There are many species of field crickets which all look alike, especially to us humans. They can usually be distinguished by their chirping, if one has good ears and have memorized how various species of crickets sound like.
Field cricket (Gryllus sp.) male.
Because only the male cricket chirps it is even more difficult to identify a single female alone. The females of some species of fields crickets can possibly be identified by the size of their ovipositor, or so some experts believe.
Field cricket (Gryllus sp.) female. Notice the ovipositor.
I took these pictures in Scarborough where the grass was loud with cricket music and probably because of the cold weather crickets were crawling all over the sidewalk to soak up some rays from the sun.
The fact that there are many species of field crickets even living in the same area, and that they are more or less indistinguishable from each other, except for the chirping sound the males make, suggests that some form of sexual selection is at play here, and the various non-interbreeding species might have come about because females refused to mate with a male whose chirping they did not like. Even if otherwise he would have been a suitable mate. Such a behavior can create an artificial barrier to interbreeding and create isolation so that speciation can take place.
Further Readings:
Genus Gryllus – Field Crickets on BugGuide.Net.
Field Cricket.
Field crickets – Gryllus spp.
In book form:
Richard Dawkins: The Ancestor’s Tale — the chapter entitled the Grasshopper’s Tale.