A spider in our biggest terrarium molted this week. It took all day to do it.

Like other arthropods, spiders have their skeleton on the outside, in the form of hard plates. To grow, the spider has to shed its small skin and then expand while the new skin underneath is still soft. But getting the old skin off ain’t at all easy, since it fits quite tightly.

The legs are a particular problem — if you’ve ever tried to take off tight-fitting gloves and had the fingers turn inside out you will begin to understand the spider’s situation. It has eight long, jointed legs that must be carefully drawn out, like fingers from a tight glove, from the old exoskeleton. Break off a leg and it has to make do with seven until the next molt, provided there is a next molt.

To make this delicate and dangerous process as easy as possible, the spider hangs upside down to get the help of gravity. The old skin splits away on the body, but the spider then spends hours extracting its legs.

We think this spider is Steatoda grossa, sometimes called the “false black widow.” When it first emerged, it was pale and translucent. After some time in the air, its color darkened and it went on its way.

These aren’t the best photos ever, but I had to improvise. I took them with my phone using a hand held jeweler’s loupe for magnification.



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