Like all arthropods, tarantulas moult in order to allow them to grow and also to replace old, tattered skins. In essence, the smaller the tarantula is, the more often it may moult. In addition, the amount of food that a tarantula consumes will go some way to defining how often a tarantula moults.
Youngsters of fast growing species may moult every 4-6 weeks while growing fast while adults will typically moult once per annum, typically at about the same time each year.
This regularity of moulting in adult tarantulas is useful because it makes breeding tarantulas in captivity far easier. For example whenever a female tarantula moults, she also sloughs off the inside of her reproductive organs, which in essence makes her a virgin again.
This means that if a female tarantula mates, but then moults shortly afterwards she won’t go on to lay fertile eggs. Knowing when your female tarantula last moulted – and hence when you are expecting her to moult again – makes it easy to decide when a female tarantula should be mated.
For best results, female should be mated as soon after a moult as possible (assuming they have had a few weeks to harden up and begin eating again) in order to give her the greatest chance possible of laying a fertile eggsac. Mated in this way, some females have laid multiple eggsacs in the course of a year, though the number of resultant babies typically drops with each eggsac produced.