In the postembryonic development of Mastigoproctus giganteus there are four nymphal stages, and the adult never moults again. Size ranges of the different stages are given. Moulting seems to occur once a year. Postembryonic development up to maturity therefore takes about four years. Secondary sexual characters in the form of the pedipalp appear at the adult stage. These include a strong positive allometric growth of the male palpal femur and patella which starts in the fourth nymph and becomes significant in the adult. The only reliable characters by which the sexes can be distinguished in third and fourth nymphs are sclerotized bars within the male genital atrium.
Sperm transfer is accomplished by a very complicated spermatophore. This carries two sperm carriers each of which consists of a base, a sperm package with reservoir and ejaculatory duct, and a conductor. The male, using its modified movable palpal fingers, pushes these into the female's gonopore. After mating, they are dropped to the ground and discarded.