If you’ve been a part of any tarantula forum or Facebook group, without a doubt you’ve stumbled upon posts asking users to help someone sex their tarantula. “Sexing” a tarantula isn’t what it sounds like, by the way – in tarantula keeping language, this means how to assign a gender to the tarantula.

A lot of times this isn’t clear, especially when you’re a beginner tarantula owner who bought their tarantula from a mainstream pet store or someone who had no idea what they’re doing. I myself did this when I got my first tarantula from a mainstream pet store. I spent many weeks believing that Spidey was a male, actually – until someone more experienced was kind enough to burst my bubble.

The best way to find your tarantula’s gender: How to sex a molt

In all honesty, it is very hard to find a tarantula’s gender until they reach a certain age. By far, the most reliable way to find the gender of your tarantula is by waiting for it to molt and then studying the exoskeleton. This allows you to actually accurately examine the spider’s sex organs, differentiating between male or female. The reason we say you must reach a certain age to do this is because many times sling’s molts are too tiny and fragile to do this accurately.

Once your tarantula has a molt that you can examine, you can use a tiny bit of water to get the abdomen of the molt to spread out (if it’s crispy and crinkled). Then you can locate the sex organs in between the two sets of book lungs. What you are looking for (in a female) is the spermatheca, a flap between the book lungs. This is where female tarantulas store sperm. If your tarantula is a male, this flap will not be present.

Here’s a great video by Tarantula Dan that demonstrates this:

And some more great videos about this:

Tom’s Big Spider

Mark’s Tarantulas

If you’d like a more detailed explanation of how to do this with a molt, Tom’s Big Spiders has an amazing article about this.

Physical

While molts are certainly the most reliable way to gender your tarantula, sometimes there are physical characteristics amongst species that can help when sexing a molt isn’t possible. Again, this is something that will be difficult to determine in a sling and usually isn’t able to be determined until a spider has reached some level of size and maturity.

• Size: In some species, females will generally be larger than the males. Males tend to be slimmer with longer legs, while females are more robust and have a more bulky appearance.
• Visible bulbs in males: While females have sex organs near their book lungs, male tarantulas who have reached maturity will have a circular “bulb” (palpal bulbs) at the end of their pedipalps, which are their sex organs.
• Hooks in males: Another characteristic in mature males that is visible is the presence of hooks on each of their front legs, which they use to keep the female in place and protect themselves from the female’s fangs during mating.
• Males have extra spinnerettes: Male tarantulas have what is called epiandrous fusillae, additional spinnerets which are believed to be used for spinning a sperm web where they deposit their sperm so they can then take it into the sex organs on their pedipalps. These can be seen on the underside of your male tarantula.

Lifespan

Worst case scenario, lifespan is usually an indicator. Tarantula males unfortunately have a relatively short lifespan once they get their sex organs. In many species of tarantulas, females generally live between 2-3 times longer than males so if you’ve inherited or come across a spider who is older, it just might be a female.

I hope this helps! If you have any other questions about tarantula behavior or care, be sure to check out my tarantula guide!