Mating strategy and isolation between the two forms, macrogyna and microgyna, of Myrmica ruginodis (Hym. Formicidae)

.1 It is shown that the size distribution and degree of overlap between individual queens of the two forms of Myrmica ruginodis , microgyna and macrogyna, is exactly the same in southern England as was originally described from Scotland. 2 Distinguishing colonies of the two forms is not as easy as distinguishing between individuals, both types can be polygynous and approximately 25% of colonies contain a mixture of queens. 3 Males of the two forms can be distinguished solely on the basis of size and, in general, macrogyna colonies produce larger males, workers and gynes than microgyna colonies. There is little correlation between the sizes of males, workers and gynes in colonies within the macrogyna and microgyna groups. 4 If the two forms are separate species and mixing is a parasitic association (as suggested elsewhere), then there should be breeding isolation between the forms. This is tested by examining data from nine mating‐swarms. 5 All the swarms contain a significant proportion of the microgyna form. There is no evidence of assortative mating, although larger males are more likely to get a mate than smaller ones. This behaviour, combined with the possibility that microgynes mate near to the nest, might prevent complete mixing during mating. 6 The status of the forms is discussed. Besides the possibility of a‘pre‐parasitic’relationship it is suggested that the forms might represent a polymorphism, present in all populations, the balance between them being the result of selection determined by local environmental factors.