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MONOMORPHIC MIMICRY IN NEARCTIC LIMENITIS BUTTERFLIES: EXPERIMENTAL HYBRIDIZATION OF THE L. ARTHEMIS‐ASTYANAX COMPLEX WITH L. ARCHIPPUS

Wing color patterns in Lepidoptera often exhibit remarkable divergence in closely related forms. Numerous species are polymorphic, whereas others possess seasonal or geographic differences in coloration. Related butterflies may possess quite different wing patterns, particularly when only certain species, sexes, or forms within a group are involved in mimicry complexes. Such differences are assumed to result from selective processes operating in the natural environment. Determining just how mimetic forms arise from non-mimetic ones has been the object of much research in the field of ecological genetics (Ford, 1945, 1953; Clarke and Sheppard, 1959a, b ; 1960a, b, c; Fisher, 1958; Sheppard, 1959, 1961; Mayr, 1963; Wickler, 1968; Turner, 1970) . Most studies have dealt with polymorphic mimicry complexes. The evolution of monomorphic mimicry (that is, the establishment of a single mimetic phenotype in both sexes over wide areas), has received little attention (Ford, 1971). The purpose of this paper is to suggest some of the hypothetical steps by which monomorphic mimics may have evolved from butterflies with non-mimetic phenotypes. I have carried out an extensive series of reciprocal inter-specific hybrid crosses and backcrosses in the laboratory using eastern species of the Nearctic butterfly genus Limenitis. This Nymphaline group is considered to be monophyletic (Poulton, 1909; Gunder, 1934; Hovanitz, 1949; Chermock, 1950), and yet the wing color-patterns within it are quite diverse, even though all of the species possess similar developmental morphology, life habits, and behavior (Edwards, 1879; Scudder, 1889; Holland, 1905; and Klots, 1951). The two species studied include 1) the banded purple (L. artkemis arthemis Drury), red-spotted purple (L. a. astyanax Fabricius), and their intergrade form (L. a. proserpine Edwards) and 2) the viceroy (L. archippus Cramer). These two species, including the non-mimetic, intergrade, and mimetic forms of L. arthemis, are shown in Figure 1 (in this, and all subsequent figures, the specimens are arranged in sequences which best illustrate their evolutionary transformations) . L. a. arthemis, like most Palearctic Limenitis (Seitz, 1924; Wynter-Blythe, 1957; Higgins and Riley, 1970), is considered to be disruptively colored. It possesses broad medial white bands, crossing its wings and contrasting starkly with its basic dark pigmentation. This pattern optically serves to break up the outline of the butterfly, especially when in flight. L. a. astyanax and L. archippus have lost most,