Sexing

One of the first things we want to know about our tarantulas (other than what species it is) is whether it’s a male or female, of course!

Here you will find a few topics on sexing tarantulas.

SEXING TARANTULA SPIDERLINGS / SLINGS

Unfortunately, it’s impossible to tell the sex of slings (spiderlings) or even juvenile tarantulas until they’ve reached a stage in their growth where their sexual organs have properly begun to develop. So when you buy a sling (spiderling) you are buying a 50/50 chance of getting a male or female. This is why many keepers choose to buy two or three slings of a species to increase the chance of getting a male or female depending on what they are hoping for.

GROWTH RATE

Growth rate can be an indicator of a male or female. Males will generally grow much faster than females. This is genetic programming from mother nature to ensure that there are always males available for mature females to mate with and ensure progeny (future offspring).

This is why many believe that if you have slings of a particular species that are all from the same hatch and some of them are outgrowing the others by far, that it’s fair to guess these might be the males. This is not a confirmed theory, however.

VENTRAL SEXING

Many claim to be able to sex a tarantula by observing it up against the glass from the outside (ventral sexing), but be careful with this. In some cases, it’s possible to learn and figure out that on certain tarantulas; the alignment and angle of the epigastric furrow and book lungs can symbolise a male or female, this has been observed in the Poecilotheria genus for example.

But it’s not the same for all tarantulas and it’s quite difficult to master. At MyMonsters we don’t rely on ventral sexing and prefer sexing a tarantula 100% by inspecting a fresh exuvium / molt / skin.

EXUVIUM / MOLT SEXING

The most accurate way to sex your tarantula is to sex it by inspecting the inside of it’s exuvium (molt/skin).

Remember that it’s easier to sex an exuvium the older your tarantula is because young tarantulas may not have developed their sexual organs yet and they might not be visible. So if you inspect the exuvium on a young or juvenile tarantula and you can not see any signs of it’s sex yet, don’t despair, just wait for the next molt to check again. However if the tarantula is already about 8 to 10cm in size and no spermatheca are visible, chances are good you have a male.

Generally you should be able to see the sex on a tarantula after the 5th or 6th instar (molt), but remember, on slow growing species like some of the Grammostola species, they may only start showing sexual organs after growing to at least 6 or 7cm regardless of the amount of instars/molts.

WHAT AM I LOOKING FOR?

What you are looking for is the spermatheca (Pronounced: spur-mah-theeka) structure that is present on female tarantulas which is where they store sperm inserted by the male tarantula during mating. Some call it a “flap” or “bunny ears” which is inaccurate because as you will see, spermatheca can differ quite drastically in shape and size and will not look like a flap or bunny ears at all.

Regardless, sperm that is inserted by special palpal bulbs on a male tarantula’s pedipalps called “emboli” is stored inside this spermatheca structure for later use. When the female is ready to lay eggs, the eggs pass through this structure and are fertilised with the stored sperm as they exit the female. It’s a lot more complicated than is described here but we hope our explanation makes it a bit easier to understand.

Tarantula Sexing Diagram - Exuvium Sexing - Male vs Female Tarantula
Tarantula Sexing Diagram – Exuvium Sexing – Male vs Female Tarantula
Tarantula Sexing - Sexing Diagram - Tarantula Sex Determination - How to Sex a Tarantula
Tarantula Sexing – Sexing Diagram – Tarantula Sex Determination – How to Sex a Tarantula

The spermatheca structure inside the green oval is present on the epigastric furrow or “line” between the two sets of book lungs closest to the tarantulas legs and carapace. This is called the anterior set of book lungs, andterior is just a fancy way of saying “nearer to the front”. On males, you will see no structure at all and the only thing visible is the epigastric furrow / line.

SEXING AN EXUVIUM

Here we have a perfect comparison between a male Tarantula exuvium (molt/skin) and a female. We have highlighted the spermatheca in yellow to easily identify the structure.

SEXING VIDEO

This video shows how to determine the sex of a tarantula. Here we have sexed a male exuvium which clearly show that they lack the spermatheca structure. 

Pause at 5:20 to see the difference between the male and female exuvium.

MORE FEMALE SPERMATHECA

Below are some more images of female spermatheca structures. As you can see they are quite different for each species, it will take a keen eye to spot some of them, but you’ll get it, don’t worry.

Once you get the hang of it, you will be able to sex your spiders easily with the naked eye. It just takes a little time and experience.

Good luck.

SEXUAL DIMORPHISM

Sexual dimorphisim is also a very clear indicator of a male or female but mostly when tarantulas have reached mature adulthood. With many species, especially old world species, the males look very different to the females when mature, from being half their size to being completely dull and lacking any adult colors seen on the females.

With some species, again especially with old world species, it’s possible to observe the “tiger” pattern on the abdomen and leggy appearance of males from very young.