Let’s see, what do we know about spiders… They have eight legs right? They’re not insects, but they are sort of related to insects. They’re sometimes called arachnids, but not all arachnids are spiders. We call this a spider. But, we also call this a spider, and they don’t seem to have much in common… And then sometimes we call this a spider (because it looks like one right?), and it isn’t a spider at all! So the question remains: what does it truly mean to be a spider?
Spiders, or organisms in the class Araneae, are certainly highly variable, but they all have a few things in common. First, though, let’s take a look at their higher taxonomic classifications. All spiders are part of the phylum Arthropoda (within the kingdom Animalia, of course), meaning that they are invertebrates with a segmented body, an exoskeleton, and jointed appendages. Spiders hold these features in common with other organisms, including insects, millipedes, and shrimp, that are all classified as Arthropods, as well.
From here, we find that spiders split from these other organisms into a subphylum called Chelicerata. The three groups of organisms in this category (arachnids, sea spiders, and sea scorpions + horseshoe crabs) have a number of features in common, but the name for this subphylum comes from one main feature: the presence of chelicerae. Chelicerae are, essentially, appendages near the mouth of an organism, that are used as fangs or pincers. These are similar to other Arthropod mouthparts, but mostly differ in their morphology/shape.
Arachnids are the largest class in the Chelicerata subphylum, and includes some well-known organisms like spiders, mites, ticks, and scorpions, as well as other lesser-known organisms such as whipscorpions and Harvestmen (The Other Arachnids). They are primarily distinguished from other chelicerates by their four pairs of legs, and one main body division between the cephalothorax and the abdomen.
Finally, we get to the order Araneae, which consists of 40,000+ species of spider. One feature that distinguishes spiders from other arachnids is their thin “waist” where the abdomen meets the cephalothorax (see my previous post on Daddy Long Legs for an example of this). Another is the presence of spinnerets, and thus the ability to produce silk (though not all spiders spin webs). In addition, like I’ve explained before, most spiders produce venom, with the exception of one or two families.
Well, there you have it folks. While there are other obvious and not-so obvious differences between all of these different organisms, it’s sometimes helpful to know the defining feature of a group. Now you can all sleep better at night, I’m sure…