Knowing whether your tarantula is male or female is one of the more practically important pieces of information a keeper can have, and yet it is also one of the most misunderstood. Males and females of most tarantula species look nearly identical as juveniles, which means guessing based on size, colour, or body shape alone leads to confident wrongness more often than people like to admit. The good news is that there are reliable methods available — one of which is genuinely definitive — and learning them properly removes the guesswork entirely.
Why Sexing Your Tarantula Matters
The difference in lifespan between a male and a female tarantula is staggering enough that the sex of the animal you are buying can determine whether your pet lives two years or twenty. Females of most species outlive males by a factor of four to ten, with female Texas Browns documented past thirty years and female Arizona Blondes recorded at over twenty-five. A male of either species might live three to five years after his ultimate moult before dying of the natural senescence that follows his breeding season. Buying an unsexed tarantula is a gamble on which end of that lifespan you end up with, and for expensive or rare species the stakes of that gamble are significant. Beyond lifespan, sex determines breeding potential, feeding requirements, and the behavioural changes that come with sexual maturity in males.
The Definitive Method: Sexing By Moult
Examining a shed moult — called the exuvia or exuvium — is the gold standard for tarantula sexing and the only method that gives a definitive answer for juvenile and sub-adult specimens. When a tarantula sheds its exoskeleton, the moult includes a cast of the internal reproductive structures that allow sex to be determined with accuracy. Females have spermathecae — paired internal organs used to store sperm after mating — located above the genital opening between the front pair of book lungs on the underside of the abdomen. These appear in the moult as a visible flap, lobe, or set of structures depending on the species. Males lack these structures entirely, and the same area of the moult will appear flat and smooth where a female would show the spermathecal flap.
The practical process begins immediately after the moult is found, ideally before it dries out and becomes brittle. The abdominal section of the moult needs to be gently unfolded and flattened — using a pair of fine forceps, dampening the moult slightly with water helps prevent tearing. The inside of the abdomen is then examined under magnification. A loupe or jeweller’s magnifier works for moults from larger specimens; a stereo microscope gives a much clearer view and is worth borrowing or purchasing if you plan to sex multiple animals over time. Successful sexing from a moult is possible from as early as the fourth moult with microscope assistance, and becomes straightforward with the naked eye once the spider has reached a few inches in legspan and the spermathecae are large enough to see clearly. Different species have different spermathecal shapes, which is relevant when confirming species identification alongside sex — the Arachnida spermathecae database on Arachnoboards is a useful visual reference for comparing what you find to known specimens.
Ventral Sexing: Looking At The Living Spider
If you do not have a moult available, it is possible to attempt sex determination by examining the underside of the abdomen of the living spider — a technique called ventral sexing. The principle is the same: females have visible spermathecae that can sometimes be detected as a slight bulge or shadow visible through the thin abdominal skin, particularly when the spider is backlit or positioned with a light source beneath it. Males lack this bulge and appear flat in the same area.
The honest caveat is that ventral sexing is significantly less reliable than moult sexing, cannot be used on small juveniles whose structures are too small to detect visually, and even experienced keepers have been confidently wrong using this method. The results from ventral sexing on forums like Arachnoboards are often contradicted when the same spider’s moult is later examined definitively. It is better treated as a useful supplement to moult sexing when a moult is not available rather than as a standalone reliable method.
The Epiandrous Fusillae Method
Mature males develop a secondary set of micro-spinnerets called epiandrous fusillae above the genital opening, used to create the sperm web that males produce before mating. These structures can sometimes be detected in the moult of a sexually mature male as a patch of tiny pores or micro-openings in the area where a female would have spermathecae. This method requires a keen eye and a good microscope and is considered an advanced technique rather than a beginner approach, but it is useful confirmation when the spermathecal area is ambiguous — a flat area with no spermathecal flap combined with visible epiandrous fusillae confirms male.
Reading The Mature Male
Once a male reaches his ultimate moult — the final moult of his life — there is no ambiguity whatsoever. Mature males develop a set of completely unmistakable characteristics that make sex determination instant and certain. The pedipalps — the small leg-like appendages at the front of the spider near the chelicerae — develop large, bulbous tips containing the palpal bulbs used to transfer sperm during mating. Many species also develop tibial apophyses, small hook-like projections on the front pair of legs used to hold the female’s fangs during copulation. The body becomes dramatically slimmer and the legs proportionally longer and thinner, built for wandering in search of females rather than sitting in a burrow.
Many species also show dramatic colour changes at the ultimate moult — the Arizona Blonde male turns black with copper colouration, the Johnny Cash Tarantula male becomes entirely jet black, and various other species show shifts that make the mature male look like a completely different animal from the juvenile of either sex. This is both a reliable identifier and a signal that the keeper now has a limited time window — males do not live long after the ultimate moult, and breeding introductions should be arranged promptly if breeding is the goal.
Visual Clues Before Maturity: How Reliable Are They?
Body size, chelicerae width, and general build are often cited as ways to guess sex before maturity or before a moult is available. The general pattern is accurate — females are larger, heavier-bodied, and have proportionally broader chelicerae than males — but these are subtle differences that require experience and several specimens to compare, and they can mislead even experienced keepers. Some male species are close enough in size to females that size alone is useless as a sexing tool. Treating these physical clues as hypothesis-generating rather than conclusive is the appropriate approach — they might tell you which way to lean, but only the moult tells you for certain.
Practical Tips For Successful Moult Sexing
The moult needs to be collected before the spider eats it, which means checking the enclosure after every moult rather than waiting for a convenient moment. A fresh moult is much easier to work with than a dried one, but even dried moults can be rehydrated by soaking in water for several minutes until pliable. The abdominal section is what you need — the rest of the moult can be discarded, and handling the thin abdominal skin with care to avoid tearing is the main practical challenge. Photography through a loupe or microscope and posting to communities like Arachnoboards is a reliable way to get confirmation if you are unsure what you are seeing. Our how to determine a tarantula’s age article provides additional context on the moult cycle and what moult frequency tells you about where a juvenile sits in its development. Everything you need to keep your tarantula well through every life stage is on our best tarantula products page.