Dwarfed Males

males that differ strikingly from females of the same species in size and, as a rule, in greater simplicity of organization.

Dwarfed males are an extreme manifestation of sexual dimorphism, which has been described in a number of species of rotifers, roundworms, segmented worms, mollusks, crustaceans, spiders, and insects. In some animals the dwarfed males differ from the females only in significantly lesser size. In some segmented worms the dwarfed males have a simplified organization as well. In Bonellia the males are 1– mm in length and live in the body of the female, which may be 7 cm in length, not counting the proboscis. Dwarfed male barnacles have a greatly reduced skeleton, legs, and internal organs (with the exception of the testes), and several live on the body of each female. Dwarfed males among certain rotifers have reduced intestines and excretory organs. After fertilization of the female the males die. Other species of rotifer with dwarfed males have females that are capable of parthenogenesis. The transition from dioecism to hermaphroditism brings with it the disappearance of the male. However, some species of barnacles that have made an incomplete transition to hermaphroditism have “complemental males,” which were described by C. Darwin (for example, Scalpellum vulgare). An extreme form of reduction in the male is observed among gastropod parasites of Holothurioidea, such as in the male of Entocolax ludwigi (about 0.5 mm in diameter), which in structure and position in the female’s body (which is up to 2 cm in length) is similar to a testis and has lost the nature of an independent organism.