That’s No Spider!
Eeek! Giant spiders have taken over the roof of the Seattle Center Armory, terrorizing visitors to the Space Needle!
… Or maybe not. A Mental Floss blog post yesterday includes this photo of the trompe l’oeil painting created by artist Marlin Peterson:
The painting is beautiful and well-executed and slightly creepy and awesome. But the blog got one important point wrong: the giant painted critters aren’t spiders at all. They’re daddylonglegs, also known as harvestmen. Like spiders, daddylonglegs are arachnids, but they’re classified in an entirely different taxonomic order (Araneae) than spiders (Opiliones). In other words, spiders are as about different from daddylonglegs as scorpions or ticks—which is to say, pretty different!
As arachnids, both harvestmen and spiders have eight jointed legs. But spiders can spin silk; harvestmen can’t. Perhaps the most obvious difference to a human-sized observer is that spiders have two main body segments (the cephalothorax and the abdomen), where in daddylonglegs there’s just one. You can see this beautifully in Peterson’s painting. Look closely at that neat oval body. Then look more closely, and see how many eyes you can count. Daddylonglegs have just two, while spiders come equipped with eight eyes.
The eyes are important for spiders because they predatory. (There is one known exception, a recently-discovered jumping spider dubbed Bagheera kiplingi. Presumably it was identified by a Jungle Book fan with a sense of irony. Bagheera was a leopard. Bagheera kiplingi is mostly vegetarian.) The usual spider feeding M.O. involves biting prey, injecting venom with the fangs to paralyze it. Then comes the unnerving bit. The spiders digest their prey externally, often by injecting it with enzymes that turn it into a slurpable mess of gooey liquid. It’s as if spiders carry their own portable blenders, set to pulverize an insect meal into a nice buggy smoothie.
Harvestmen, on the other hand … well, you can probably guess from the name. Most species are omnivores or scavengers, feeding on fungi, plant matter, decaying organisms, and small insects. Unlike spiders, daddylonglegs can digest solid food. They feel their way through their environment, tapping with their second pair of legs, longer and more sensitive than the others.
So if they’re so different, why do people so commonly think that daddylonglegs are spiders?
Perhaps part of the confusion stems from the fact that there is a group of true spiders called the daddy long legs spiders, often also known as the cellar spiders. I’ve always thought of them as “bathroom spiders,” myself, because that’s where I always find them, hanging out in the corners. I’ve always had a grudging truce with my bathroom spiders. I prefer that they keep away from me, but I figure that, if they’re catching enough bugs to stay alive, then they’re doing me a favor. The cellar spiders have similar proportions to harvestmen, with bodies about the size of a pushpin head and long, spindly legs.
While I tolerate bathroom spiders, I positively delight in daddylonglegs. Spiders stalk or ambush. Daddylonglegs glide. They hold themselves lightly on their feet and move effortlessly and springily over obstacles. Since I was a child, I’ve had a habit of scooping up daddylonglegs into my hands when I find them. They always scoot across my hands with the grace and ease of a Latin dancer.
Lots of people hate spiders. I’ve yet to meet anyone who hates a daddylonglegs. In fact, Marlin Peterson commented on his blog that he deliberately chose to depict Opiliones in order to avoid freaking out arachnophobes. The man knows his stuff. Too bad that Mental Floss couldn’t tell the difference.