Eumastigonus insulanus presents a problem that is seen in many widespread species (see Eumastigonus distinctior). It was originally described from Stephens Island, Marlborough Sounds and has since been found throughout the North Island and at least in the northern part of the South Island, excluding the dry area of Marlborough and Canterbury south of the Wairau. However, the further the populations are from either side of Cook Strait, the uncertainty that the population is part of the same species, increases. There appears to be gradual accumulation of differences which could be interpreted both as a clinal change within one species or as discontinuous discrete changes, each form representing a separate species. These populations perhaps were once contiguous and adjacent ones would interbreed; a typical clinal situation. Now that humans have modified the environment so much, populations are no longer contiguous with no genetic exchange and through that we have lost much basic information on which a species could be defined. The answer may not even be found with DNA/RNA analysis.