Let’s talk about spider anatomy, shall we? You probably remember from high school biology that they’ve got eight legs, eight eyes, two body segments, and are horrible. This is all true. Mostly. (The little monsters from yesterday that go around spitting on everything only have six eyes, for example.)
They don’t have antennae, though they will occasionally pretend to if they’re trying to trick things into thinking they’re ants.
Above: The Kerengga antlike jumper (Myrmarachne plataleoides) being kind of a dick about things.
The cephalothorax (cephalo:head::thorax:torso) is where they keep their venom glands, and their abdomen is where they keep their silk glands. Silk is dispensed through the spinnerets, with the scytodids having another batch of them in with their venom glands because everything is awful. The legs are referred to as the first legs through the fourth legs, with the first legs being those closest to the fangs and the fourth legs being the closest to the spinnerets. So the jumper up there is using her first legs to mimic antennae, and most spiders will use their third legs to hold onto prey while wrapping with their fourth legs.
Eye arrangement is so variable that it’s typically described in terms of location, with eyes that are some combination of anterior (front) or posterior (back) and lateral (side) or median (middle).
What you’re looking at in that top picture is the dorsal (back) view.
Think “dorsal fin.”
The belly view is called the ventral view.
Above: Brown widow (Latrodectus geometricus). Words cannot adequately describe how not-down I am with brown widows. The wretched things spin little tents and hide in them during the day.
This isn’t something you normally have to worry about, but if you’re trying to figure out what the horrible spider hiding in your shower stall is, this will come up. Like, if you’re seeing “dark brown violin-shaped mark on the dorsal side of the cephalothorax,” in your little guide, you know you’re looking for this:
And also that whoever decided brown recluses’ marks look like a damn violin was high. If you’re looking for “red hourglass on the ventral side of the abdomen,” see above.
Spider-sex is complicated by the female’s reproductive tract (in purple) hanging out more toward the anterior portion of her little spider body. Male spiders use their palps (or, more specifically, the palpal bulb at their tips) as a secondary sex organ to inject sperm into the females’ receptacle.
Above: Spider balls.
As you may have noticed, the palps are on the male’s face, which leaves him frequently just waving his abdomen right in the female’s general fang-region. It doesn’t help that female spiders, like a lot of other female arthropods, have basically evolved little sperm cellars in their abdomens, so they don’t need to keep mating in order to keep producing viable offspring.
You see a lot of strategies to get around this kind of risky behavior, from prenuptial gift-giving (the female is probably not going to ignore a preferred prey item to eat the male) to being a teeny-tiny and thus stealthy little thing in comparison.
Above: Orb weavers are hilarious.
And now you know more about spiders than you ever wanted to! Congratulations!
[Top image from here]