The Head of a Harvestman

What am I looking at?

A harvestman’s two eyes are located on a turret-like bump called an ocularium, located on the upper surface of its body. This is a depth-coded frontal cross section through the ocularium. The two glowing orbs are the lenses (1). Toward the middle of the image, the cyan, blue, and purple dots are the nuclei of the cells within the ocularium, some of which form conspicuous concave layers (2). The red, yellow, and green rod-like structures between the layers of nuclei are photoreceptor cells, which form the cup-shaped retina (3). The two tubes at the center bottom of the image are the trachea that provide oxygen to the eyes (4).

Biology in the background

Harvestmen can be found all over the world except in Antarctica. Most are omnivores, feeding on a variety of organic material, and many are scavengers, feeding on dead plants and animals. Some feed on smaller insects and are mainly ambush predators – staying still until prey comes close and then pouncing. Unlike spiders, which can only drink the internal fluids of their prey, harvestmen can also swallow chunks of solid food. They do not produce silk and therefore do not build webs.

They also don’t have venom and thus aren’t poisonous if a predator eats them, but they do use a variety of behavioral tactics to confuse predators into thinking they’re not worth attacking. These tactics include bobbing (so it’s hard for predators to pinpoint where their body is), freezing (so they blend into the background), and secreting foul-smelling chemicals (so they smell and taste bad). When caught or handled, they often sever one of their legs, a process called autotomy; an autotomized appendage twitches for a while, providing a distraction.

Harvestmen vary in size considerably; the largest can have a leg span of up to 34 centimeters (13 inches).

Technique

These images were created using confocal microscopy.