It is most apparent that a spider's legs get longer after molting. Here are two photos I took of a spider in 2011. Both photos are of the same spider, taken two days apart. The bottom photo is of the spider prior to molting, and the top is of the spider after molting. The measurements given are the lengths of the first legs. The two photos are proportioned correctly relative to each other, so you can visually compare the before and after sizes.
The spider is a male Mecaphesa dubia, which is a kind of crab spider (family thomisidae). In most spiders, the legs get just a little bit longer with each molt, but in male Mecaphesa crab spiders the front legs instantly become very long legs upon molting into adulthood. This spider's front legs became 60% longer in a single molt. I'm not sure how it helps them to have such long legs, but I can attest that the adult males can jump high and far to get away from danger. The long legs may also help the spider to keep a grip on the female while mating.
Mandy Howe, a highly knowledgeable "amateur" arachnologist and the spider smarts behind the public resource www.spiders.us, offered this explanation back in 2011:
I think the new exoskeleton is folded up inside the old one, then when the old is shed off, there is a period when the new exoskeleton is rubbery and expandable... the spider pumps hemolymph into it and expands itself to full size, completely filling out that new exoskeleton that grew all curled up under the old one.... at least that's how I think it happens.
Since 2011 I have seen spiders molt many more times. It wasn't until I watched a large spider molt that I could see much of what was going on. I have now seen Mandy's explanation in action. I photographed a large Holconia immanis huntsman spider molting, which you can see in this photo series. Here is one photo from the series:
Here is how a spider gets bigger by molting:
- The spider first fattens up its abdomen. A spider's abdomen does not have a hard exoskeleton like most insects do. Instead, it is soft and expandable. The more the spider eats, the bigger the abdomen can get.
- The outer exoskeletons separates from the newly formed inner exoskeleton. When this happens, the spider becomes inactive. The exoskeleton also tends to darken when this happens, at least in crab spiders. Notice how dark the legs are in the above photo of the Mecaphesa prior to molting.
- The spider hangs from a thread. Not all spiders do this, but the araneomorphae that I study do. Tarantulas do not hang from threads, and I don't know the particulars of their process.
- The spider pumps hemolymph from the abdomen into the cephalothorax. "Hemolymph" is just bug blood. The "cephalothorax" is the spider's head. A spider's legs are on its head.
- The pressure from the hemolymph in the cephalothorax causes the top of the cephalothorax—the carapace—to lift off. In particular, a seam at the back of the carapace breaks, while the front of the carapace (around the eyes) remains attached and becomes a hinge. You can see the carapace hinged forward in the above photo of the Holconia immanis.
- The spider pulls out of the old cephalothorax while continuing to pump hemolymph into the cephalothorax and legs. In a big spider, such as the one above, you can see a pulsing of the pumping action, although this could also be the spider regularly convulsing muscles to pull free. New, soft legs pull out of the old legs. Because the new cephalothorax and legs are soft, pumping hemolymph into them causes them to expand. Hanging from a thread allows gravity to assist the spider in pulling free.
- Once free of the old exoskeleton, the spider hangs limp, with the legs below it, presumably to allow the spider to continue pumping hemolymph into the legs to expand them to full length.
- The spider then repeatedly flexes and extends the legs, presumably to mobilize the joints.
- Finally, the spider takes a normal spider pose and waits for the new exoskeleton to harden up. The soft exoskeleton is normally light or pale, and it darkens as it hardens. During this phase, the spider is prone to being eaten by predators, because it can't readily defend itself. I have to be careful not to let crickets near a molting spider, because the crickets may eat it all up.
You can see many of the above steps in the photo series I posted. At the end of this process, the spider has longer legs and a bigger cephalothorax, but because it has been pumping hemolymph from the abdomen into the cephalothorax, the abdomen is much smaller. As you can see in the before and after photos of the Mecaphesa celer, above, the abdomen is much smaller after molting. Notice also that not only did the Mecaphesa's legs get longer, but its cephalothorax got bigger too.
This post is preparation for my next post, which will be about how spiders regrow lost legs.
(Some notes: Because I don't study tarantulas, I don't know how their process differs. I do know that they often molt by lying upside down, not from a thread. Tarantulas also molt once per year as adults. I don't know if this is true of mygalomorphae in general, which is the group to which tarantulas belong. Also, butterfly caterpillars molt periodically in a similar but less spectacular process.)