As the caterpillar feed and feeds, it little realises it is eating not just for one but for many and not its own. Swimming inside the caterpillar’s ever more bloated body, parasites live on the blood of their host. They are growing too. These uninvited guests – larvae impregnated by a parasitic wasp – will soon become masters of their unwillingly accommodating slave. Before long, their teeth will be sharp enough for them to slice their way out of the caterpillar – eating it alive from the inside out – while their host becomes a zombified bodyguard, protecting and ultimately dying for its murderous, newly hatched brood.
To begin the morbid process, the adult female parasitic wasp, Cotesia glomerata impregnates a cabbage white caterpillar chosen as the host for its hungry little wasps to-be, thrusting its needle-like ovipositor through the victim’s skin and pumping her eggs into the body cavity. With the eggs hatched inside the caterpillar, up to 60 individual larvae will soon develop. After twelve days of the caterpillar gorging and providing nourishment, the larvae within will each have grown to the size of a grain of rice. They can even be seen, squirming beneath the surface of the caterpillar’s skin like a trick of the light.
The caterpillar continues to feed and balloon, consuming one and a half times as much food as would one of its un-parasitized kin, with more than 30% of its body weight made up by the larvae. The burgeoning body invaders are clever, however, for while they drink their host’s blood, they are careful not harm its vital organs. They thus keep it alive while they are still steadily growing. The caterpillar’s role is not yet fulfilled. When fully mature after two weeks, the larvae are ready to break out from the protection of their surrogate womb, and their newly formed saw-like teeth are called into cutting action.