SIBLING SPECIES,CALL DIFFERENCES,AND SPECIATION IN GREEN LACEWINGS (NEUROPTERA: CHRYSOPIDAE: CHRYSOPERLA)

Green lacewings of the morphologically homogeneous carnea-plorabunda-downesi group within the chrysopid genus Chrysoperla produce unique, species-specific, substrate-borne songs during courtship and mating; both sexes sing, and partners must reciprocally exchange their acoustical signals before copulation will occur. Two widespread, sympatric North American representatives of this complex, the sibling species pair C. plorabunda and C. downesi, hybridize readily in the laboratory but not in nature. This species pair has been presented as exemplifying sympatric speciation by disruptive selection. Here, it is shown from tape-playback and female-choice experiments that calls represent bona fide reproductive isolating mechanisms between the two species. Furthermore, call analyses of F₁, F₂, F₃, and backcross progeny of the two species confirm polygenic control of call expression, in which different alleles at each of several loci are fixed in each taxon. Sex linkage of traits is absent, but the various features of the calls are not completely independent of one another in their patterns of inheritance. These and other life-history data cast doubt on several major premises of the sympatric speciation hypothesis and suggest that call alteration might have triggered the speciation event giving rise to the siblings. A complex of cryptic “song morphs” physically and ecologically identical to C. plorabunda and C. downesi, but singing different songs, exists in the mountains of western North America, while the Alps of central Europe harbor populations of C. carnea that have undergone call differentiation in an analogous but independent manner. It is proposed that call divergence may in itself be driving the speciation process within this section of Chrysoperla, by greatly accelerating the rate at which full reproductive isolation between populations can be achieved.