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Genus Gryllus in North America north of Mexico
Species of the genus Gryllus (field crickets) are the most-studied of North American singing insects. They are large, easy to rear and handle, and diverse in their acoustic behavior, ecology, and life cycles. Their early taxonomic history is instructive. Many species were described in the 1800s, but when Rehn and Hebard (1915) intensively analyzed 1,500 pinned Western Hemisphere specimens, they concluded that all native American Gryllus belonged to a single "exceedingly plastic" species, Gryllus assimilis. This conclusion held for more than 30 years, until B. B. Fulton (1952) studied, in the field and in captivity, living Gryllus in North Carolina. He showed there were four reproductively isolated populations that differed in their calling songs, habitats, and seasonal life cycles. Because he found no defining morphological differences, Fulton did not give scientific names to the species he had discovered. R. D. Alexander (1957) extended Fulton's studies of Gryllus to the Midwest, found a fifth species, and showed that most species pairs were separable by morphological characters, at least in one sex. He assigned scientific names, thus confronting museum curators with the fact that their trays of Gryllus assimilis probably contained mixtures of species none of which was likely to be G. assimilis. More importantly, differentiating and naming species of North American Gryllus made them inviting subjects for studies of evolution, behavior, and physiology.
Since 1957, six more Gryllus species have been recognized in the eastern United States and 5 additional ones have been found in the West. David Weissman is currently working on a revision of Gryllus from the western and central states that will probably more than double the number of recognized species.
Studies of calling songs and life histories are still the predominant means of recognizing new species of Gryllus. Once a new species is recognized, morphological differences may be easy or difficult to find.
Seasonal life cycles
Species of Gryllus from the same geographic locality may have very different seasonal life histories. In fact, the two most abundant field crickets in the Northeast are separable chiefly by their life histories. Gryllus veletis and G. pennsylvanicus (spring and fall field crickets) do not differ in song or habitat and differ morphologically only in the average length of the ovipositor relative to the body length. However, G. veletis overwinters as mid-sized juveniles and matures in spring, whereas G. pennsylvanicus overwinters as eggs and matures in fall. The two species co-occur as adults, in very small numbers, only briefly in midsummer. Thus, except for the occasional specimen collected during the period of overlap, the date of collection is sufficient to distinguish adults of the two species.
Gryllus veletis and G. pennsylvanicus, like many other Gryllus, have a single annual generation. Some species have two discrete generations per year (e.g., G. rubens) and others have generations that overlap (e.g. G. assimilis). Gryllus firmus in Gainesville, Florida, has remarkably varied responses to seasons. It overwinters in all stages except small juveniles and the eggs of a single female may hatch over a five-month period, with the resulting juveniles maturing over a nine month period!
The evolutionary branching sequence (phylogeny) that produced the species of North American field crickets is important to those wishing to understand the evolution of the differences among the species. For example, the similarity of G. pennsylvanicus and G. veletis, except in their seasonal life cycles, led to the assumption that the two were sister species and to a theory of how the ancestral species might have produced the two species sympatrically by allochronic speciation (Alexander and Bigelow 1960). Subsequently, studies of chromosomes, allozymes, and mitochondrial DNA indicated that the split between egg and juvenile-overwintering species occurred early in the phylogeny of North American Gryllus and that G. pennsylvanicus and G. veletis were on different branches of the Gryllus family tree.
Huang et al. (2000) reconstructed the phylogeny of ten species of North American Gryllus based on the DNA sequences of two mitochondrial genes. Trios of closely related species were firmus-ovisopis-pennsylvanicus, texensis-rubens-lineaticeps, and fultoni-integer-veletis. Gryllus assimilis was not closely related to any of the other species, but it may form a monophyletic group with fultoni-integer-veletis.