Small Creatures, Smart Design: Insect Anatomy
So how are they put together, these tiny creatures with whom we share our planet? The following section is a crash course in insect construction. It also shows that, despite their modest size, insects can count, teach and recognise both each other and us humans.
Six Legs, Four Wings, Two Antennae
What exactly is an insect? If you’re in any doubt, a good rule of thumb is to start by counting legs. Because most insects have six legs, all attached to the mid-section of their body.
The next step is to check whether the bug has wings. These are also on the mid-section. Most insects have two pairs of wings: forewings and hindwings.
You have now indirectly grasped one crucial hallmark of insects: their bodies are divided into three. As one of many representatives of the phylum Arthropoda, insects are formed of many segments, although in their case, these have merged into three pretty clear and distinct sections: head, thorax and abdomen. The old segments still appear as indentations or marks on the surface of many insects, as if somebody had cut them with a sharp implement – and in fact, that is what gave this class of animal its name: the word ‘insect’ comes from the Latin verb ‘insecare’, meaning to cut into.
The front segment, the head, isn’t so unlike our own: it has both a mouth and the most important sense organs – eyes and antennae. While insects never have more than two antennae, their eyes can vary in number and type. And just so you know: insects don’t necessarily only have eyes on their head. One species of swallowtail butterfly has eyes on its penis! These help the male to position himself correctly during mating. The female also has eyes on her rear end, which she uses to check she is laying her eggs in the right place.
If the head is the insect’s sensory centre, the mid-section – the thorax – is the transport centre. This segment is dominated by the muscles needed to power the wings and legs. It is worth noting that, unlike all other creatures that can fly or glide – birds, bats, flying squirrels, flying fish – insect wings are not repurposed arms or legs but separate motor devices that supplement the legs.
The abdomen, which is often the largest segment, is responsible for reproduction and also contains most of the insect’s gut system. Gut waste is excreted at the rear – usually. The minute gall wasp larvae, which live out their larval existence in the completely closed structure the plant builds around them, are extremely well brought up. They know it’s wrong to foul your own nest and since they are trapped in a one-room flat without a toilet, they have no choice but to hold it in. Only once the larval stage is complete are the gut and the gut opening connected. See also Chapter 7, here.
Living in an Invertebrate World
Insects are invertebrates – in other words, animals without a backbone, skeleton or bones. Instead, their skeleton is on the outside: a hard yet light exoskeleton protects the soft interior against collision and other external stresses. The outermost surface is covered in a layer of wax, which offers protection against every insect’s greatest fear: dehydration. Despite their small size, insects have a large surface area relative to their tiny volume – meaning that they are at high risk of losing precious water molecules through evaporation, which would leave them as dead as dried fish. The wax layer is a crucial means of hanging onto every molecule of moisture.
The same material that forms the skeleton around the body also protects the legs and wings. The legs are strong, hollow tubes with a number of joints that enable the insect to run and jump and do other fun things.
But there are a few disadvantages to having your skeleton on the outside. How are you supposed to grow and expand if you’re shut in like this? Imagine bread dough encased in medieval armour, expanding and rising until it has nowhere left to go. But insects have a solution: new armour, soft to start with, forms beneath the old. The old, rigid armour cracks open and the insect jumps out of its skin as casually as we’d shrug off a well-worn shirt. Now it’s crucial for the insect to literally inflate itself to make the new, soft armour as big as possible before it dries and hardens. Because once the new exoskeleton has hardened, the insect’s potential for growth is determined until another moulting paves the way for new opportunities.
If you think this sounds tiring, it may be a consolation to hear that (with a few exceptions) the lengthy moulting process only occurs in insects’ early lives.
Time of Transformation
Insects come in two variants: those that change gradually through a series of moultings and those that undergo an abrupt change in the process of developing from child to adult. This transformation is called a metamorphosis.
The first type – e.g. dragonflies, grasshoppers, cockroaches and true bugs (see also here) – gradually change in appearance as they grow. A bit like us humans, except that we don’t have to shed our entire skin in order to flourish. For these insects, the childhood stage is known as the nymph stage. The nymphs grow, cast off their exoskeleton a few times (just how many times varies by species, but often three to eight times) and become increasingly like the adult versions. Then, finally, the nymph moults one last time and crawls out of its used larval skin equipped with functioning wings and sex organs: voilà! It has become an adult!
Other insects undergo a complete metamorphosis – an almost magical change in appearance from child to adult. 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