The distribution of two forms of the meat ant, the typical Iridomyrmex detectus (F. Sm.) (purpureus (F. Sm.)) and an undescribed blue form, was studied within 260 km of Adelaide, South Australia. In this area, rainfall varies from more than 1000 mm per annum in the Mt. Lofty Ranges to less than 200 mm in the Murray Basin. The factors that limit the occurrence of both forms are detailed and their distributions are described. The blue form occupies the lower- and typical I. detectus the higher-rainfall parts of the area, and they meet along a boundary in the mallee close to the 300-mm isohyet. In the Murray valley, where moisture is available, typical I. detectus penetrates into areas of lower rainfall. It appears to be dominant to and able to displace the blue form but its distribution is governed, directly or indirectly, by rainfall and the availability of water. The high rainfall limit of the arid-tolerant blue form seems to be determined primarily by competition from typical I. detectus. Closer analysis of the boundary zone reveals other factors that may influence the distributions of the two forms. These are their relative population densities, soil type, the productivity of different habitats and local distribution of rainfall.
