We know, in part, what makes the insects different. Those other first animals tended to their young, as do most of their descendants, such as birds, reptiles, and mammals, which still bring their young food and fight to protect them. Insects, by and large, abandoned these ancient traditions for a more modern life.
Insects evolved hardened eggs and with them a special appendage, an ovipositor, which some use to sink their eggs into the tissue of Earth. Lift a stone and you will find them. Split a piece of wood, and they are there, but not only there. Birds struggle to find good places to nest, yet insects evolved the ability to make anything—wood, leaves, dirt, water, even bodies (especially bodies)—a nursery. If there is a single feature that has ensured insects' diversity and success, it is the fact that they can abandon their young nearly everywhere and yet have them survive—because of those eggs.
They began simply, smooth and round, but over 300 million years, insect eggs have become as varied as the places where insects reign. Some eggs resemble dirt. Others resemble plants. When you find them, you might not know what you are seeing at first. The forms are unusual and embellished with ornaments and apparatuses. Some eggs breathe through long tubes that they extend up through water. Others dangle from silky stalks. Still others drift in the wind or ride on the backs of flies. They are as colorful as stones, shaded in turquoises, slates, and ambers. Spines are common, as are spots, helices, and stripes. More than biology, their designs suggest the work of an artist left to obsess among tiny forms. They are natural selection's trillion masterpieces; inside each is an animal waiting for some cue to break free.
The basic workings of insect eggs, however, like the basics of any egg, are recognizable. The egg develops its shell while still inside the mother. There the sperm must find and swim through an opening at one end of the egg, the micropyle. Sperm wait inside the mother for this chance, sometimes for years. One successful sperm, wearied but victorious, fertilizes each egg, and this union produces the undifferentiated beginnings of an animal nestled inside a womblike membrane. Here eyes, antennae, mouth parts, and all the rest form. As they do, the creature respires using the egg's aeropyles, through which oxygen diffuses in and carbon dioxide out. That all of this occurs in a structure typically no larger than a grain of raw sugar is simultaneously beyond belief and ordinary. This is, after all, the way in which most animals ever to have lived on Earth had their start.