I’m going to go out on a limb and say that you’ve probably seen a spider before. Well, that is, unless you’ve been living your entire Earthly existence on the barren, ice-blasted wastes of the Antarctic interior…in which case, I don’t have conversations with murderous, shape-shifting aliens, kthnxbai. Spiders comprise the largest and most successful order of arachnids, with nearly 46,000 species living everywhere from rainforest canopies, to scorching desert sands, to pitch black caverns that wind miles underneath the ground. They are exceedingly varied in their lifestyles and habits. Some can glide like a frisbee from high up in trees. Some slink along the surface of standing water, and nab minnows like some kind of mini, eight-legged, catfish noodlin’ Jesus of Nazareth. Some specialize in the most dangerous prey…other spiders…and net them with a toxin-laced phlegm that they spray from their mouthparts.

Yes, with all their diversity and ubiquitousness in both ecological and human cultural settings, spiders are unquestionably the flagship arachnid group (although many other fascinating orders exist; see the first entry in my lesser-known arachnids series for information on the Class Arachnida in general). Whether we love them or hate them, spiders, as a group, are familiar to us, and many people are aware that spiders come in a huge range of shapes and sizes. For example, it is no secret that black widow spiders (various Lactrodectus species) are among the more dangerously venomous species in North America….or, that the fuzzy, hamster-sized tarantulas (family Theraphosidae) are the biggest spiders on Earth. But what is much less widely known is that far more than size and venom differ between tarantulas and black widows.

While it’s perhaps instinctual to think of spiders as existing in a big monolith of vague “spideriness” with minor tweaks on a common, eight-legged theme, spiders actually divide up into distinct groups with important biological differences, the result of deep evolutionary rifts that stretch back hundreds of millions of years. The truth is that there are several very distinct branches of the spider family tree.

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