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Mepraia spinolai is an endemic species in Chile that lives in wild and domestic habitats. It is the only species of the Reduviidae family that shows alate polymorphism; females are always wingless, but males can be found with and without wings. The M. spinolai karyotype consists of 10 pairs of autosomes and a complex sex determination system. Males from the northernmost regions I and II (latitude 18 degrees-26 degrees South) are always winged (braquipterous) and are X1X2Y, with a large Y chromosome. From region III to the metropolitan region (latitude 26 degrees-33 degrees South), males may be either winged or wingless but appear to be polymorphic for a small neo-Y chromosome, which may have originated by fracture of the large holocentric Y chromosome found in populations from farther north. Experimental crosses suggest that the genes for wings are linked in the Y chromosome and also that there are two cytologically indistinguishable types of neo-Y chromosomes. One form (Y-1) bears a gene or genes for wings while the other (Y-2) lacks such genes. Males that are X1X2Y1, X1X2Y1Y1 and X1X2Y1Y2 are winged, while the absence of Y-1 (X1X2Y2 and X1X2Y2Y2) results in a wingless male. These chromosomes and morphological changes are correlated with a shift of the southern population into more arid habitats of the interior in the metropolitan region and region III.