B. boehmei cast skin showing fused spermathecae.

Sub adult male B. boehmei cast skin.

These photographs show the well defined sex organs of both male and female tarantula Brachypelma boehmei. From an early stage in the spiders development, the signs of a female are quite obvious and although the actual spermatheca (sex organs which receive and store the sperm) aren't fully formed, successful determination is possible with the appearance of a heavily folded area known as the uterus externus (the large, upward and curved piece of semi-transparent 'skin') and a large lower lip. This lip is present before any real signs of the spermatheca proper and there is a marked difference in the immature male. Inspection of a suspected male cast skin is again obvious if the specimen is fairly large as none of the above features are present. 

If determination is required earlier in the spiders development, then things become slightly more complicated and I recommend either sending your spider to registered sexing practitioner (details can be found in the Journal of the British Tarantula Society) or reading further on the subject. "Sex determination of immature theraphosids from their cast skins" by Kathleen and John Hancock for example.

Matching these photographs to the line drawings in the above mentioned book will give a better idea of what the females spermathecae and epigynum area actually look like in relation to the other features (book lungs etc., seen here as the large white areas on either side of the central epigynum). Now all you have to do is wait for your young tarantula to moult!

Another sexing technique is to inspect the underside of the tarantulas abdomen as there are slight differences between males and females. The important area on the underside of the abdomen is called the epigastric furrow and this is located between the first pair of book lungs nearest the pedicel or waist (see photographs below).

Adult female epigastric furrow (left) and sub-adult male epigastric furrow (right).

Adult and sub-adult tarantula males have structures called epiandrous glands located between the first pair of book lungs and are surrounded by a dense patch of hair that can sometimes be seen with the naked eye. This area between the book lungs is also much narrower in males and is lacking the deep fold found in females. In some species it can be difficult to see this area so although not as accurate as using cast skins, this method can prove useful when a recent moult is not available and (with a little practice) positive sex can be determined.

Once adult, sex determination becomes simple as males go through some important physical changes when they go through their final moult. For a more information on the mature male, click here.

Sexual dimorphism in adult tarantulas P. formosa - female left, male right.