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. Though insects never have more than two antennae, their eyes can vary in number and type. And just so you know: insects don't necessarily have eyes just on their head. One species of swallowtail butterflies 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 that she is laying her eggs in the right place.
If the head is the insects' sensory center, the midsection—the thorax—is the transport center. This segment is dominated by the muscles needed to power the wings and legs. It is worth noting that, unlike those of all other animals that can fly or glide—birds, bats, flying squirrels, flying fish—insects' 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 gall a 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 apartment without a toilet, they have no choice but to hold it in. Only after the larval stage is complete are the gut and the gut opening connected.
LIVING IN AN INVERTEBRATE WORLD
Insects are invertebrates—in other words, animals without a backbone, skeleton, or other bones. 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 on to 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, 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 armor, expanding and rising until it has nowhere left to go. But insects have a solution: new armor, soft to start with, forms beneath the old. The old, rigid armor cracks open, and the insect jumps out of its skin as casually as we'd shrug off a shirt. Now it's crucial that it literally inflate itself to make the new, soft armor as big as possible before it dries and hardens. Because once the new exoskeleton has hardened, the insect's potential for growth is fixed until another molting 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 molting process occurs only in insects' early lives.
This excerpt ends on page 4 of the hardcover edition.