Saddle Up, Pardner

What am I looking at?

These are five saddleback slugs – larvae of the saddleback slug moth (Acharia stimulea). You can see the distinctive saddle-like pattern on their green backs: a brown spot outlined in white (1). The stinging hairs are located on fleshy knobs called scoli (2) near their front and rear. Their head is tucked under the front of their body, facing upward, and is invisible from above. Also invisible are their highly reduced prolegs; the underside of this caterpillar is reminiscent of the “foot” of a snail or a slug, hence its common name.

Biology in the background

These moths live in eastern North America and Mexico. The caterpillars are polytrophic – meaning they feed on many different plants, from broadleaf trees to conifers to palm trees. During their first few instars (developmental stages), they feed gregariously, staying close together. They disperse as they mature.

Their scoli – the plump bumps near their head and tail – are covered with hairs that produce venom in hollow canals. The hairs are urticating, which means that their fine, long tips break off in the skin of any animal that touches them. The venom in the embedded hair may then cause mild to severe skin irritation, a persistent rash, and even nausea in more sensitive individuals. This is an excellent defense mechanism that helps shield these caterpillars from many predators, but sadly it is powerless against their main nemesis – parasitoid wasps.

Interestingly, these very distinctive caterpillars will develop into rather unremarkable moths – so it is from the caterpillar that the species’s common name is derived.

These caterpillars can grow to be about 2.5 centimeters long, or a bit bigger than a human thumbnail.

Technique

These images were created using macrophotography.