Determining the sex of your tarantula can be challenging. However, several key anatomical differences will make the job easier.
Generally, tarantula females are larger and bulkier than males. If these are your first tarantulas or if you do not have a spider of the opposite sex for comparison, sexing based only on your spider’s size can be difficult. Similarly, female tarantulas have broader chelicerae, her jaws or fangs, than males do in proportion to their body size. But without previous familiarity with tarantulas and/or other spiders for comparison, you can't rely on this detail alone to sex your spider.
Mature Male Differences
If you wait until your tarantula matures, sexing him or her will be easier. Mature males have a hook on the underside of their front leg. These hooks keep the female's fangs away from the male during mating. Mature males also have bulbs on their pedipalps, the two shorter, leg-like feelers located near their fangs. These bulbs are the male spider's sex organs.
Molted Skin Differences
For owners who want to sex their tarantulas early, the task requires taking a closer look at the spider’s molted skin.
You need the skin to be as recently discarded as possible so it still retains some moisture. Otherwise, the skin may break or crack when you examine it.
Carefully, smooth out the skin around where the spider's abdomen would have been. You should see four white spots at the top of this area. Those spots represent the discarded lining of the tarantula's lungs. Between those spots you should see a crease. If the crease has a roundish bump emerging from it, you are looking at the skin of a female tarantula. The bump is the female's spermathecae where she stores the male's sperm after mating. Male spiders will not have that bump.
Another difference you can often see in male tarantulas is the presence of epiandrous fusillae, additional spinnerets 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. If you gently flip your tarantula over and examine its abdomen, you will see a line called the epigastric furrow across the abdomen. Just above that line, you may see an arched or semicircular area. That area is epiandrous fusillae, which means your tarantula is male.
In large tarantulas, you can usually see this spot clearly in good light. However, if you have smaller tarantulas, you may need to use a handheld or pocket microscope to see the area.