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What is a spider?

Most people think they know the answer to this question, but when challenged they usually say more about what a spider is not. Like, it’s not an insect, or not what you want to find in your lunch. So here’s my attempt to say what a spider really is.

Golden Orb Weaver

Yep, that’s a spider. Or more specifically, a Golden Orb Weaver (Nephila plumipes)

A spider is an eight-legged critter that eats other critters. Where the term ’spineless’ is considered a bit of an insult for humans, among spiders it invites a sense of belonging because they don’t have a spine at all. Instead, they carry their squishy bits around inside a hard armour-like skin called an exoskeleton.

Now, apart from the bit about having eight legs I could have been describing a lot of other critters, including several insects here, so I’ll now mention some more things that make them different from insects.

The differences between and insect and a spider

You’ll see in the cartoony pictures above that the spider doesn’t have wings. That means spiders can’t fly, although some of them do manage tricks which allow them to either glide or be carried by the wind. You’ll also see that the insect has its body divided into three main bits (head, thorax and abdomen) while the spider has only two bits. That’s because the spider’s head and thorax are combined into one section called a cephalothorax.

Red Velvet Mite

Not a spider: This little critter has eight legs like a spider, but it’s not a spider. My guess is it’s a Red Velvet Mite. This one was about 3 mm long. Mites are closely related to spiders.

And here’s another thing about spiders: they’re not able to chew their food.

Wolf Spider shedding its skin

Wolf Spider shedding its old skin

Outgrowing their skins

I mentioned earlier that spiders wear a tough skin called an exoskeleton to hold their soft bits together. That system works well for spiders but it has its limitations too. The exoskeleton is not able to keep expanding as the spider inside it gets bigger. So as a spider grows up it sometimes has to produce another skin and shed the old one. The picture at right shows the discarded skin of a Huntsman spider.

Discarded spider skins are more interesting than most people realise. If you look at the close-up picture alongside it you’ll see it’s covered with lots of stuff like hairs and spines. So the spider sheds a lot of stuff with that skin. Of course I made sure there wasn’t a spider wearing the skin before I picked it up. When you see the size of its fangs you’ll understand why.

The bits that bite

Spiders have fangs which can inject venom. Despite that fact, most spiders are not dangerous to people. However, because some spiders can be dangerous I advise people to treat all spiders with caution, especially if you have trouble knowing which ones are the most venomous.

Venomous or poisonous?

You might have noticed that I said that spiders inject venom from their fangs. That’s why spiders are described as venomous instead of poisonous.

Now I could get into a bit of trouble here because this subject is complicated, but a rough guide goes something like this: something is considered poisonous if it’s dangerous to eat it. For example, many types of wild mushrooms are poisonous.

And a critter is considered venomous if it puts venom into you by either biting or stinging.

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