DEVELOPMENTAL RATES AND DIAPAUSE IN ACHETA PENNSYLVANICUS (BURMEISTER) AND ACHETA VELETIS ALEXANDER AND BIGELOW (ORTHOPTERA:GRYLLIDAE)

Tendencies to diapause were not observed in nymphs of Acheta pennsylvanicus (Burmeister), but diapause was noted in a high proportion of the nymphs of Acheta veletis Alexander and Bigelow. On the average, nymphal development was more rapid in pennsylvanicus than it was in veletis nymphs. Embryonic diapause always occurred in pennsylvanicus but never in veletis eggs. These differences in developmental physiology are genetically determined and the failure of the two species to produce hybrids may be due to conflicting lethal interactions of the two genotypes in hybrid embryos. Both species die out after several generations of laboratory rearing at continuous high temperatures, and the evolution of cold hardiness has apparently involved a reduction in tolerance to constant high temperatures. Reproductive isolation may have been achieved by these two species without geographical isolation. Nymphal development is more rapid in northern than in southern veletis populations.