Extreme Insects. Richard Jones

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      The European snail beetle, Drilus flavescens, is small (4 to 7 mm) and brown; it has a black head and thorax, and feathery antennae – at least the male has. The female, by extreme contrast, is a large, soft, flabby, caterpillar-like creature, 50 times as large as the male. The males fly on hot sunny days, but the females lack both the normal hard beetle wingcases and also the functional membranous flight wings. The distribution of the males shows that the species is fairly widespread on limestone or chalk soils, but despite this the female is virtually unknown. In fact, the female of this peculiar species is so rarely seen that there was no reliable published picture of her until this mating pair was photographed in 2003.

      The larvae of Drilus eat small snails. Despite being a widespread insect, the rarity (or perhaps the secretiveness) of the females and larvae meant that the beetle’s life cycle was not worked out until 1903. Quite why males and females of Drilus should be so very different is still a bit of a mystery, although many female glow-worms (also beetles but in a completely different family) are also wingless, and their larvae, too, are snail predators.

Most mixed-up sexuality

NAME	bilateral gynandromorph various species, but particularly prominent in butterflies
LOCATION	this example was bred in captivity
ATTRIBUTE	half male and half female

      Insects are usually either wholly male or wholly female. In extremely rare situations, however, there appears an individual that is exactly half one sex and half the other – a bilateral gynandromorph – and nowhere is this more striking than when it involves a butterfly. In butterflies, as in most animals, sex is determined by the chromosomes. Females have two X chromosomes (XX) and males have just one (XO). Butterfly sperm contains either an X or no-sex chromosome.

      In this marsh fritillary butterfly (Euphydryas aurinia) the sperm that originally fertilised the egg contained an X chromosome so the offspring was due to be XX, female. But after the very first cell division into two, one of the XX cells (female) somehow lost an X and became XO (male). Throughout the many millions of further cell divisions in the growing caterpillar and during metamorphosis in the chrysalis the right-hand side of the insect stayed female while the left-hand side had become male. When the final adult butterfly emerged from its pupa, it continued to be right half female and left half male.

      Gynandromorphs are very rare and unlikely to survive. Neither male nor female sexual organs are functional. Some striking butterfly specimens occur where males and females have different wing patterns. In the case of the marsh fritillary, males are significantly smaller than females. This specimen was reared as part of a genetic study. In the wild all it could have achieved in life would have been a terminal spiral flight.

Most bloated insect

NAME	honeypot ants Myrmecocystus species, Camponotus inflatus and others
LOCATION	Australia, New Guinea, South Africa, western USA to Mexico
ABI LITY	use their stretched bodies to store sugar and water

      For most aboriginal peoples, honey from bees was the only source of sweetness for thousands of years. But in Australia, western USA, Mexico, South Africa and New Guinea, they could raid another source – the hugely bloated honeypot ants.

      Honeypot ants have grossly distended abdomens. Their job is to hang immobile from the roofs of nest burrows and fill up with the goodies brought back by their nest-mates, the workers – nectar and honeydew (aphid excrement little changed from the liquid plant sap these insects suck out). This behaviour has evolved in several different genera around the world, usually in desert habitats where the storage of food against hard times allows the colony to survive in the harshest of environments.

      The storage ants, called ‘repletes’, can expand their bodies by a factor of many hundreds compared to the normal workers. Their translucent bodies vary in colour from almost clear, through yellow-brown to dark amber. The darker bodies contain the sugars glucose and fructose. The palest and heaviest repletes contain very dilute sugar solutions.

      The evolution of repletes is thought to be linked to a system that exploits the unpredictable food sources provided by desert flowers. The volume of the repletes is built up in cool, moist weather, and they are then tapped by the rest of the colony during hot, dry times. The change from building up to tapping happens at about 30-31°C (86-88°F), suggesting that the real purpose of the repletes is to store water against drought.

Most seasonally dimorphic insect

NAME	map butterfly Araschnia levana
LOCATION	widespread across mainland Europe
ATTRIBUTE	alternating generations of completely different-looking butterflies

      The European map butterfly, Araschnia levana, gets its name from the pretty patterns that mark the undersides of its wings. The mottled browns and oranges of its background are criss-crossed with bright white lines reminiscent of the radiating compass marks superimposed on old maps and nautical charts. However, it is the patterns of the upper sides that are most remarkable.

      Spring butterflies, emerging from chrysalises that have remained dormant through winter, are bright orange above, marked with a series of black spots and blotches. Their eggs produce caterpillars that feed quickly on their nettle host-plants, and the summer generation of butterflies that emerges a few weeks later has a completely different colour pattern – jet black, with a strong white flash down each wing (shown right). So different are these colour forms that they were long thought to be two distinct species.

      This extreme dimorphism (meaning ‘two forms’) has attracted a lot of research from entomologists, and the factors that decide which colour pattern will be produced are now well understood. The final adult morph is decided by the effects of day length and temperature on the feeding caterpillar. Short days and cold, enough to induce winter torpor, produce the spring orange form levana while long hot days produce the black and white summer form prorsa. Experiments have shown that caterpillars from either generation can be raised under artificially altered temperature and daylight regimes to produce the ‘wrong’ adults.

      It is still not known why the map butterfly shows such stark changes between its two generations. The scene is further confused by the fact that more northerly and montane populations have only one generation (form levana) each year, while in the south there is a partial third generation with intermediate СКАЧАТЬ