Hayley Weir, of Balclutha Primary School, asks :-

How do caterpillars of Monarch Butterflies stay on the plant without falling off? Do they have special feet?

Samantha Botting, a zoologist in charge of the Tropical Forest at Otago Museum, responded.

You are right, Monarch caterpillars do have special feet to stop them from falling off plants. They are called prolegs. These are fake legs that have tiny hooks on them and work like suction cups. Monarch caterpillars use these eight little ‘feet’ to hold on tight to their plant while they munch on the leaves.

Prolegs are found in almost all caterpillar species. Some have more or fewer prolegs, depending on their size. Monarch caterpillars also have six ‘true’ legs. The true legs are used by the caterpillar to help guide food to its mouth. They are often very small and close to its head, so can be tricky to see.

The caterpillar’s six true legs become the legs you see the butterfly walking on once it emerges from its pupa. Monarch butterflies have evolved their own special way to hold onto plants on windy days. Generally, butterfly legs are made up of five segments, and at the very base of these are tiny claws called tarsal claws. These allow the Monarch butterfly to keep a grip on different surfaces and stop it from slipping off plants while feeding on nectar.

There is also experimental evidence showing that if a caterpillar damages its true legs or if the pupae do not form properly, the butterflies can emerge without their legs or tarsal claws. This can be very harmful for the butterfly. Without them it can’t hold onto plants to feed, or climb away from danger, or if its wings become wet from rain it can’t hold itself upside down to let them dry.

Our Tropical Forest at Otago Museum will be reopening in December as part of the new science centre, so come along and see just how special caterpillars’ prolegs are, and how terrifically adapted butterflies are for holding onto plants.

Send questions to: Ask-A-Scientist, PO Box 31-035, Christchurch 8444 Or email: questions@ask-a-scientist.net