EPOW Ecology Picture of the Week -- Mysteries of the Hooded Harvestman

Explanation: This is a rather bizarre, tiny invertebrate from the conifer forests of the Cascade Mountains in southern Washington state. Called hooded skunk spider or hooded harvestman, this is only one of four known species of the genus Dendrolasma. This one is endemic to the Pacific Northwest, U.S.

Three's a crowd under a low-power microscope.

Actually, this is not a true spider, but a "harvestman," so named because they were originally observed during harvest time. They belong to class Arachnida as do true spiders, ticks, mites, scorpions, and pseudoscorpions.

Its two eyes or ocelli are located on either side of a stalk on top of the head, like a periscope that can see two ways at once. The stalk widens into a hood where it attaches to the head.

But what makes this species so odd is the whitish structure jutting from its head. It is called a hood apparatus and its function is unknown but may serve as a sensor, perhaps to detect prey.

The white structre on the right extrudes from the
head, and is not a spinneret. It is the
so-called hood apparatus.

A closeup of the hood apparatus.
No other spider sports this feature.

Hooded harvestmen function as predators on other insects in forest litter but their specific diet is largely unknown. In fact, little is known at all about their ecology.

Now this is interesting.

I photographed this harvestman, apparently a Dendrolasma, in a minimal web about a meter off the ground along a streambed in northwestern California. It was inspecting a prey insect caught in the web.

It's interesting because references suggest that all members of Opiliones, including Dendrolasma, do not spin webs. In particular, Dendrolasma harvestmen are predators within vegetation litter on the forest floor. Maybe it took over this web and its prey from another species...?

Acknowledgment: My thanks to entomologist Andy Moldenke, Oregon State University, for the identification.

EPOW - Ecology Picture of the Week