Asked by: Adrian Lewis, Exeter
The sex of a caterpillar is fixed at the moment the egg is fertilised, but most species don’t show any sex-specific features until they turn into butterflies.
The immature reproductive organs are still present in the larval stage though, and in a few species with translucent bodies, you can see the red or dark yellow testes of the males.
Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.