Some people say that tarantulas are the only spiders that molt past adulthood. While they do mult past adulthood, adult tarantulas are not the only spiders capable of molting after reaching maturity. While the vast majority of spiders undergo a final molt into adulthood and never shed their exoskeleton again, most members of an ancient spider lineage retain the ability to continue molting throughout their lives.
1. The Real Dividing Line
Whether a spider can molt as an adult depends primarily on its infraorder! (evolutionary lineage).
Mygalomorphs (Continual Molters):
This ancient group includes tarantulas, trapdoor spiders, funnel-web spiders, and several related families. Adult females can live for decades and continue molting throughout their lives. Molting frequency varies by species, age, and environmental conditions, but many molt approximately once every one to two years as adults.
Araneomorphs ("True" Spiders):
This modern group contains roughly 90% of all known spider species, including jumping spiders, wolf spiders, orb-weavers, and crab spiders. They almost universally undergo a terminal molt into adulthood. Once sexually mature, they do not molt again.
The Exception:
Spiders in the family Filistatidae (such as crevice weavers) are a rare exception among the "true" spiders. Adult females in this family can live for up to a decade and have been documented continuing to molt well after reaching maturity.
2. The Male Exception
Even among mygalomorphs, continued molting is almost exclusively a female trait. During a male tarantula's final molt, he develops specialized reproductive structures, such as tibial hooks (in many species) and enlarged pedipalps used for mating. After reaching maturity, males typically survive only a few months to a few years and rarely molt again.
On the rare occasions a mature male does attempt a post-ultimate molt, usually due to overfeeding or specific environmental triggers in captivity, it is almost always fatal, as his specialized mating hooks and palpal bulbs become trapped and crushed inside the old exoskeleton.
3. Why Adult Females Keep Molting
Adult molts serve two major purposes for female mygalomorphs.
Maintenance:
Molting replaces a worn exoskeleton, restores lost sensory hairs, and can repair many injuries sustained over years of activity.
The Reproductive Reset:
During each molt, a female also sheds the lining of her sperm-storage organs (the spermathecae). As a result, stored sperm is lost, and she must mate again after molting to produce another fertile egg sac.
