EVOLUTION IN THE FIELD CRICKET, ACHETA ASSIMILIS FAB.

Abstract

Progeny of northern spring field cricket adults lay non-diapause eggs, undergo nymphal diapause, and overwinter as nymphs. Progeny of northern fall adults lay diapause eggs, do not undergo nymphal diapause, and overwinter as eggs. The two populations cannot interbreed freely in the field owing to a temporal difference in breeding seasons; they did not interbreed in the laboratory. Rearing experiments show that the developmental differences are genetically based rather than environmentally conditioned, and it is, therefore, unlikely that hybrids would be viable even if they were produced in the field. Consequently these two populations behave as good species. Field crickets from Virginia developed much more rapidly than did spring crickets from Quebec. Quebec spring males and Virginia females produced hybrids with developmental rates intermediate between those of their parents. More female than male hybrids were produced, and the females developed more rapidly than did male hybrids. Offspring were produced by hybrid females and Quebec spring males, but not by hybrid females and Virginia males. Partial, but incomplete reproductive isolation exists between Quebec and Virginia field crickets. A possible mechanism of sympatric speciation in insects is discussed.