The "destruction or alteration of (the host's) gonad tissue by parasites" (Noble and Noble, 1971) is widely referred to as parasitic castration. It is often accompanied by a modification in the expression of the host's secondary sexual characteristics and may also have a variety of other physiological and behavioral effects. That its effects may have considerable influence upon the life-history of the parasite has been less appreciated. Parasitic castration has attracted scientific attention ever since it was first reported by Maln (1881) and studied by Giard (1886, 1887, 1889; Giard and Bonnier, 1887). The phenomenon has been reviewed for various groups of host organisms, e.g., Crustacea (Reinhard, 1956; Charniaux-Cotton, 1960), Insecta (Wiilker, 1964), and Mollusca (Koie, 1969; Cheng, 1971; Wright, 1971), as well as for groups of parasites, e.g., Nematoda (Welch, 1965) and Turbellaria (Jennings, 1971). Its importance has recently gained recognition in a number of parasitology textbooks (e.g., Caullery, 1952; Cheng, 1972; Noble and Noble, 1971). Several explanations of parasitic castration have been proposed. McClelland and Bourns (1969), for example, consider it to be primarily an adaptive response of the host, and they use a group-selection argument for their explanation: