Alan's arthropods & trips

The sex of a spider is best determined from a cast skin (exuvium). The sexual organs are situated between the first pair of booklungs (picture 1).
The female exucium contains the spermathecae (picture 2). The male exuvium has a flat area on that same place.
Every species has a different shape of spermathecae, see drawings below.

Female Brachypelma smithi:

picture 1

picture 2

Spermathecae of different species:

Click a picture to see a larger view:

Source and owner of drawings: Mikhail F. Bagaturov.

Article from Rick West on birdspiders.com: How to determine the sex of your tarantula: < Link >