For those in the know, tarantulas can be sexed from their skins from quite an early age. The moulted skin is turned inside out and the reproductive linings which are shed as part of the moult are examined under a microscope. With practise one can sex tarantulas from when they are only a few centimetres long.
However this takes time and effort to learn and for all but the most obsessive tarantula keepers you may be best to find a professional who will sex immature tarantulas for you.
However sexing mature tarantulas is far, far easier and is something that anyone can learn very quickly and easily.
In general adult female tarantulas are far more bulky than males. The males may infact have a larger legspan than the females but are typically far more lightly-built. The females, in contrast, have far larger abdomens and generally look chunkier.
As well as being more spindly, adult make tarantulas develop special reproductive organs which can be easily seen when you know what you’re looking for and when you see these you can be absolutely certain that your tarantula is infact a sexually mature adult ready to start breeding with any females you can lay your hands on.
The first thing to look at is the presence of special palpal bulbs on the end of the pedipalps. When you look at a tarantula it often looks like they actually have five pairs of legs rather than four. This is because the first, smaller pair are actually modified mouthparts rather than being functional legs. Your tarantula may use these, for example, to gently flip prey into it’s fangs.
However in adult male tarantulas this “extra pair of legs” – the “pedipalps” or just simply “palps” – are used in the act of copulation. The adult male develops a small “bulb” on the end of each of his palps which is used when mating with females and these bulbs look very different to the tips of normal palps.
Rather than being hairy and “leg-like” these bulbs typically look dark, shiny and hard and are easy to spot. Just take a look at the photo below of an adult male Pink Toe (Avicularia avicularia) and hopefully you will see those shiny “knobs” on the end of his palps.
Seeing these guarantees an adult male tarantula.
There is also a second element you can look out for but this one can be less reliable than the previous method. The reason it is less reliable is that whilst all adult male tarantulas have palpal bulbs, only some possess this feature. Frankly, a lot of the common tarantula species do possess this feature but it’s important to be aware that not all do.
The second feature when sexing an adult male tarantula is the presence of hooks or spurs on the underside of the first pair of legs. These are typically part way along the leg and are used by the male to hook the females fangs out of harms way during mating, thus reducing the chances of him becoming dinner!
Again, take a look the following photo of the same adult male Pink Toe (A. avicularia) shown earlier but this time note that one of his “tibial spurs” as they are know as clearly visible.
So there you have it. One foolproof way to tell if a male tarantula is sexually mature and a second “back-up” method that works with many though not all species. So now you know, do you have an adult male? 
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