A caterpillar's medicine cabinet

Becoming an adult butterfly or moth is a high-stakes obstacle course.

One of the biggest threats a caterpillar faces is the risk of being parasitized.

Wasps and flies inject their own eggs into a caterpillar's body for growing larvae to feed off its internal organs.

But the woolly bear caterpillar (Grammia incorrupta) is one of the only recorded species able to fight back against these unwelcome attackers.

The first line of defence

Cells known as haemocytes play an important role in the caterpillar immune system. Like our white blood cells, these help development and growth.

Some haemocytes can envelop and destroy tiny microbial parasites that invade the insect's body.

For many caterpillars, however, this immune response is not enough to kill off invaders determined to take over their bodies.

But the woolly bear caterpillar is far from powerless. When a parasite hits, this species takes direct action.

The fight begins

Scientists think that by deliberately eating plants laced with toxic alkaloids, the caterpillars either poison the invaders inside them or boost their own immune systems.

A study found that woolly bear caterpillars suffering from parasitic infection ate more alkaloids than their healthy counterparts.

In doing so, they were also more likely to survive and grow into adult tiger moths.

But eating the toxins also comes with a risk. If healthy caterpillars eat a lot of the alkaloids, they are more likely to become ill and die. This means the insects must find a healthy balance in their diet.

Such discoveries suggest that insects are able to self-medicate in the same way some mammals and birds do.

Insect doctors

Researchers think Grammia incorrupta caterpillars have an immune system that can detect and react to parasites inside the body.

A parasite's presence stimulates a chemical in the part of the brain that controls taste, driving the caterpillar towards medicinal plants.

Dr David Lees, Museum micromoth curator, says, 'These are not the only insects that act as doctors.

'Female monarch butterflies have a different strategy. They can become infested with the spores of a potentially deadly parasite that often passes to their offspring.

'When this happens, they make sure to lay their eggs on the most toxic plants to help kill the infection and protect their young.

'They seem to sense their infection levels and choose the best medicine for their future offspring.

'So both species can ward off these debilitating parasites.'