The abdomen, which is less sclerotized than the thorax, consists of 10 segments with membranes in between, allowing for articulated movement. The sternum, on the first segment, is small in some families and is completely absent in others. The last two or three segments form the external parts of the species' sex organs. The genitalia of Lepidoptera are highly varied and are often the only means of differentiating between species. Male genitals include a valva, which is usually large, as it is used to grasp the female during mating. Female genitalia include three distinct sections.