of which were new to science. Bates continued collecting and making observations in Brazil, only returning home eleven years later when his health became compromised. He lived the remainder of his life in London, passing away there in 1892.
Bates wrote of his life in the jungle in The Naturalist on the River Amazons, a book he was encouraged to prepare by Darwin, who praised it highly when it appeared in 1863. Bates was a strong proponent for Darwin’s theory of evolution, his vast experience with species in the tropics having given him empirical evidence as to its veracity. Most importantly, the mechanism of evolution by natural selection explained perfectly a phenomenon he had uncovered while in Brazil. Bates had found complexes of butterflies whereby different species had virtually identical patterns of coloration. Sometimes the similarities were so strong that even under close inspection one could be fooled. He had discovered that some species advertised themselves to predators, signaling their toxicity as a defense, while hypothesizing that the others lacked such defenses but had safety conferred upon them owing to their resemblance with the other. He realized that natural selection resulting from the differential survival of color variants could, over time, produce mimics—species that were fully tasty to a hungry bird but colorfully disguised to prevent their easy distinction by a predator, or even some entomologists! Today, this kind of disguise is known as Batesian mimicry.
Bates published his hypothesis of mimicry in butterflies in 1861, and today, this mechanism of “false-advertising” coloration as an antipredator defense is known to be widespread across the animal kingdom. Given that insects are so ancient, so varied, and such champions of evolutionary success, it is perhaps not surprising that they have been a frequent source for informing us of evolution’s underlying principles, even when the very process itself is meant to deceive.
During his exploration through the Amazonian region, Henry Bates was taken with the convergent patterns of coloration among unrelated butterflies within a common area, and he explored how such patterns could have evolved. For example, the pair at lower left are tiger mimic white (Dismorphia amphione, second from bottom) and disturbed tigerwing (Mechanitis polymnia, bottom), while the pair at lower right ar
e clearwing white (Patia orise, second from bottom) and giant glasswing (Methona confusa, bottom). From his paper “Contributions to an Insect Fauna of the Amazon Valley,” in the Transactions of the Linnean Society (1862).
Detail of an illustration of various orchid bees from Jules Rothschild, ed., Musée entomologique illustré: histoire naturelle iconographique des insectes (1876–1878) (also see page 186).
“To make a prairie it takes a clover and one bee,—
One clover, and a bee,
And revery.”
—Emily Dickinson
Complete Poems, 1924
Visit any meadow in bloom and you will invariably find butterflies flitting about and bees abuzz. There is perhaps no more familiar or intimate relationship in the biological world than that between insects and plants. Long before there were flowers, insects spent eons perfecting ways in which to feed on plant tissues—everything from the roots to the shoots, the seeds to the leaves. Plants evolved mechanisms by which to deter these herbivores—noxious chemicals, sticky resins, tougher and more abrasive tissues, even mimicry—and insects responded in kind. The numerous varieties of mouthpart specializations among insects reflect this evolutionary back and forth. Around 140 million years ago, the first flowers appeared, and although it would take another 50 million years before they would begin to achieve considerable dominance, eventually flowering plants would become ubiquitous. The ecological rise of flowering plants was partly fueled by their marriage to insects, and many groups of insects owe their own success to their floral hosts.
Innumerable Insects Page 17