Obviously, before you can breed your tarantulas, you need to have a male and a female. Getting a pair together can be tough with most tarantulas for several reasons. One problem is that males are relatively short lived compared to females, and they tend to mature before females in the same clutch mature. This is because males require fewer molts before maturity.
Thus, even if the eggs of a clutch produce half males and half females, all those males probably will die long before their sisters mature. Assuming inbreeding is harmless in tarantulas (which probably is the case, as it must happen many times in burrowers with restricted distributions in small areas of habitable land), trying to raise both sexes from one egg sac seldom works. Generally, the hobby breeder tries to start out with a mature female (either raised from a spiderling or purchased as a juvenile or an adult) and buys, rents, or borrows a mature male of the right species when the owner feels the female is ready to mate.
The other problem is that sexing tarantulas is difficult. As a rule, males are recognizable only once they go through their last molt and suddenly develop visible secondary sex organs on the pedipalps (and sometimes the first legs). Females gradually become mature, and there is no true way of determining just when a female moves from immature to mature, so a lot of guesswork is involved.
Let’s start with males. Males as a rule are a bit smaller than females of the same species and population, and they tend to have longer, thinner legs and a smaller, sometimes almost shrunken-looking abdomen. Occasionally, their colors and patterns are slightly different in details, and possibly the details of some areas of climbing bristles on the legs (the scopulae) may differ between sexes. When the male makes its molt to the sexually mature form—usually sometime between one and three years of age—this is the end of its growth, and the spider will not successfully molt again. With this molt, it develops fairly obvious external characters that allow it to be sexed.
First, the tarsus of each pedipalp becomes strongly modified. The segment generally becomes widened or twisted into a clawless structure called the cymbium that is scooped out underneath to hide an oval sperm bulb that ends in a hollow process called the embolus. When not in use, the sperm bulb is held in a horizontal position against the cymbium and may not be easily visible, but you should be able to detect that the segment itself is not normal and then notice the swollen bulb. In the majority of common tarantulas, the mature male also develops a large blackish hook or spine at the end of the tibia of each first walking leg. This is known as the tibial apophysis (plural, apophyses). Though sometimes partially hidden under long bristles under the legs, if you look closely, you can see the apophyses.
Additionally, in many male tarantulas, you can see a shallow, straight epigastric furrow running between the two anterior book lungs. Running from the furrow forward toward the pedicel is a pair of nearly parallel indented lines, usually with a slightly raised, whitish oval area between them. This is the ventral spinning field of the male, from which it spins some of its sperm web.
Mature females tend to be a bit larger than mature males, with thicker legs and larger abdomens. Maturity is gradual in females, and it may take three to six years for some larger tarantulas to mature. To determine the sex of a living tarantula that lacks obvious sperm bulbs, you must turn it over (carefully) and observe the base of the abdomen under magnification and good light. As in the male, the epigastric furrow runs between the two anterior book lungs, but in a mature female it is a raised, clifflike area that often is obvious in lateral view. As a rule, the higher the cliff, the more likely that the tarantula is mature. There generally is a trapezoidal area (not a square as in males) lacking a pale spot in the center in front of the epigastric furrow.