Our granddaughter Melanie spotted this spectacular little caterpillar on our back deck. It is the larva of an inconspicuous little (1.5 inch wingspan) dark brown moth, Acharia stimulea. This one is about one inch long, which is near their maximum size.
The legs and prolegs of this caterpillar are well hidden under the body. As you can see it is well armed with spines. It reminds one of a porcupine. The spines are hollow and contain a potent hemolytic venom. According to David L. Wagner in “Caterpillars of Eastern North America”, the sting from these spines may be the most potent of any caterpillar in North America. The venom can cause a severe rash, migraines, rupturing of red blood cells, and anaphylactic shock. It is perilous even to handle the cocoon, because there may be some stray spines embedded in it. The bright colors of the larva are an advertisement to potential predators that they will encounter a problem if they try to eat this creature.
The caterpillar will feed on a wide variety of host plants including maple, oak, cabbage, blueberries and corn.
The front end of the caterpillar is shown in the photograph to the left above. Its small dark head just protrudes from the mantle of skin. Several small tubercles with stinging hairs can be seen. The rear end of the caterpillar is shown to the right above. It looks strikingly like a face with two large eyes. This pattern might serve to fool predators into thinking that this is the head end of the creature. Or it might remind them of a serpent’s head and deter attacks. The rear end is also well armed with spines.