Sac Spiders
I’ve got two spiders for you today, that I think are both the same species, but that were found at different times. The first was caught near our porch light on June 13, 2015, and looks to be a fairly substantial male spider (the body was about a quarter of an inch long).
His big pedipalps clearly mark him as a male.
He’s also missing a leg on his right side, but he seemed to get around just fine with only seven.
The second one we found over a month later, on July 20, 2015. While it was in the house at the time, it was our own fault – I carried it in with a bucket of mulberries[1]. When I picked it up, it rappelled off of my finger on a silk thread, so this first shot is of it hanging in mid-air.
Since it is smaller than the first one, and has a paler carapace, I think this one is immature. And the thin pedipalps suggest that she is a female. If she isn’t the same species as the male, she is at least a close relative.
The eye pattern is the same for both spiders: a row of six eyes, with two up above that are the same size as all the others.
The light fuzz on her abdomen also seems to be patchy. Although, that could just be from getting wet when I washed her off the mulberries.
From the eyes, general build, and habits, I believe that these are Leaf-Curling Sac Spiders in the genus Clubiona. These appear to be roaming hunters, not web-spinners, although from what we see with the one dangling from my finger, they obviously will put down a safety line to keep from falling. And since the second one came out of a tree, it is safe to say that they are probably more arboreal than wolf spiders (which I mostly see on the ground). When a female is ready to lay her eggs, she will close herself up in a chamber she makes by rolling up leaves with silk. She then stays in there to guard the eggs (mostly from other spiders of her species, evidently), and dies before they hatch. And when her babies hatch, they eat her corpse. Well, it’s better than being eaten alive, I guess.
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[1] Our mulberries are quite tasty, but tedious to pick one at a time, and the tree is tall enough that the majority of the berries are too high to pick without a ladder in any case. So I harvest them by putting a tarp on the ground, and whacking the branches with a stick. This is really fast (I can get a couple of gallons of mulberries in about 10 minutes), but does tend to collect a fair number of little crawly things that we then have to pick out.
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I wonder why there isn’t more interbreeding between spider subspecies until they’re an indistinguishable, polyglot population.
I think it comes down to three things: location, timing, and sexual selection (and these apply to pretty much all animals, not just spiders).
First the location: different spider species are in very different locations. Some are in the grass, some prefer the leaf litter, some like tree bark, some are up in the tree foliage, some like swamps (and may even skate around on top of the water), and spiders in one habitat really don’t mix that much with spiders in the others. These habitats might seem really close together to us, but to a few-millimeter-long spider getting from one to the other is a major trek. And since different body shapes do better in one habitat than another, they have the incentive to stay in the habitat they are best adapted to. Spiders that wander into the wrong one are likely to get eaten.
Next the timing: spiders have mating seasons, and they don’t necessarily overlap. Species that mate in the fall aren’t going to mix much with the ones that mate in the spring. We have an example of this sort of speciation happening right now, only with crickets instead of spiders: the “fall field crickets” sing in the fall, lay their eggs to overwinter, and then hatch out in the spring, while the “spring field crickets” sing in the spring, have their eggs hatch in summer, and overwinter as nymphs. The split happened so recently that they are practically indistinguishable from each other, and could probably still mate successfully if you just managed to synchronize their mating periods, but as it is they will now diverge more and more over time.
And when we get to sexual selection, there is a matter of geometry. Because of the way spider sex works, the male has to get his pedipalp to match up with the female epigyne, and if it doesn’t fit, mating doesn’t happen. And the different spider species differ so much in their pedipalp/epigyne geometries that this is one of the main traits used to tell them apart. So, once two species no longer fit together, they are never going to interbreed again. And then there are other sexual traits, such as the exact maneuver the male has to use in approaching the female to avoid getting eaten. Wrong approach, and zip! No mating. So if you get two females that prefer different approaches, they are going to diverge pretty fast.
I realize this is all sounds really simple and obvious, but that’s what evolution is: variations over time (sometimes unexpectedly large variations) due to accumulation of simple, obvious effects.