Forbes’s Tree Cricket

by Carl Strang

DuPage County is one of the few documented places where Forbes’s tree cricket occurs. That is not because it is particularly rare. It probably is quite widespread and common. The problem is that at present the only way to identify a tree cricket as a Forbes’s is to measure the number of pulses per second in its song (which is a long, nonstop, fairly loud trill). Forbes’s tree cricket is a sibling species of the better known black-horned tree cricket. In fact, where both are known to occur together the only way to distinguish them is by song pulse rate.

Black-horned or Forbes's 2b

Sibling species don’t interbreed, but visually are indistinguishable or nearly so. Black-horned and Forbes’s tree crickets are able to avoid interbreeding because the different pulse rates are recognized by females, who approach only males with the correct song for their kind. Sibling species are a recurring theme in all our major groups of singing insects: crickets, katydids and cicadas. That fact underlines the importance of song as opposed to visual recognition in these groups.

Black-horned or Forbes's male 1c b

You can’t distinguish their songs by ear; we’re talking about pulse rates that can approach 90 per second on a warm day. I use my home studio equipment to slow the recorded song, step by step, until the pulses are countable.

Forbes’s tree cricket was recognized as a species by Thomas J. Walker of the University of Florida, who runs the Singing Insects of North America website . As far as I know he has not published this distinction outside that website, but in his 1963 monograph The nigricornis Group of the Genus Oecanthus  he showed data that demonstrate black-horned tree crickets have two very distinct song pulse rates. With further study after that publication he concluded that these were in fact being produced by separate, sibling species. The “fast-trilling nigricornis” now is labeled Oecanthus forbesi, Forbes’s tree cricket.

These two species are distinguished from other tree crickets by being relatively dark, though they are quite variable and individuals at the paler end of the spectrum have to be studied with a hand lens focused on the pattern of spots on the two basal antenna segments (partially visible in the photo below).

Black-horned or Forbes's male 1a b

These two species prefer prairies or meadows with lots of forbs (flowering herbs other than grasses, sedges and the like), though some sing from vines or shrubs within such meadows. Given their recent recognition as separate species, little is known about their variability and possibly distinct habitat preferences. It is painstaking work, as you have seen, but in coming years I hope to build a database on this species pair in DuPage County. In my limited study so far I have found black-horned tree crickets at Blackwell Forest Preserve, and Forbes’s at Danada Forest Preserve and Tri-County/James Pate Phillips State Park.

1 Comment

  1. August 17, 2010 at 5:56 am

    […] Until I saw the photo I didn’t expect this. The cricket overall was very pale, and I had ruled out the black-horned/Forbes’s sibling species pair because they typically have dark areas on the head, pronotum (top of thorax), and underside of the abdomen. If I had to choose, though, based on antennal spotting alone, I would say this individual was a black-horned or Forbes’s tree cricket, albeit at one extreme end of their range of variation (for more on that species pair, see my post of last autumn). […]

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