ALLOCHRONIC SPECIATION IN FIELD CRICKETS, AND A NEW SPECIES,ACHETA VELETIS

Abstract

McNeill (1889) was apparently the first investigator to recognize that the most abundant and widely distributed field cricket in northeastern North America is actually composed of two populations, one overwintering as a late instar nymph and maturing in spring and one overwintering in the egg stage and maturing in middle or late summer. McNeill also noted (1) that the adult males of the nymph-overwintering population more often occupy burrows and are characteristically more solitary and more aggressive than those of the egg-overwintering population, and (2) that the ovipositors of the females in the nymphoverwintering population are usually shorter in relation to the length of the body than those of the females in the eggoverwintering population. Later investigators, such as Blatchley (1903, 1920), Walker (1904), Criddle (1925), Urquhart (1941), Cantrall (1943), Fulton (1952), Alexander (1957), and Bigelow (1958) have corroborated and refined McNeill's observations on these two populations without materially altering his conclusions. It is surprising that in spite of the confusion in field cricket taxonomy, the relationships of this pair of populations have been fairly well understood by field biologists for about seventy years. Prior to Fulton's work, various names had been applied to these two forms, either as binomials or as trinomials. Fulton, Alexander, and Bigelow did not separate the two populations with formal nomenclature. Fulton remained quite uncertain as to their status, being able to deal only with the southernmost fragments of their ranges in the northwestern part of North Carolina. Alexander, puzzled by the apparent identity of the two populations in song, habitat, and distribution, noted that most females could be separated on the basis of ovipositor length, and stated (p. 592), These two broods may interbreed in mid-summer, or possibly in fall in the southern part of their range, or it maybe that they have been isolated such a short time that no noticeable differences have yet appeared between them. Certainly more investigation is needed to, clarify their relationship. Bigelow, on the basis of differences he had discovered in the developmental rates of the two' poptulations, and the differences in diapause stage, stated (p. 147), The distinctive differences between these two' populations are more likely to become further consolidated than they are to break down through any future gene exchange. Therefore, these two populations should be regarded as distinct species, however similar they might be morphologically. Recently, we have pooled our information on these populations and have concluded that a more detailed discussion of their relationships, and their recognition as distinct species, is in order. We believe that these species have become reproductively isolated through a seasonal' separation of adults initially imposed by'