Identification
The degree of black on these two species (O. forbesi and O. nigricornis) can vary greatly, and the antennal markings also vary within their own species. Apparently the only way to know for certain which is which is by the pulse rate of the song of a male at a known temperature.
Regarding the difficulty in separating O. nigricornis from O. forbesi based on photographs: A gal who has done extensive studying of tree crickets (including song analysis, mating trials, and DNA sequencing) once wrote to me: In the eastern United States, I have found O. nigricornis, but not O. forbesi (in NJ and three sites in NY). In Ohio, I have found both O. forbesi and O. nigricornis in the same field, although the sites where I have found this have been in eastern Ohio (Akron and Canton). In central Ohio (Columbus area) and west from there, I have found O. forbesi but not O. nigricornis (OH, IL, WI, MI, IN, SD).
As a general rule, Forbes' is found west of Ohio; Black-horned is found east of Ohio. They are best ID'd by the rate of the male's song
It's probably wise to use this broad range for now in separating the two species. Unless something is clearly outside the area encompassing the states of OH, IL, WI, MI, IN, SD -- photos should probably go in the nigricornis vs forbesi taxon page.
Range
Regarding the difficulty in separating O. nigricornis from O. forbesi based on photographs: A gal who has done extensive studying of tree crickets (including song analysis, mating trials, and DNA sequencing) once wrote to me: In the eastern United States, I have found O. nigricornis, but not O. forbesi (in NJ and three sites in NY). In Ohio, I have found both O. forbesi and O. nigricornis in the same field, although the sites where I have found this have been in eastern Ohio (Akron and Canton). In central Ohio (Columbus area) and west from there, I have found O. forbesi but not O. nigricornis (OH, IL, WI, MI, IN, SD).
As a general rule, Forbes' is found west of Ohio; Black-horned is found east of Ohio. They are best ID'd by the rate of the male's song.
[O. forbesi is abundant in northern Wisconsin]