Observations on the free-living adult stage of Gordius dimorphus (Nematomorpha: Gordioidea).

A total of 119 adult hairworms, Gordius dimorphus, was collected on 2 occasions from a small mountain stream in New Zealand's South Island. Size dimorphism between male and female worms was much greater than previously reported. The average sizes of worms of either sex found in mixed-sex groups were no different from those of worms occurring either alone or in single-sex groups. In a sample taken in spring, the sex ratio was female biased, whereas in a sample from early summer the sex ratio was strongly male biased. A greater proportion of males was found in mixed-sex groups in early summer than in spring. Although laboratory experiments are needed, from the little evidence currently available it would appear that hairworms form mating groups more or less at random.