Trego's Mountain Ear

This past week, some folks in our community Facebook page wanted to know if something was a tick or a spider. The comments section got a bit heated, and the offending post seems to have been censored. There were differing opinions, as there often are on such things, and opinions held with no shortage of confidence.

Here’s how you can actually tell the two critters apart.

Both spiders and ticks are arachnids, and have two body regions – the “cephalothorax” (a combination of old Greek words for “head” and “chest”) and the “abdomen” (same as in English).

In arachnids, the cephalothorax is what all the limbs are attached to, and where the mouthparts and muscles are. The abdomen holds most of the non-muscular organs, and is where breathing and digestion take place.

In spiders, there is a strong pinch between these two body regions – it looks like you could cut the abdomen off with a lasso of dental floss.
In ticks and mites, however, these same two body regions are “completely fused” or “broadly joined”. Functionally, this means it’s hard to see when one part begins and the other ends, much less separate the two.

This is pretty much all you really need to tell the two apart, but there are a few more differences.
Mouthparts:
Spiders have paired fangs (chelicerae, pronounced “Kelly-Sir-Ee”) to pinch their prey and inject venom.
Ticks and mites have mouthparts like hypodermic needles, and their chelicerae have saw-like teeth to help them cut their way through your hide before injecting chemicals to keep your blood from clotting.
Legs:
Spiders (well, uninjured spiders) all have eight legs, even when they are newly hatched.
Ticks, however, start out as larvae with six legs. They gain another pair of legs after their first molt.

Medical Importance (are these dangerous?)

Spiders:
In our area, northwest Montana, we have only one spider of medical significance. That’s the western black widow. Personally, I’ve always found Western Black Widows to be very docile – I’ve known quite a few kids to keep females as pets, even ones with eggsacs. Next time you’re at a rodeo and there are crates as extra seating, flip one over – the odds of finding a black widow are decent.

While we have Hobo spiders (aka “the aggressive house spider”) up here, their venom isn’t anything to get worried about. They move quickly – that’s about the only reason for their bad reputation. They need to move quickly, though – they don’t have much at all in the way of venom, and they don’t make sticky webs, so they rely on speed to catch their prey. If you want to control them, I’d recommend using sticky traps.

Ticks:
There’s a lot more to be concerned about with ticks – while their feeding itself isn’t generally harmful in our neck of the woods, they can transmit harmful diseases in other locations.
Southern Montana is home to several tick-borne diseases you should be aware of.
The American Midwest and East are where you’re most likely to catch Lyme disease. Some west-coast deerticks in coastal regions of Washington and Oregon, can spread it, too.