Extreme Insects. Richard Jones

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СКАЧАТЬ ATTRIBUTE the longest head of any insect

      It will come as no surprise to discover that some males have big heads. Big heads can be attached to big jaws (see page 60) or house big eyes (see page 56). But the male giraffe-necked weevil of Madagascar has the most awkward-looking head imaginable. And what does he use it for? Nodding.

      The male’s long, slender head takes up about 10 of his 25 mm (1 in) length. The neck is another 7 mm, making the insect’s head and neck over 70 per cent of its entire body length. It holds them angled up from its squat body, like a miniature construction site crane. The female’s head and neck are also relatively long, but only about half her total body length.

      The male uses his stretched form for no practical purpose. The nodding, however, is very important to other giraffe-necked weevils. It seems that the males contest one another, trying to out-nod their opponents in ritualised fights. After head-to-head nodding competitions, one male will retreat. It also appears that the females choose the best nodders with which to mate. Thus, over evolutionary time, the males with the longest heads (better for nodding) have been selected.

      The irony is that it is the female who really needs a long head. Trachelophorus belongs to a group of beetles called leaf-rolling weevils. She chews through both sides of a leaf blade to the mid-rib. The leaf now has a tendency to curl, a property that she harnesses using her angled neck and head to roll the partly severed plant into a small cigar. She lays an egg inside, and the grub is protected from predators and parasites while it feeds.

Most streamlined insect

NAME	water pennies in the family Psephenidae
LOCATION	worldwide
ATTRIBUTE	shaped to withstand rushing torrents

      Despite rolling boulders and white water, life continues beneath the surface of fast-flowing rivers. There, attached to the stones in the water, live water pennies. So named because they are roughly the size of a one-cent coin (a penny), these creatures are the larvae of beetles. The adult beetles are terrestrial, but their larvae are wholly aquatic.

      A water penny is multi-segmented, with each segment flattened into a flange that surrounds its body, hiding head, legs and gills beneath a smooth carapace. It clings tight to rocks and stones using its clawed feet. If it cannot get a purchase, then even slow-moving water can wash it away. The larvae spend most of the time under stones or pressed into small cracks in the rocks, feeding on microscopic algae. But they must leave the water to pupate, and at such times they are exposed to the force of the water.

      In rapidly moving water, there is a boundary layer of calmer water at the bottom, slowed by friction with the river bed. Small and flattened, water pennies can sit within this layer, but they cannot afford to be complacent. As well as clinging on tight with their feet, they use hydrodynamics to hold fast. By pumping water out through the gaps between their segments and at the tail end of the body, they can reduce turbulence to creep slowly through the force of the flow.

Loudest insect

NAME	dog day cicada Tibicen pronotalis
LOCATION	North America
ABILITY	makes the loudest noise of any insect

      Insects are generally small, secretive and quiet. Most are reluctant to draw attention to themselves, but the cicadas are an exception. Along with crickets, katydids and grasshoppers, the cicadas use sound to communicate with each other, and they do so in the loudest manner possible.

      On each side of the first abdominal segment is a large round organ called the tymbal. The tymbal, just like a drum, has a stiff elastic membrane held taught by a rigid circular frame. Inside the insect’s abdomen, a large muscle is attached by a narrow thread to the centre of the membrane. When the muscle contracts it distorts the tymbal membrane, causing it to buckle suddenly, creating an audible snap. When it relaxes, the membrane clicks back to its resting position. By vibrating the membrane at 4,000 to 7,000 times a second, the clicks become merged into a continuous whining buzz. Inside the abdomen, two air sacs (modified breathing tubes) are tuned to the natural frequency of the tymbals and act as amplifiers. The noise made is astounding, easily competing with loud power tools, lawn mowers or motorcycles. Cicadas on the motorway verge can often be heard from inside fast-moving cars, or through dense forest from over 1 km away.

      The volume of a noise is measured using sound pressure level meters. The loudest sustained volume recorded for an insect is for an African cicada, Brevisana brevis, which clocked up 106.7 decibels. Human hearing is damaged by prolonged exposure to this volume and the recommended limit is less than two hours per day. Brevisana keeps it up all day long. The loudest peak cicada call ever recorded was for one of the North American dog day cicadas, Tibicen pronotalis, which reached 108.9 decibels during an alarm call. The normal purpose of cicada ‘songs’ is for males to call to females and announce territoriality to each other. On the whole the largest cicadas make the most noise, so everyone knows who is the biggest. Alarm calls are made as a defence against birds, and at these volumes the sound is truly repellent.

Best hoverer

NAME	big-headed flies in the family Pipunculidae
LOCATION	worldwide
ATTRIBUTE	superb stock-still hovering

      When, on 29 September 1907, the French aviation pioneer Louis Charles Breguet lifted off the ground in an erratic prototype helicopter, Gyroplane 1, he was trying to emulate a flight technique long mastered by insects – hovering. The ability to hang in mid-air, even for just a moment, is of paramount importance if an insect wants to land on a leaf or a flower, as there are no runways for a glide-down descent.

      Because insects can flex (twist) their wings, thrust and lift can be generated by both backwards and forwards strokes. In the fastest insects, the power stroke (pushing backwards) and the recovery stroke (pulling the wings forwards again) generate nearly all thrust, with just enough lift to keep level flight. In hoverers, thrust and lift are directed straight down, with just enough power to support an insect stationary.

      Among the best-known hovering insects are the hawkmoths and bee-flies, which hover apparently motionless while drinking nectar from a flower. Others include the hoverflies, named for their habit of hovering in a shaft of light, over a flower, or in a woodland clearing. That hovering is important to these large and brightly coloured insects is demonstrated by the fact that they have huge eyes; in the males there is СКАЧАТЬ