Updated: Jul 1, 2020
Why do jumping spiders molt?
Jumping spiders molt to grow. They have to shed their exoskeletons in order to get bigger. Adult jumping spiders do not molt, hence their last molt takes them to maturity.
How do I know if my jumping spider is about to molt? What are the signs of premolt?
Before juvenile spiders molt, they will enter a dormant period called "pre-molt". A spider in premolt makes a very thick opaque nest that is different from their regular sleeping hammock. If disturbed during premolt and having to make a new web, their new premolt web will be more see-through and more like their regular resting retreat. Other signs also point to premolt behavior, such as a very fat abdomen and not eating. Some spiders may wander about for a bit, but will go back into their nest for the majority of the time. They usually refuse food offered to them.
Here's what a premolt web looks like for a small spiderling, note that the web is very thick and lush:
Here's what a thinner resting retreat looks like; it is very thin and see-through:
Here's the same spider after molting, shown with her molting/premolt nest and the old exoskeleton:
Sometimes jumping spiders will go into what may look like a false premolt. They build a thick nest and are more reclusive. However, they won't molt yet, and will still eat. Hence, I always still continue to offer harmless prey like flies that can't burrow, hide very well, or bite.
How do I care for a jumping spider in premolt?
Humidity is very important for spiders in premolt. You want to make sure there's a bit of substrate, such as coco fiber, in the enclosure, to hold humidity. This helps them molt easier. You should also mist finely onto their nest, or right beside their nest, to offer them a chance to drink if they need to.
Most spiders do not eat during premolt. Except for my P. otiosus. They seem to begin to build a thick premolt nest, but will still eat for a few days until they're actually in premolt. They will then refuse food offered. Therefore, I always make sure there's a fly or small prey (that can't damage their premolt web or the spider during the delicate process of molting), even when the spider is in premolt.
The majority of the prep work for a molt is done beforehand in what they eat. A juvenile spider needs to eat sufficiently to be hydrated enough to withstand being in premolt and molting.
How do I know if my jumping spider molted?
You can tell if a jumping spider has molted when it looks like there are two spiders in the nest - one of which is the old exoskeleton. It helps to hold or shine a light behind the nest to see. Some spiders will push their exuvia out of the nest after molting. (This is a very thin premolt/molting nest, and not really the standard. This spider was moved shortly before molting, hence a more lackluster premolt/molting web).
How soon should I handle and feed my jumping spider after they've molted?
Jumping spiders are very delicate after a molt. Their new exoskeletons need time to harden. If you've accidentally disturbed a newly molted jumping spider, you'll see their chelicerae are are dull and their legs look almost transparent. Once they've regained color and luster returns to their chelicerae, they are ok to handle.
The larger the spider gets, the longer it takes to harden and dry. Young spiders to smaller juveniles may eat within the day or the next day after molting. Sub-adults and adults may take up to 3 - 4 days to eat again. If spiderlings/young juveniles take more than 2 days to eat, or if adults take longer than 4 days to eat after molting, I will do what I can to stimulate appetite, (more about that here).
How often do jumping spiders molt?
There isn't a set time between molts. Immature jumping spiders will molt as often as their growing speed, to an extent, until adulthood. How often they eat will determine how fast they grow, and hence how often they molt. The bigger, closer to maturity, the spider gets, the longer the stretches between molts. It's not unheard of for a small sling to molt once a week, until they get bigger.
What are instars, L1, L2, L3..., larval stadiums?
Scientists label invertebrates as instars or larval stadium numbers (L1, L2, L3, L4 so on so forth) based on how many molts they've had. Hobbyists and breeders have picked that up as well. However, it is somewhat problematic due to inconsistent definitions of when the 2nd instar/L2 begins. Second instar is more commonly known as when the spider first emerges from the nest, capable of life outside. A first instar would then have molted their egg membrane, but has immature eyes, claws, mouth, and cannot survive outside the nest. But, there are also many that believe the spiderlings emerge as first instars, hence creating a discrepancy.
The general spider buyer also usually has no concept how big a 4th instar jumping spider is (it's tiny, fruit fly sized). And many buyers purchase a 4th/5th instar thinking they're getting a decent sized spider, but instead receive a tiny spiderling that is rather fragile to care for. Furthermore, a 4th instar of one species, or one spider, may not be the same size as another 4th instar.
My personal preference is to label spiders by the prey size they are eating, sub-adult (if I can make an educated guess), or adult.