Extreme Insects

Introduction

Insects are the most extreme organisms on Earth, and despite their diminutive stature, they wield inordinate power. With the exception of the polar ice caps, every terrestrial ecosystem on Earth is colonised by insects and to some extent controlled by them (and they have made inroads onto the open ocean, too). They dominate the middle ground of virtually every terrestrial food chain or food web.

Insects are extreme in numbers. A single leaf-cutter ant nest, the size of a large camper van buried in the soil, may contain 8 million individuals working together as a single giant super-organism. In the tropical rainforests, termites can reach densities of 10,000 per square metre, a higher density of animal mass than in the largest wildebeest herds of the Serengeti. To produce such numbers, insects are extreme in their fecundity. Egg loads can be counted in thousands and generation times in days. If conditions are right, plagues of biblical proportions can appear as if spontaneously.

Insects are extreme in diversity. Even the experts cannot agree whether there are 3 million different species of insect on the Earth, 10 million, 30 million or 80 million. Compare that to the mere 5,400 known species of mammal. About four-fifths of all the animals yet discovered on our planet are insects: that’s over 1 million species at the last count. And there are many times that number out there awaiting discovery.

Insects are extreme in form. Evolved into the most peculiar shapes and colours, with strange structures and beautiful patterns, even the smallest of these wonderful creatures is magnificent under the microscope. Each has adapted to solve the extreme pressures that arise in the struggle to survive in a world that is dangerous, competitive and unforgiving.

Extreme Insects is divided into three chapters, exploring the nature of the insect universe and looking at some of the most extraordinary creatures in existence.

Extreme Form. In addition to the biggest, smallest, and largest wingspan, we take a look at extreme shapes: spikiest, furriest, shiniest, flattest. Why have such forms evolved? What benefit do they give to the insects that possess them?

Extreme Evolution. Some parts of insect anatomy can appear completely alien to the human eye. They have evolved to allow their possessors a special tool, weapon or means of escape. They have allowed certain insects to survive in extremely difficult or dangerous circumstances. Insects are complex creatures that interact with each other, with their food and with enemies who see them as food. And they get up to some very strange things. They seem to be dancing, skulking or hiding. They brave danger or run like cowards. Some nurture and some murder; some commit suicide. They may appear very clever or extremely dim. Some steal and some give gifts. What is the biological explanation behind these apparently odd behaviours?

Extreme Impact. Humans now reckon themselves to be the dominant life form on Earth, but we have been around for only a few hundred thousand years. Insects were here over 300 million years earlier. Humans, the mere junior upstarts, now come into conflict with a much older and better-established group of organisms. And despite our modern sophistication, we cannot escape such tenacious and apparently determined animals. They invade our fields, our houses and even our bodies. Some we can tame for our own uses, but with others we are still at war.

Insects are both awful and awe-inspiring, certainly worthy of our respect and our study. They give us a window on the natural world through which we can see, and attempt to understand, the environment in which we live, indeed of which we are an integral part. The huge numbers of insects, and their depredations on human food and health, are sometimes bemoaned. In reality, they form a vast biomass, and it is a wasteful shame that insects form an insignificant part of the human diet. We may not eat them very often, but insects offer a more philosophical sustenance – food for thought. In their study, there is a veritable feast for the mind.

Richard Jones

London, September 2009

Extreme Form

Oldest insect • Biggest insect • Longest insect • Whitest insect • Shiniest insect • Slimiest insect • Biggest blockhead • Most sexually dimorphic insect • Most mixed-up sexuality • Most bloated insect • Most seasonally dimorphic insect • Highest number of wings • Flattest insect • Most back-to-front insect • Longest ovipositor • Widest head • Brightest light generation • Most variable colour pattern • Bloodiest insect • Most beautiful insect • Longest head • Most streamlined insect • Loudest insect • Best hoverer • Ugliest insect • Largest jaws • Largest wingspan • Best camouflage • Most transparent wings • Hairiest legs • Snappiest jaws • Prettiest eyes • Most elegant eggs • Largest eye markings • Lightest footstep • Furriest insect • Most poisonous insect • Most heavily armoured insect • Longest wing tails • Best burrower • Smallest insect • Heaviest insect • Fastest flier • Fastest runner • Longest tongue • Smelliest insect • Most subterranean insect • Fastest wing-beat • Smallest egg • Largest egg • Spikiest insect • Biggest feet • Largest claws

Oldest insect

Most insects are very small, very delicate and very edible, so the fossil record they have left behind is extremely poor. The majority end up as prey for other animals, bitten, chewed and digested away. Where their remains are not eaten, there are no large bones to be preserved, and even the toughest of insect shells are made of highly biodegradable compounds. As a result, it takes some pretty special circumstances for insect fossils to form, and they are thoroughly scrutinised when found. Or at least they should be.

Until recently, the oldest acknowledged insect-like fossil was an ancient relative of modern springtails. These are wingless soft-bodied creatures that are not now classed as insects but as a sister group within the subphylum Hexapoda (six-legged arthropods). The fossil was found in 1919 by the Reverend W. Cran in the old red sandstone deposits (also called the Rhynie cherts) at Rhynie in Aberdeenshire, Scotland, which date from 407-396 million years ago. It was finally described in 1926 by three paleontologists, S. Hirst, S. Maulik and D.J. Scourfield, who aptly named it Rhyniella praecursor.

Two years later the rock sample was re-examined by the Australian entomologist Robin Tillyard. He identified what had been thought to be a broken fragment of a Rhyniella head capsule as belonging to a different creature, which he named Rhyniognatha hirsti.

The specimen lay untouched in the Natural History Museum, London, until 2004, when it was examined again by evolutionary entomologists Michael Engel and David Grimaldi. Using modern microscopes, they were able to see the fossilised jaws in much greater detail, and made an astonishing discovery. The shape of the jaws – toothed, broadly triangular, with two bulges where they articulated against other sections of the mouthparts – showed that they were not from some ancient springtail, but from a true insect and probably one with wings.

The Rhynie cherts formed in an area of hot springs and active geysers, which contained fluids rich in dissolved silica. As the water cooled the silica crystallised out of the water to form the fossils for which the area is now renowned. Hot water is very damaging to insect wings and other soft tissues, so it is not surprising that only the tough jaws of this insect have been preserved.

Biggest insect

In 1771, the Swedish naturalist Carolus Linnaeus described a giant beetle, and named it using his new scheme of binomial (two names) nomenclature: one name for the genus (Titanus) and one name for the species (giganteus). This name could not have been more apt for an insect that regularly reaches 17 cm (6.7 in) long. Linnaeus never saw the beetle itself. He coined the name after seeing an engraving of it in an encyclopedia.

The reason Linnaeus never saw one is that this was one of the rarest insects then known. During the 18th century, specimens were occasionally washed up dead on the shores of the Rio Negro, near Manaos in Brazil. The first living beetles were not found until 1958, when they were attracted to the street lights which were newly installed in the towns and villages in the area. Its early stages and life history are still unknown, but similar species have maggot-like larvae that feed in rotten logs.

There is still some doubt as to whether Titanus giganteus truly is the ‘largest’ insect. Few reliable measurements of living specimens have been taken. There is also little data regarding its weight – usually regarded as the key indicator of size by record-measuring organisations. As a result, four other beetles are contenders for the title. These are the shorter but stouter ‘Elephant’ beetles from South America – Megasoma actaeon (13.5 cm) and M. elephas (13.7 cm) – and the Goliath beetles from Africa – Goliathus regius (11 cm) and G. goliathus (11 cm).

Longest insect

One of the best ways to avoid being eaten is to hide, and one of the best ways to hide is to blend in with the surroundings using camouflage. Stick insects (also called walking sticks) have taken this to an extreme, and their pencil-thin bodies and pin-thin legs perfectly resemble the twigs through which they climb. So good at hiding have they become that some stick insects have acquired a trait that is a common evolutionary result of having no (or virtually no) predators – they have become very large.

For nearly 100 years, the record for the longest insect in the world was held by a specimen of a giant stick insect from Borneo, Phobaeticus (formerly Pharnacia) kirbyi. Its body was 328 mm (12.9 in) long, and from the tip of the outstretched front leg to the end of the back leg it measured 499 mm (19.6 in). This specimen had long been misidentified as the closely related P. serratipes, and it was only shortly after its true identity was established in 1995 that another huge stick insect was discovered. Ironically, this time it was a specimen of the true P. serratipes, found in Malaysia. It had a total length of 555 mm (21.9 in), although its body alone was slightly shorter than the famous P. kirbyi specimen.

Measuring lanky insects is fraught with difficulties, and this could have been the point at which some controversy arose. Most size measurements for insects deliberately ignore legs, antennae, tails and snouts because they vary tremendously within a population, especially between male and female of the same species. It has long been known that the leg lengths of stick insects vary, even when measured on different sides of the same specimen. However, the matter was settled in October 2008, with the description of a new species of stick insect from the Malaysian state of Sabah on the island of Borneo. Phobaeticus chani was named after the entomologist Datuk Chan Chew Lun, who donated the largest of three specimens, found by a local collector, to the Natural History Museum in London. With a body length of 357 mm (14 in) and a total length of 566 mm (22.3 in), it takes the record no matter which way it is measured.

Whitest insect

White is not a common insect colour, as it makes an insect stand out to predators in a natural world dominated by browns and greens. Perhaps the best-known white insects are cabbage whites (Pieris species). Like other butterflies, they use their colour patterns to recognise each other when mating. But they fade into insignificance against ghost beetles in the genus Cyphochilus.

Ghost beetles are found throughout Southeast Asia, where they are sometimes regarded as a pest in sugar cane plantations. Ghost beetle larvae feed in fungi, and the beetles’ whiteness is thought to be a camouflage against this rare white foodstuff. On close examination, the whiteness is caused not by the beetle’s exoskeleton (its tough outer shelllike body), which is dark brown and almost black, but by a dusty coating of pure white overlapping scales, which cover its body, head and legs. Each scale is minute, measuring only 250 by 100 μm and just 5 μm thick (a μm or micrometre is one thousandth of a millimetre).

The scales were first studied by Pete Vukusic, an optical physicist at Exeter University in the UK, who discovered that the beetles’ whiteness is caused by a random network of tiny filaments, 0.25 μm in diameter, inside the scale. The random arrangement of the filaments means that the different rainbow colours in natural white light are scattered simultaneously, equally and highly efficiently, with no single colour predominating. The beetles are among the whitest objects found in nature – much whiter than teeth and milk.

Shiniest insect

Insect colours serve many purposes. Greens and browns act as camouflage against living and dead leaves, tree trunks, branches and twigs. Bright yellow, orange and red, often marked with black, warn that an individual is poisonous or might sting. But the brightest and most spectacular colours do neither. Metallic glints of bronze, blue, green, red and violet occur in many beetles, bees, wasps, flies and, of course, butterflies (see page 48). The most astonishing of these are the brilliantly shining golden chafers, Plusiotis species, of Central and South America.

Metallic sheens are not colours in the conventional sense of a pigment or colourant on the surface of the animal. The red of a ladybird, for instance, appears because the yellow, green and blue wavelengths in sunlight are absorbed and only red light reflects back into the eye of the beholder. The metallic shine of the golden chafers, by contrast, is caused by the white sunlight being broken, much as it is when shining through a diamond, to give a series of rainbow glints.

Seen through an electron microscope, the surface of the beetle is revealed to be covered with minute parallel grooves. These reflect certain portions of the light at the precise angle to shine like polished metal, while absorbing and scattering other wavelengths.

Shining colours are not just for showing off to a potential mate, although this is important for many butterflies. One of the main purposes, ironically, may be to avoid attention. In bright sunlight, against wet mud or in the dripping rainforest canopy, metallic glints are surprisingly confusing to the eye of a predator, which searches for images based on shape.

Slimiest insect

Contrary to popular opinion, insects (like snakes) are not slimy. Slime or, to give it its more technical term, mucus, is a sticky secretion used especially by molluscs and vertebrates. Snails and