Spider Molting: How Spiders Shed Their Exoskeletons

A spider lying on its back is not dead. It's molting. This misunderstanding sends more than a few tarantula owners into a panic every year, and it reflects how unfamiliar most people are with one of the most fundamental biological processes in arachnid life. Molting is how spiders grow, repair structural damage, and in some cases replace lost limbs. Every spider alive has done it many times.

For a comprehensive overview, see our Complete Guide to Spiders.

What Is Molting?

Spiders, like all arthropods, have an exoskeleton: a rigid external structure that provides support and protection but cannot expand as the animal grows. To increase in size, spiders must periodically shed this exoskeleton entirely and produce a new, larger one. This process is called molting, or ecdysis.

The shed old skin is called an exuvia (plural: exuviae). It is a complete, hollow cast of the spider in its previous form, including legs, chelicerae, fangs, and spinnerets. Exuviae found in corners or on web surfaces are frequently mistaken for dead spiders. The real spider nearby will be soft, pale, and temporarily unable to defend itself.

Molting is not optional and cannot be interrupted midway without consequence. A spider that encounters problems during the molt, whether from dehydration, inadequate humidity, external disturbance, or nutritional deficits, may fail to complete the process and die.

How Often Do Spiders Molt?

Molting frequency depends on the species, the life stage, and environmental conditions. General patterns hold across most species:

• Spiderlings and juveniles: Molt frequently, sometimes every few weeks, as they grow rapidly after emerging from egg sacs.
• Subadult spiders: Molt at longer intervals, typically one to three months depending on species and food availability.
• Adult females of most species: Continue to molt periodically throughout their lives, though frequency decreases with age. Molting allows females to repair damage and, in some species, replace worn reproductive structures.
• Adult males: In most species, stop molting once they reach sexual maturity. This is one of several reasons why males have significantly shorter lifespans than females.

The broader spider life cycle, from egg to adult, is well covered in our spider life cycle guide. Tarantulas are instructive examples because their molts are more easily observed in captivity than those of most wild species. Young tarantulas may molt five or six times in a single year. Adult females of long-lived species such as the Mexican red knee (Brachypelma hamorii) may molt only once every two to three years.

According to UF IFAS, molting frequency is strongly influenced by temperature and prey availability. Spiders maintained in warmer conditions with abundant prey grow faster and molt more frequently than those in cooler, food-limited environments.

The Molting Process: Step by Step

The complete molting event spans several days to weeks from early preparation through full recovery.

Pre-Molt Signs (Days to Weeks Before)

A spider approaching a molt shows recognizable behavioral and physical changes that become easier to identify with experience:

• Reduced feeding or complete food refusal, often the earliest reliable sign in captive tarantulas
• Darkening of the abdomen, visible through the cuticle as the new exoskeleton begins developing beneath the old one
• Construction of a silk mat or sealed retreat, particularly common in mygalomorph spiders such as tarantulas
• Increased time spent stationary in one location, often upside-down on the enclosure wall or lid

The Molt Itself

The spider lies on its back or side and may appear entirely dead. The old exoskeleton splits along the carapace (the dorsal surface of the cephalothorax), creating an opening through which the spider slowly extracts itself. This process typically takes 15 minutes to several hours depending on species size.

The spider pumps hemolymph (blood) into its body, increasing internal hydraulic pressure to help crack the old exoskeleton open and push the new, softer body through the split. This is why humidity is so critical during molting: inadequate moisture makes the old exoskeleton brittle, effectively trapping the spider inside it.

Post-Molt Recovery

After extraction, the spider is soft-bodied (the technical term is "teneral") and highly vulnerable to physical injury and desiccation. The new exoskeleton requires time to harden through a chemical cross-linking process called sclerotization.

• Small house spiders may harden within a few hours
• Large tarantulas may remain soft for one to three weeks post-molt

During this recovery window, the spider should not be offered live prey. Crickets and other live feeders left in an enclosure have been documented killing post-molt tarantulas that were too soft to defend themselves.

Limb Regeneration During Molting

One of the more remarkable properties of spider molts is the capacity for limb regeneration. A spider that has lost a leg, whether to a predator, an accident, or voluntary autotomy (self-amputation to escape capture), can regenerate that limb over subsequent molts.

The regenerated limb emerges at the next molt as a smaller, paler version of the original. It grows progressively closer to normal size with each additional molt. Complete regeneration typically requires two to four molts depending on species and the age at which the limb was lost. Younger spiders with more molts ahead of them regenerate more completely than older individuals.

According to the AMNH, limb autotomy combined with molting-based regeneration is particularly well-developed in spiders relative to most other arthropod groups. The ability to sacrifice a limb to escape a predator while retaining full regenerative capacity represents a significant adaptive advantage, especially for species with long lifespans.

Molting Patterns by Spider Type

What to Do If You Find a Molting Spider

Finding a spider on its back indoors or in a captive enclosure is disorienting if you don't know what you're looking at. The correct response is straightforward:

• Do not touch it. Physical contact during molting can tear limbs or rupture the soft cuticle.
• Remove any live prey from the immediate area. Crickets in particular pose a genuine danger to teneral spiders.
• Ensure adequate humidity if the spider is in a captive setting. A lightly misted wall or a shallow water dish helps maintain the moisture the spider needs.
• Wait. Most spiders complete the process successfully if left undisturbed.

If a spider appears genuinely stuck mid-molt, a cotton swab moistened with warm water can be used with extreme care to soften dried membrane around legs or the abdomen. This should be a true last resort, attempted only after the spider has been visibly stuck and immobile for more than an hour.

Learning about baby spiders helps put early molting patterns in context: spiderlings emerge from the egg sac and begin molting almost immediately, with the frequency that characterizes their rapid early growth.

In my 15 years in pest management, I've received calls from alarmed tarantula owners who were convinced their spider had died overnight, only for them to send me a photo of a freshly molted spider sitting beside a perfect, intact exuvia. The spider lying on its back is almost never actually dead. The best advice I can give is to leave it alone, come back in a few hours, and resist the urge to intervene.

Molting is one of the more dramatic biological processes that happens in ordinary homes, whether in a tarantula enclosure or a dusty garage corner. Understanding it removes the alarm and replaces it with something considerably more useful: genuine appreciation for what arthropod biology can do.

Main Causes

Indoor spiders activity reflects two drivers — a hospitable indoor environment and a sufficient supply of insect prey. Spiders enter through gaps under doors, around windows, utility penetrations, and any opening leading to attics, basements, garages, or crawl spaces. Once inside they settle wherever undisturbed corners, low light, and easy prey access converge. Cooler weather pushes outdoor species inside in late summer and fall as they seek mating sites or shelter. The most important upstream driver is the indoor insect population — homes with active fly, gnat, moth, or other pest activity sustain larger spider populations than homes without prey. Cluttered storage areas, accumulated webbing, and outdoor lighting that draws nocturnal insects all amplify the indoor pressure.

How to Identify

Identification matters because risk and control differ significantly by species. Most household spiders — cellar spiders, common house spiders, jumping spiders, wolf spiders — are harmless and beneficial. Two species in North America warrant caution: the black widow with its shiny black abdomen and red hourglass marking, and the brown recluse with its violin-shaped marking and uniform tan-brown coloring without leg banding. Check webs for shape and structure: tangled cobwebs in corners indicate cellar or common house spiders; funnel-shaped webs near ground level indicate funnel-web species; sheet webs across grass are usually grass spiders. Single sightings without webs are usually transient outdoor species and do not indicate an infestation.

Risk and Severity

From a pest management standpoint, discovering spider exuviae (shed skins) indoors is informative rather than alarming for most species. An intact exuvia confirms that a spider has successfully completed at least one molt in that location, indicating stable long-term harborage. For harmless house spiders, exuviae in corners or basements require no treatment beyond standard cleaning. The concern escalates with species identification: a black widow exuvia found in a garage — recognizable by the distinctive globular abdomen proportions — confirms an established population. Per CDC guidance, black widow venom is medically significant and professional intervention is warranted when the species is confirmed. A freshly molted spider of any venomous species is temporarily defenseless and may strike when contacted in a confined space, since it cannot flee. Consult a physician immediately if a bite from a suspected black widow or brown recluse occurs. Multiple exuviae in one location indicate repeated use of that harborage over an extended period.

Solutions and Actions

For most spider species the goal is removing webs and reducing prey rather than chemical treatment. Vacuum or sweep down all visible webs weekly, including egg sacs, in garages, basements, attics, eaves, and exterior corners. Reduce indoor insect populations by maintaining screens, sealing entry points, and addressing any active pest issue — fewer insects means fewer spiders. Apply a residual insecticide barrier to the foundation perimeter, around windows and doors, and in eaves to deter newly arriving spiders. For confirmed black widow or brown recluse populations in storage areas, use professional pest control, wear long sleeves and gloves when handling stored items, and shake out shoes and clothing left in garages or basements. Single sightings indoors without webs are usually transient and need no chemical response.

Prevention

Preventing spider molting problems is a function of preventing spiders from establishing long-term harborage in the first place. Exuviae found repeatedly in the same location indicate a spider has used that site across multiple molt cycles — eliminate the harborage by removing clutter, sealing gaps, and reducing prey insects. Regular vacuuming of baseboards, corners, and storage areas removes both spiders and cast skins before populations grow. In garages and sheds where black widows commonly shelter, quarterly inspections and removal of visible webs and exuviae interrupt the reproductive cycle. Reduce clutter that creates the undisturbed, sheltered conditions supporting long-term spider residence. Per UC IPM, sticky traps placed in storage areas and along baseboards provide early detection of established populations before multiple molt cycles have occurred. For captive tarantulas, maintain appropriate humidity — typically 60 to 80 percent depending on species — to prevent fatal molting complications.

Frequently Asked Questions

How do I know if my spider is dead or molting?

A molting spider lies on its back or side and may be completely motionless for extended periods. Gently touch a leg with a soft object: a molting spider will react slowly or twitch, while a dead spider will not respond at all. The presence of a shed exuvia nearby is the most definitive evidence of a recently completed molt.

Can a spider die during molting?

Yes, molting is an inherently vulnerable process. Spiders can die mid-molt from dehydration, physical damage from disturbance, nutritional deficiencies that weaken the new cuticle, or predation during the teneral period. In captive tarantulas, inadequate humidity is the most common cause of molting mortality. In wild spiders, the primary risk is predation while soft.

Should I remove the exuvia after my spider molts?

You can leave it or remove it without harming the spider either way. The exuvia has no nutritional value. Some keepers leave it in the enclosure since the spider will sometimes rest against it during the hardening period. Remove it if mold develops or if you suspect it may be attracting mites or other pests.

What should I recheck first for spider molting?

Recheck the exact place, timing, and repeated signs connected with spider molting before changing your plan. A single sighting or old web can mean something very different from fresh activity in several rooms. Confirm whether insects, clutter, moisture, gaps, or stored items are supporting the issue, then match the response to what you actually found.