Male Steatoda Spider

2023 January 1

We found this spider in the bathroom on November 6, 2022. It was a smallish spider, with a legspan of maybe half an inch and a body about the size of a grain of uncooked rice. It is obviously a male, as we can tell from his enormous, hairy pedipalps that look kind of like spiky boxing gloves.

Some of his eyes were also catching the light from my flash and reflecting it back, making it look like they were glowing.

Only two of the spider’s eight eyes are glowing, though. That’s because this is one of the spiders where the posterior median eyes (the two back ones in the center) have a “tapetum”, like a cat’s eyes do. The tapetum is a reflective layer which sits behind the retina. When light comes into the eye, if it passes through the retina without being absorbed and sensed, it then reflects off of the tapetum and goes back through the retina. This gives it a second chance to be sensed, and so significantly boosts the light-sensitivity of the eye. It also makes the eyes light up like beacons if you shine a bright light on them. The downside is that the reflected light gets a bit scattered in the process, and so the image is kind of blurry. The tapetum is therefore a tradeoff between light-sensitivity and image sharpness.

But, spiders have up to eight eyes, so they have the opportunity to both have eyes with tapetums (for seeing in the dark) and without tapetums (for seeing sharply in brighter light). So their anterior median eyes (the two center ones in the front) and the lateral eyes (the ones on the sides) generally don’t have tapetums and so don’t reflect light from camera flashes and flashlights.

In this case, the lack of a tapetum makes the other eyes really hard to see, even in this extreme closeup/, because they are just black on a black background.

They are a bit more visible in this side photo, where the light is coming in from the side and you can see more of the eyes as transparent bubbles.

The other issue was he kept scrunching up his legs over his face whenever he was sitting still. I did get a shot with the legs out of the way while he was scurrying around in the ceramic dish I had him in, but he was moving fast enough that I’m not sure it helped much.

I also got a shot of one of his legs that was good enough to show the hooks on the end. These hooks are what spiders use to hang from their webs, when they are on one.

So, he is definitely one of the cobweb-weaving spiders in the genus Steatoda, and I think he’s most likely a Steatoda borealis. These are one of the most common spiders to find up here in the northern areas. They are a native spider that normally lives in crevices of trees and under rocks and the like, but they really like basements and areas under furniture, too. Completely harmless to humans, of course. Even if they manage to break the skin (which I am inclined to doubt, they aren’t very big), their venom is not particularly toxic to humans, and they don’t have a lot of it to inject in any case.

Anyway, he wasn’t on his web because he was out looking for females. Which is why so many of the spiders I photograph turn out to be male – they are the ones that wander about and get in the house where we can see them, rather than hanging out in safe places outdoors.

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