In our human world, we have to turn to fairy tales and fantasy for examples of this sort of shapeshifting, like kissed frogs turning into princes, or J.K. Rowling’s Professor Minerva McGonagall shapeshifting into a cat. But for insects, kissing and spells aren’t the cause of the change: the metamorphosis is driven by hormones and marks the transition from child to adult. First, the egg hatches into a larva that looks nothing like the creature it will ultimately become. The larva often resembles a dull, pale rectangular bag, with a mouth at one end and an anus at the other (although there are honourable exceptions, including many butterflies). The larva moults several times, growing bigger on each occasion but otherwise looking pretty much unchanged.
The magic happens in the pupal stage – a period of rest in which the insect undergoes the miraculous change from anonymous ‘bag creature’ to an incredibly complicated, ingeniously constructed adult individual. Inside the pupal case, the whole insect is rebuilt, like a Lego model whose blocks are pulled apart and put back together again to make an entirely different shape. In the end, the pupa splits and out climbs ‘a beautiful butterfly’ – as described in one of my all-time favourite children’s books, The Very Hungry Caterpillar. Total transformation is brilliant and undoubtedly the most successful variant. Most insect species on the planet, 85 per cent of them, undergo this type of complete metamorphosis. This includes the dominant insect groups, such as beetles, wasps and their relatives, butterflies and moths, and flies and mosquitoes.
The ingenious part of it is that insects can exploit two totally different diets and habitats as child and adult, concentrating on their central task in each phase of their lives. The earthbound larvae, whose focal point is energy storage, can be eating machines. Then, in the pupal stage, all the accumulated energy is melted down and reinvested in a totally new organism: a flying creature dedicated to reproduction.
The connection between larvae and adult insects has been known since Ancient Egyptian times, but people didn’t understand what was happening. Some thought that the larva was a stray foetus that eventually came to its senses and crawled back into its egg – in the form of the pupa – in order to be born at last. Others claimed that two totally different individuals were involved, the first of which died and was then resurrected in a new form.
Only in the 1600s did Dutch biologist Jan Swammerdam with his new invention, the microscope, demonstrate that the larva and the adult insect were one and the same individual throughout. The microscope enabled people to see that if a larva or pupa was cut open carefully, clearly recognisable elements of the grown insect could be found beneath the surface. Swammerdam enjoyed displaying his skills with scalpel and microscope before an audience, and used to demonstrate how he could remove the skin from a big silkworm larva to reveal the wing structure beneath, complete with the characteristic veined patterning on the wings. Even so, this did not become general knowledge until much, much later. In his journal, Charles Darwin notes that a German scientist was charged with heresy in Chile as late as the 1830s because he could transform larvae into butterflies. Experts continue discussing the exact details of the metamorphosis process even now. Luckily, there are still some mysteries left in this world.
Insects don’t have lungs and don’t breathe through their mouths like we do. Instead, they breathe through holes in the sides of their bodies. The holes run, like drinking straws, from the surface of the insect into its interior, branching out along the way. Air fills the straws and the oxygen then passes out of them and into the body’s cells. This means that insects don’t need to use their blood to transport oxygen to the various nooks and crannies of their bodies. However, they do still need some kind of blood – known as haemolymph – to carry nutrition and hormones to the cells and to clear them of waste material. Since insect blood doesn’t transport oxygen, there is no need for the ferrous red substance that colours our mammal blood red. Consequently, insect blood is colourless, yellow or green. That is why your car windscreen doesn’t end up looking like a scene from a bad crime novel when you’re driving along on a hot, still, summer afternoon but is instead covered in yellowish-green splatters.
Insects don’t even have veins and arteries: instead, insect blood sloshes around freely among the bodily organs, down into the legs and out into the wings. To ensure a bit of circulation, there is a heart of sorts: a long dorsal tube with muscles, and openings on the side and at the front. Muscle contractions pump the blood forward from the rear, towards the head and the brain.
Insects’ sensory impressions are processed in the brain. It is tremendously important for them to pick up signals from their surroundings in the form of scent, sound and sight if they are to find food, avoid enemies and pick up partners. Although insects have the same basic senses as us humans – they sense light, sound and smell, and can taste and feel – most of their sense organs are constructed in a totally different way. Let’s take a look at insects’ sensory apparatus.
The Fragrant Language of Insects
The sense of smell is important for many insects, although, unlike us, they lack a nose, doing most of their smelling through their antennae instead. Some insects, including certain male moths, have large feathery antennae that can pick up the scent of a female several kilometres away, even in extremely low concentrations.
In many ways, insects communicate through smell. Scent molecules allow them to send each other various kinds of messages, from personal ads such as ‘Lonesome lady seeks handsome fella for good times’ to ant restaurant recommendations: ‘Follow this scent trail to a delicious dollop of jam on the kitchen counter.’
Spruce bark beetles, for example, don’t need Snapchat or Messenger to tell each other where the party is. When they discover an ailing spruce tree, they shout about it in the language of scent. This enables them to gather together enough beetles to overpower a sickly, living tree – which ends its days as a kindergarten for thousands of beetle babies.
We miss out on most of these insect scents, which we simply can’t smell. But if you wander beneath the greenery of ancient trees on a late summer’s day in the town of Tønsberg, southern Norway, you may be lucky enough to pick up the most delightful aroma of peaches: it is the hermit beetle, one of Europe’s largest and rarest beetles, wooing a girlfriend in the neighbouring tree. The substance it uses rejoices in the thoroughly unromantic name of gamma-Decalactone and we humans produce it in labs for use in cosmetics, and to add aroma to food and drink.
The scent is very helpful to the hermit beetle, which is heavy and sluggish and seldom flies, or not far at any rate. It lives in ancient, hollow trees, where its larvae gnaw on rotted wood debris, and it’s a real homebody: a Swedish study found that most adult hermit beetles were still living in the same tree they were born in. This lack of interest in travel complicates the business of finding new hollow trees to move into, and the situation is hardly helped by the fact that old, hollow trees are an unusual phenomenon in today’s intensively exploited forest and farmland. As a result, the species, which is scattered across Western Europe, from southern Sweden to northern Spain (though not the British Isles), is decreasing all over its range and is protected in many European countries. In Norway, it is considered critically endangered and can be found in only one place: an old churchyard in Tønsberg. Or two places to be precise, because some individuals have recently been moved to a nearby oak grove in an effort to secure the survival of the species.
Flowery Temptresses
