Hemileuca Stonei

Hemileuca stonei is the Hemileuca most likely to look brown, as distinct from drab or reddish, and to have significant amounts of bare chitin uncovered with fur on its body. It is considered closely related to H. grotei and H. (grotei) diana, but has consistent visible differences.

Photo from Flickr.

Living in the desert along the border between Arizona and Sonora, H, stonei has received little attention from humans. Its caterpillars eat leaves of a few Mexican varieties of oak trees. It has no economic significance. It is only occasionally a nuisance.

Photo from Southwestdesertflora.com. Even the host plant of the caterpillars is prickly. This is Quercus emoryi, Emory's oak; young H. stonei also eat Q. oblongifolia, Mexican oak.

Mature caterpillars are blackish gray and may have a conspicuous blue or purple undertone. Younger caterpillars were probably black. Young Hemileuca caterpillars have relatively long, thin, sometimes simple bristles that contain relatively little venom, and try as much as possible to live in a cluster of caterpillars with a sibling on either side. Older caterpillars, who leave the family group as they get too big to feed side by side on one leaf, and eat whole leaves by themselves, have shorter bristles with branching brittle tips. Each tip contains about as much venom as a bee sting and may stick in the skin, oozing venom into the skin, for days. The final caterpillar skin has flattened rosette-shaped bristles on the back, allowing more bristle tips to irritate the skin.

Photo from Rtphill1. 

Most Hemileucas hatch when leaves of their host plant are relatively new and tender, but stonei hatch when oaks are blooming, so their first few meals are oak blossoms (which can look a bit like caterpillars). As the caterpillars grow bigger and tougher they find fewer blossoms and have to gnaw on oak leaves.

Photo by Tom Van Devender. A "mature" stonei caterpillar is smaller than most stingingworms, but it can still sting. The bristles sting mechanically, with no conscious action necessary on the part of the caterpillar. The caterpillars instinctively curl up with all their bristles facing out when they fall, but it's not clear whether their primary motive is to drop onto something and sting it, or just to let their bristles absorb landing shock. All stingingworms are so easy to hate that it's hard for humans to imagine what the animals feel or think, whether they have any idea how nasty they seem to everyone but themselves. 

They spend the hottest part of the year pupating in the fallen leaves under the host trees, where the caterpillars usually place what is described as a loose cocoon, or the foundation for a cocoon, on a few leaves they pull together for shade before they pupate. For silk moths, all the Hemileucas produce very little silk.

Adult moths are most recognizable when they have a dark brown color, but their color can be olive.

Photo by Jmbearce.

Others are charcoal gray:

Photo by Blisowsky.

Though less furry than some Hemileucas, with their little flat heads seldom covered by long hair on the thorax, the bodies often show small patches of bright red, yellow, or orange hair. Males have plumy antennae; females have smooth hairlike ones. (Males apparently use the extra nerve endings on their plumy antennae to trace the subtle scents of females, which humans can't smell.)

Males average a little smaller than females and are more likely to have lighter-colored wings and a tuft of bright-colored fur on the rear end. Females are more likely to be described as black and have black fur on the rear end. Males can be blackish, too, and the tips of their tail ends aren't always much more colorful; the antennae are the reliable sex characteristic. Wingspans are usually more than two inches but not known to be all of three inches.

At https://mothphotographersgroup.msstate.edu/species.php?hodges=7744.5 , Ken Osborne has photos of a typical pair. The female's wings are darker and her wingspan is 62 mm; the male's wingspan is 45 mm. The male's wings are rounder, though, and his body is much longer. After laying her eggs the female's abdominal section, which is used almost entirely for holding eggs, is still broader than the male's but much shorter. 

Like other silk moths, they don't eat or drink after pupating. They fly just long enough to reproduce before they die. Hemileucas spend most of a year, or years, in wingless forms. They can fly for more than a week, but most probably live only a day or two after they get their wings. And when stonei do fly, it's in autumn--September to November, after the hottest weather in their range is over.

So, when the adult moths crawl out of their pupal shells, even as they stretch and expand their crumpled new wings the males are sniffing the air and the females are pumping out their scent. Males fly in the daytime and are often mistaken for butterflies. They fly fast, somewhat erratically, following scents on the wind. Females are uncomfortably full of eggs when they eclose; they may prefer young unmated males, but it's all about unloading eggs and, once a male arrives and offers to help with that task, they don't wait for a better offer. 

The process of preparing eggs for unloading is, however, leisurely for Hemileucas. They send some time snuggling and caressing before and after fertilizing the eggs, and often mate face to face. 

Photo by Ksacco, proving that this pair were so focussed on each other that they didn't even seem to mind that a human hand was curving around them to document how small these members of the "giant" silk moth family really are. Possibly their sensitivity to pheromone scents gave them a clue that they were going to be included in a selfie rather than crushed and eaten. Chemical analysis has not been done for all the Hemileucas, but several Hemileuca scents include traces of fragrances used in soaps and hand lotions...is it possible that Ksacco's hand smelled good to these moths? About half the photos of living stonei online document that, if not positively attracted to humans, these moths can easily be persuaded to perch on at least some humans' hands.

("Do people really spend their time and money analyzing the scents of insects humans don't even smell?" Of course they do. For the bigger Hemileucas that live closer to humans, there has been considerable interest in synthesizing their scents in order either to lure them into traps or to guide them to mate and reproduce as far from humans as possible. But stonei naturally do live in places where few humans choose to be, so we've shown less interest in them.)

After spending about an hour with their mates, females usually spend a quarter to a half hour flying about, even if they had time to find a suitable place to lay eggs before coupling. They lay eggs by ones,  taking time to place each one, usually placing them close together in a roughly ring-shaped cluster around a twig that seems likely to produce the right kind of leaves for their caterpillars.