SEXING YOUR TARANTULA BY THE CAST EXOSKELETON

The sexing of your Tarantula is quite a  simple process, once you know the basics. Although there are many books about Tarantulas only few of the better ones explain how to sex your specimen. Here is how to do it.

This method uses the examination of the internal genital parts cast skin. If your specimen is of a large sub adult size the sexing process can be easily done with your naked eye. If you are wishing to sex a juvenile you may require a hand held lens. When you are confident in what to look for you can sex very young juveniles but you will need a good microscope to sex the smallest size, but remember the smaller the spider the harder it will be. I have found you can sex a Brachypelma smithi with a leg span of an 1.5 inches with ease, with other species this will be more difficult due to different shapes and sizes of their seminal receptacle ( spermathecae ). Here is what you are looking for when sexing your spider.

The red arrows point out the key area of interest to us, the genital structures as seen from the inside of the cast skin. The four pale areas, are the book lungs, between the first set of lungs, nearest the join to the abdomen, there is a fold of skin this is the ephigastic furrow. This is where we find the genital structures of our specimen and what we are using to determine the sex of your immature tarantula. 

(1) FEMALE

If there is a prominent and distinct structure ( spermathecae ) visible as shown by the red arrow here . These are the seminal receptacles, then this will be a female, the size and shape of the spermathecae will very from species to species see the diagrams below. These deferent shapes are often used in the determination of the genus of a newly discovered tarantulas.

(2) MALE

If there is no visible structures of any kind, and just a faint crease or line between the book lungs then your specimen will mature into a male, seen here. Great care should be taken here as young females have a small seminal receptacles, the more mature the specimen the more pronounced the structures. Use a hand held lens if unsure, and on smaller tarantulas use a good microscope.

 
Please note the diagrams (1) and (2) are of a Brachypelma smithi and the the structures do very from species to species

The sex determination process is best done with a fresh skin, but if the skin is old and dried out but in good condition you can re-hydrate the skin for a better look. This is how I do it, use a flat bottomed container the type used to keep food fresh, such as Tupperware© and big enough to take the fully spread skin. Then add two drops of washing up liquid and add hot water from a kettle, make sure its not boiling, add only a small amount enough to cover the bottom of the tub and not the whole skin. Then seal the lid down and wait ten minutes to check the skin, it should now be soft and pliable. Carefully remove and place on a few layers of paper towel. Then with some tweezers gentle ply apart the abdomen skin, once the book lungs and ephigastic furrow are visible leave to dry. This should not take to long as the hot water will quickly evaporate off into steam. Once you have seen for yourself the obvious differences, sexing your tarantulas will be easy, allowing you to plan future breeding and perhaps stopping the unfortunate loss of a mature male whilst trying to mate him with a large presumed female but immature male. Hopefully this has given you some idea of how to go about sexing your tarantula. The next step would be to compare some of your tarantulas cast skins with the above illustrations or find a skin of your freshly moulted male and compare that to a tarantula you suspect, or know to be a female.

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This page has been online for a very long time so, some of the info and termonolgy my need updating.
Regards
Mark