A novel pathway is proposed to explain the occasional breakdown of prezygotic (chemical-courtship) isolating mechanisms in mimetic ithomiine butterflies (Mechanitis polymnia and M. lysimnia). In dense dry-season populations, mixed groups of males have been observed to court a single female. She becomes receptive through recognition of the pheromone produced by the conspecific majority males, but may be mated by a transpecific minority male in the courting group, leading to accidental interspecific hybridization. All five interspecific matings observed in Sumare, Sao Paulo, Brazil, were between a female of the more-abundant species, M. polymnia casabranca, and a male of the less-abundant M. lysimnia lysimnia. The Fl hybrid can probably backcross to both parental types, as many recombinant phenotypes are represented in the 48 presumed hybrids known from six localities, among over 36,500 parental types sampled or marked during seven years. The small interspecific introgression may occasionally be important in the evolution of these mimetic butterflies. THE BREAKDOWN OF SPECIES-ISOLATING MECHANISMS in animals can lead to the possibility of introgression and even reticulate evolution of the sort which is better studied in plants. Such hybridization represents a threat to the taxonomist, a challenge to the evolutionary biologist, and a joy to the geneticist who can thereby study the interaction of separate adaptive lines in a single individual. Normally, sympatric and closely related animal species possess very efficient barriers against gene flow. That such barriers are occasionally ineffective, however, is well known (Mayr 1970: 69-81; Lowe, Wright, Cole, and Bezy 1970; Patton, Selander and Smith 1972; Jones 1973; Woodruff 1973; White 1978). In extreme cases, especially after environmental modification, complete breakdown has been observed with massive interspecific introgression and even local formation of new isolable adaptive sets, in a background of strongly heterozygous populations (White 1978: 333-336). Hybridization may be a far more important factor in species multiplication and adaptive radiation than is generally admitted by the school of allopatric speciation (Templeton 1981). Still, the breakdown of isolating mechanisms seems to be very infrequent in nature, and the opportunity for observation of its pathway and the confirmation of its results has been rarely offered to biologists. We report here preliminary observations on a probable new pathway which can lead to interspecific hybridization in locally dense populations of Mechamitis polymia and M. lysimnia (figs. 1-12), two warningly colored ithomiine butterflies which commonly occur together (Brown 1977, 1979) from central Mexico to southern Brazil. M. lysimnia, more tolerant of cold climates, extends farther south to central Argentina, but is absent from much of the central Amazon Basin. The two species are biologically distinct: in our study area in Sumare, Sao Paulo (22?51' S., 47'16' W.) (Brown and VasconcellosNeto 1976, Vasconcellos-Neto 1980), they oviposit on different foodplants, shown temporal dissociations in population dynamics and reproductive peaks, and have morphologically distinct larvae (VasconcellosNeto 1980) as well as differently patterned adults (fig. 1). Previously, presumed cases of interspecific hybridization in Ithomiinae (Brown 1977) were ascribed to the well-known pathway (Mayr 1970:78) of extreme scarcity of one of the species near a spatial or temporal limit, leading to acceptance of a transpecific mate by an individual (presumably female) of the rare species which has had no success in locating a conspecific consort. How might hybridization come about in dense, sympatric butterfly populations? We have observed in the field some of the steps of the presumed hybridization process, reported here. COURTSHIP AND MATING BEHAVIOR IN Mechasnitis. -In diurnal butterflies with similar mimetic patterns, chemical communication must be especially important in courtship recognition (Scott 1973). This conclusion is supported by the exceptional development of scent organs in the members of Mullerian mimicry complexes (Muller 1877a, 1877b, 1878; Brower, Brower, and Collins 1963; Brower, Brower, and Cranston 1965; Owen 1971: 158-163). Many years ago, the hair-brushes located on the hindwing costa of male Ithomiinae were suspected to function in scent diffusion during courtship (Miiller 1878, 1879; Kaye 1914); recent studies substantiating this 288 BIOTROPICA 14(4): 288-294 1982 This content downloaded from 157.55.39.221 on Mon, 03 Oct 2016 05:27:25 UTC All use subject to http://about.jstor.org/terms