Participants in last week's Ohio Plant Diagnostic Workshop looked at but didn't touch, the Smaller Parasa (Parasa chloris). They kept their distance because the deceptively named caterpillar packs a venomous punch that's far from small. As with many creatures in Nature (e.g. crocodilians, mamba snakes, grizzly bears, etc.); these caterpillars should not be handled.
The Smaller Parasa belongs to the "Slug Caterpillar" family, Limacodidae. These unusual moth caterpillars don't actually look like slugs (order Gastropoda); they are so-named because of the way they move. The caterpillar's thoracic legs are shortened to almost nonexistence and their prolegs are condensed to simple sucker-like structures. The lack of obvious legs makes these unusual caterpillars look like they're creeping along on their stomachs like a gastropod (gastro = stomach; poda = foot).
The Limacodidae moth family includes some of Nature's most venomous caterpillars. Some of the most notorious that are found in Ohio include the Saddleback Caterpillar; (Acharia stimulea); Crowned Slug (Isa textula); Skiff Moth (Prolimacodes badia) caterpillar; and my favorite based on one of its common names, the Hag Moth (a.k.a. Monkey Slug) (Phobetron pithecium) caterpillar which looks like a piece of leaf debris; albeit a very dangerous piece of debris.
Limacodid slug caterpillars aren't the only venomous moth caterpillars found in Ohio that shouldn't be handled. A number of species that belong to the family Megalopygidae (flannel moths) can also deliver a painful sting including the White Flannel Moth (Norape ovina) caterpillar which is covered in spines and hairs and the Black-Waved Flannel Moth (Lagoa crispata) caterpillar with early instars that look like a "living Q-Tip."
The Io Moth (Automeris io) belongs to the so-called silkworm moth family, Saturniidae. The caterpillars of many members of this family sport threatening-looking non-venomous bristles. A good example is the caterpillar of the Royal Walnut Moth (Citheronia regalis) which is known as the Hickory Horned Devil for its scary-looking but non-venomous spines. However, the bristles on the Io Moth caterpillars are the real deal. They can deliver a seriously painful sting.
A Hairy Defense
The vast majority of lepidopteron (moths and butterflies) caterpillars, including many with hairs and spines, do not present a threat. However, there are some with hairs that are modified for defense. These are collectively called urticating hairs from the Latin urtica meaning "nettle." Indeed, the hairs on stinging nettle (Urtica dioica), which are called trichomes; provide a good model for the venom injecting mechanism used by some of the more serious urticating hairs found on caterpillars.
In general, caterpillar urticating hairs may be divided into two groups: long, flexible hairs; or stout bristles. These two groups can be further divided into hairs that puncture the skin, with some types breaking off to cause mechanical injury or hairs that puncture the skin and inject venom (envenomating hairs).
Saddlebacks are capable of launching a two-pronged defense response. All of its stout urticating bristles may break-off to remain embedded in their assailant. Some are envenomating bristles and they contain two types of venoms: vesicating (blistering) and hemolytic which breaks down red blood cells. Stings from these hairs are intensely painful and are described by hapless victims as sharp and burning. The pain may last for a long time and often spreads over a large area. Although saddleback stings are not generally life-threatening, it is best to seek medical attention. As with bee and wasp stings, some people are particularly sensitive to the saddleback venom. Of course, the saddleback's bristles don't protect it from all enemies as illustrated by the parasitoid wasp cocoons sprouting from one hapless saddleback in the image below.
Smaller Parasa caterpillars hide their venomous armament. Their envenomating bristles remain almost hidden from view until needed for the caterpillar's defense. If disturbed, the retractable structures bristling with bristles pop out to ward off potential predators or intransigent entomologists.
Not all caterpillars armed with defensive hairs are hazardous to all people. Tussock moth caterpillars (family Erebidae) can have both long, flexible hairs and stout non-venomous bristles. However, the effects may be enhanced by a person's overall sensitivity. Some people have a severe allergic response to hairs that simply penetrate and break-off. They may suffer extensive rashes and localized swelling causing them to believe they have encountered envenomating hairs when they have not.
The bottom line is that unless the hairy or bristly caterpillar is identified as innocuous, don't handle them. It's best to err on the side of caution.