Eight Things That Are Not Spiders

Eight legs does not mean spider. Arachnida contains eleven orders and spiders are one of them, so a great deal of what gets called a spider is a relative rather than a member.

The test

Two questions separate spiders from most of the rest.

How many body segments? A spider has two — a cephalothorax and an abdomen — joined by a narrow waist. Harvestmen, mites and ticks have one fused body with no visible waist.

Is there silk? All spiders have spinnerets and produce silk. Nothing else on this page does.

Adding a third: spiders have venom glands and fangs. Most of the animals here do not.

Harvestmen (Opiliones)

Long thin legs, a single oval body, and no waist. Two eyes on a small turret, not eight.

No silk. No venom glands at all. They eat detritus, fungi, decaying material and small invertebrates, and they are not predators in the spider sense.

They are called daddy long-legs, and so are cellar spiders and crane flies, which is how the most persistent myth in the subject got started — the claim that the most venomous spider in the world cannot bite you. It fails for all three animals, for a different reason each time, and the daddy long-legs myth works through each.

Camel spiders (Solifugae)

Present in the desert southwest. Fast, pale, with genuinely enormous chelicerae relative to the body — they can be a third of the total length.

No venom. No silk. They kill with the jaws.

Almost everything circulated about them is false: they do not reach half a metre, do not chase people, do not run at the speed of a car, do not scream, and do not eat camel stomachs. The photographs responsible for the reputation were taken with the animal close to the lens.

They are worth a look if you find one. The jaws are remarkable.

Whip scorpions and whip spiders

Vinegaroons (Thelyphonida) — southwestern US, dark, with a thin whip-like tail and heavy pincers. No venom. They spray a concentrated acetic acid mixture, which smells strongly of vinegar and stings in the eyes.

Tailless whip scorpions (Amblypygi) — flat, with spined grasping front limbs and one pair of very long thin antenna-like legs used as feelers. No venom, no silk, and entirely harmless. Common in tropical and subtropical caves and buildings, and alarming out of proportion to anything they do.

Ticks and mites (Acari)

One body segment, no waist, eight legs as adults — six as larvae, which is why a larval tick is regularly assumed to be an insect.

These are the arachnids that actually matter medically, and not because of venom: ticks transmit Lyme disease, anaplasmosis, babesiosis, spotted fever and more. Removal within 24 hours substantially reduces transmission risk for several of them.

A tick embedded in skin is the arachnid encounter worth acting on, and it is the one that gets least attention relative to spiders.

Scorpions

Obviously distinct, and included because the body plan is a useful comparison: pincers, a segmented tail with a stinger, two body segments like a spider but with the abdomen drawn out into the tail. Venom in the tail rather than in fangs.

In the US, only the Arizona bark scorpion is medically significant.

Pseudoscorpions

Tiny — 2–8 mm — with pincers and no tail, found under bark, in leaf litter and in houses, often in old books. They have venom glands in the pincers and are entirely harmless to people.

They are also completely beneficial indoors, eating dust mites and booklice, and they are one of the more pleasant things to find under a microscope.

Woodlouse spiders

Included because they are a genuine spider that people assume is not, on account of the enormous forward-pointing fangs and the plum-coloured body. Dysdera crocata eats woodlice, needs those fangs to get through the shell, and is harmless. It has six eyes, which puts it briefly on the recluse suspect list until anything else is checked.

Why it matters

Beyond accuracy: the response differs. A tick needs removing and watching. A spider generally needs leaving alone or moving. A harvestman needs nothing at all.

And identifying the group first is what stops an alarming-looking arachnid becoming a recluse report, which is where a very large share of them end up.