How to Sex Male vs. Female Tarantulas

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Sexing a tarantula can be confusing or intimidating at first. Unlike mammals, tarantulas lack obvious external genitalia, and differences between males and females are subtle until maturity. Once you understand the biology and where to look, identifying a tarantula’s sex becomes much easier. Learning this essential skill helps with breeding, growth, or simply understanding your spider better.

This guide explains the most reliable sexing method, clarifies common misconceptions, and shows how sexual dimorphism can change a spider’s appearance as it matures.

Image from nswfmpa.org

What Sexing a Tarantula Actually Means

When we talk about “sexing” a tarantula, we’re referring to identifying whether the spider is biologically male or female. The differences between them are exclusively internal until maturity.

Females possess reproductive structures known as spermathecae and a uterus externus located inside the abdomen. These structures store sperm and allow them to produce viable egg sacs, often for months after a single mating.

Males lack these structures. Instead, mature males develop tibial hooks or spurs in some species, bulbous pedipalps used to transfer sperm, and slender bodies built for traveling in search of a mate.

A juvenile male looks almost identical to a juvenile female. The differences only become obvious after the male’s ultimate molt, when he transforms dramatically into his mature form.

Knowing your tarantula’s sex is helpful for several reasons:

Lifespan
- Females live significantly longer than males. Some females live twenty years or more, while males often live only a few years after maturing.
Growth Rate
- Males grow faster and reach maturity earlier.
Behavior
- Mature males wander more, refuse food, and become restless. Females remain bulkier, calmer, and settle into their enclosures more reliably.
Breeding
- Accurate sexing is essential for pairing tarantulas, preventing accidental male-to-male introductions, and ensuring responsible captive breeding.
Enclosure Planning
- Females reach larger adult sizes and may require more permanent setups. Mature males often do not need long-term, elaborate enclosures.

Image from Ty Dye Exotics

The Most Reliable Method: Molt Sexing

While there are several ways to guess or speculate, there is only one truly reliable way to sex a tarantula, and that is with a molt.

Sexing by molt is the gold standard. When a tarantula sheds its exoskeleton, the molt includes the internal reproductive structures that allow us to determine sex with accuracy.

Females will show a visible flap or set of lobes inside the abdomen portion of the molt. These structures are the spermathecae and the uterus externus.

Males will not have this flap. Instead, the area will appear flat and smooth or may show only a simple fold with no internal structure.

This method is accurate even in juveniles, long before mature male traits appear. However, molts can be delicate, curled, or damaged, and learning to read them takes some practice.

Step-by-Step: How to Sex a Tarantula Using a Molt

1. Collect the Molt Before It Dries Completely

Fresh molts are softer and easier to manipulate. If the molt has already dried, you can still soften it.

2. Hydrate the Molt

Lightly mist or soak the abdomen portion, especially the belly-side plate. Let it sit a few minutes until pliable.

3. Gently Unfold the Abdomen

Using tweezers or a soft brush, open the abdomen plate like a tiny book. The area you need is the thin flap right between the first pair of book lungs.

4. Identify the Structures

• Females: Look for a raised flap, lobes, or distinct internal shapes.

• Males: Look for a flat, smooth patch with no internal development.

5. Species Variations

Some genera, like Avicularia, Caribena, and Ybyrapora, have extremely thin reproductive structures, making them harder to see. Others, like Pamphobeteus and Grammostola, have much easier-to-read spermathecae.

6. If You Can’t Tell

Don’t assume. Wait for the next molt, photograph it under strong lighting, or seek a second opinion.

Troubleshooting Difficult or Damaged Molts

Molts are fragile and often fall apart during handling or hydration. Here are some common scenarios and how to deal with them:

Torn abdomen: If the epigastric area is missing, you cannot determine sex from that molt.
Collapsed molt: Rehydrate longer and gently reshape with soft tools.
Stiff molt: Soak the abdomen in warm water or use a light damp paper towel for several minutes.
Curled abdomen: Carefully unfold it from the edges, not the center, which is more likely to tear.

Sometimes, even a perfect molt fails to reveal the sex. In that case, patience is your friend. The next molt will offer another opportunity.

Ventral Sexing: Helpful, but Not Always Reliable

Many keepers attempt ventral sexing by examining the underside of the tarantula while it rests on the glass or sits still during routine care. This can sometimes offer clues, but it is not definitive.

What you can sometimes see:

The shape of the epigastric furrow
Slight differences in the surrounding setae
Possible indicators of juvenile females

What you cannot rely on:

Color
Abdomen patterns
Behavior
Size
The myth that "males always have smaller abdomens" in juveniles

Some species are simply too fuzzy, too dark, or too small to ventral sex accurately. Arboreal species such as Avicularia and Caribena are notoriously difficult to read this way.

As a real example, I once had an Omothymus violaceopes that I was convinced was female based on ventral sexing. The spider molted, matured, and transformed from a rich purple tarantula into a bright gold, incredibly leggy mature male. That experience is a perfect reminder that ventral sexing is an educated guess at best, never a guarantee. Years ago, I was posting pictures of my Poecilotheria metallica, and dozens of people were commenting that it was a male based on the pattern of the abdomen. However, after all these years, she is still growing, has not matured out as a male, and has been confirmed female by examining two different molts. This exemplifies how quick visual assessments, especially on the internet, are rarely correct.

If you want accuracy, molt sexing is the only reliable method other than waiting for the male to mature and have his ultimate molt, or the final molt of his life signaling his reaching maturity and reaching the final stage of growth.

Identifying a Mature Male

Once a male tarantula completes his ultimate molt, he undergoes significant physical changes:

Tibial hooks: Present in many species, especially terrestrials. Used to hold the female’s fangs during mating.
Longer legs and slender body: Mature males become leggier and lighter for traveling.
Shortened lifespan: Males typically live only a few months to a couple years after reaching maturity.
Increased wandering: They pace the enclosure, attempt to climb, and may refuse food.
Bulbous pedipalps: The tips of the pedipalps enlarge into sperm-transfer organs. They almost look like little boxing gloves.

These traits appear only after the ultimate molt and are definitive indicators of sex.

Sexual Dimorphism in Tarantulas

Sexual dimorphism refers to differences in appearance between males and females of the same species. In many tarantulas, males and females look nearly identical until the male matures. In others, the differences can be dramatic.

These differences may appear in color, pattern, body shape, leg length, and overall size of individuals.

Examples of Sexual Dimorphism:

Poecilotheria metallica

Females retain vibrant blue coloration throughout their lives. Mature males often lose much of this color, taking on washed-out or dull tones.

Omothymus violaceopes

Females display intense purples and blues. Mature males transform into bright gold with very long, slender legs. This was the species that fooled me early on when ventral sexing misled me into thinking I had a female.

Monocentropus balfouri

Females hold bold blue and cream contrasts. Males are pale silver or faded tan.

Pamphobeteus species

Females remain dark and earth-toned. Mature males often exhibit metallic purples, pinks, or blues that look nothing like their juvenile form.

Psalmopoeus irminia

Females retain bold black bodies with bright orange patches. Mature males become grayish, washed out, and much leggier.

Psalmopoeus cambridgei

Females show deep green and brown hues. Males molt into pale beige or tan spiders.

Tapinauchenius species

Mature males of many species lose nearly all color, becoming slim, fast-moving, and ghostly pale.

Sexual dimorphism can be helpful, but only after the male matures. Before that point, color patterns mean very little. This is why sexing juveniles based solely on appearance is notoriously unreliable.

Common Misconceptions and Mistakes

Here are some of the most common errors keepers make when trying to determine sex:

Assuming color equals sex
Believing males are always smaller (only true after maturity)
Mistaking a plump abdomen for a female
Thinking hooks appear before the ultimate molt
Assuming all species have tibial hooks
Treating ventral sexing as a guarantee
Using feeding habits alone to guess sex

Sexing tarantulas requires patience. Rushing to label a spider too soon leads to confusion and incorrect assumptions.

Final Thoughts

Sexing a tarantula is one of the more rewarding skills you will develop in the hobby. It takes practice, patience, and a willingness to accept that sometimes you simply cannot know until a molt gives you the answer. The anatomy is small, delicate, and easy to miss, which is why molt sexing remains the most accurate and dependable method available.

Ventral sexing can help narrow things down, and sexual dimorphism can offer clues in certain species, but neither should replace a confirmed molt. Every keeper eventually misgenders a spider. It’s part of the learning curve, and even experienced hobbyists get fooled from time to time, especially with arboreal species that mature into completely different colors.

With time, careful observation, and a few molts to practice on, these once mysterious differences start making sense. And when you finally identify your tarantula’s sex with confidence, it adds a whole new level of understanding and appreciation to the experience of keeping these remarkable animals.

Written by Richard Stewart of The Tarantula Collective 12/11/2025

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