How to determine gender

This is often one of the biggest questions that goes through a keepers mind when acquiring a new tarantula. Females are prized among hobbyists for their larger size, longer lifespans and ability to produce multiple eggsacs. Determining the sex of a tarantula can be tricky though, especially when it is younger as male and female tarantulas will often look and act identical until mature.

When a male matures (has his final or "ultimate" molt) he gains some distinguishing characteristics such as "tibial hooks" (present in most species) and bulbous pedipalps which are often referred to as "boxing gloves"

Mature Male C. cyaneopubescens (Green Bottle Blue)

Boxing gloves are circled left & tibial hooks are circled right

Also notice the lankier overall appearance, including longer legs and smaller, more narrow fangs of the mature male...

Mature Male C. cyaneopubescens (Green Bottle Blue)

...when compared to a mature female of the same species?

Mature Female C. cyaneopubescens (Green Bottle Blue)

Before males mature they do not have these easy-to-spot physical characteristics. You must sex the specimen by looking at the exuvium also known as a cast skin or molt. "Molt-sexing" is the best and most reliable way to confirm the sex of a specimen. Carefully examine the molt to look for the presence of a spermathecae. This reproductive organ is where sperm is stored until the eggs are laid and is only present in females. A male can be identified by confirming the absence of a spermathecae.

Female G. porteri (Rose Hair)

The spermathecae is being manipulated with a toothpick 

Female G. porteri (Rose Hair) approx. 5 1/2" DLS

The spermathecae is highlighted in red

The spermatheca is located between the first pair of book lungs. It is usually only visible on a larger tarantulas and only if the molt is intact. Young spiderlings are far too small to determine gender. Typically we have to wait until the specimen is minimum 2-3" leg span until the molt will be large enough to "read." For the best chance of recovering a sexable molt remove it from the tarantulas enclosure as soon as possible after it sheds. Be very careful not to disturb the soft and vulnerable tarantula!

The male A. avicularia/Pinktoe (pictured below) is completely flat between the first set of book lungs. There is no flap/presence of a spermatheca.

Male A. avicularia (Pink Toe) approx. 3 1/2" DLS

Has no spermathecae