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macrogyna: females and males larger, the largest sexual females with the head dark brown above and orange-brown below the eyes, with longer, more tapering ovarioles, of which the terminal filaments account for only onethird the total length in virgins, eyes less prominent but of the same size, colonies usually monogynous with relatively large aggressive workers, the average worker head-width less than the average female head-width, colony reproduction by dissemination of fertile females which found colonies either alone or in small aggregates; microgynca: females and males smaller, the smallest sexual females with head brown above and yellow-brown below the eyes, with shorter, less tapering ovarioles of which the terminal filaments account for two-fifths of the total length in virgins, eyes more prominent but of the same size, colonies polygynous with relatively small and docile workers, the average worker head-width similar to the average female headwidth, fertile females return to their nest (and possibly other nests) and colonies reproduce by fission.

Evolution of polygyny in a species whose young require parental care, to which the male can contribute directly, presents certain interesting problems. With each additional mate, the male may have more young to care for simultaneously; less time being devoted to each may influence their survival. Even if polygamy increases the male's total contribution to future generations, as compared to monogamy, the reverse would probably be true for each of the females mated to him. Since it is generally accepted that in birds females select mates from among available males, it seems unlikely that selection would favor those females that bred with males already mated. This has probably led to the general belief that polygamy in avian mating systems is a by-product of an unbalanced sex ratio (cf. Chapman, 1928; Kendeigh, 1941a; Mayr, 1941; Ryves and Ryves, 1934; Skutch, 1935; Williams, 1952), an assumption which is not necessarily true but which has probably led to a stagnation of thought concerning evolution of polygamy in birds. Fisher (1958) and Kolman (1960) indicate that sex ratio at fertilization would be adjusted by selection so that average parental expenditure to rear males equals that to rear females. Willson and Pianka (1963) point out that if sexual selection results in sexual dimorphism expressed during the period of parental care, there may be a change in average expenditure per sex with a consequent change in sex ratio. Thus, mate preference, which in my opinion should have a profound influence on the mating system, may alsohave an influence on the sex ratio. This does not preclude the possibility that sex ratios may