Field crickets wait for spring beneath cow pies in Boulder’s Open Space. Click on picture for larger image.
They call from the entrance of their burrows, ready to dive for cover if calling attracts a predator rather than a mate
By Jeff Mitton
Curious to see which species were stirring in early spring, I hiked from the Coalton Trailhead on CO 128, looking for flowers and insects. Easter daisies hugged the ground, but no other flowers were up. I glimpsed a few spiders scrambling in the dry grass, but nothing else stirred.
On a hunch, I used the toe of my boot to flip a cow pie and was pleased to see four crickets scrambling in the unexpected light. I flipped about a dozen pies and found crickets beneath most of them.
I have always enjoyed the company of field crickets. They were in the basement and garage when I was growing up and I enjoyed the challenge of catching them by hand. Here in Boulder, their chirping is pervasive on summer nights.
Field crickets are in the genus Gryllus, which has over 70 described species. Field crickets in Boulder Open Space are Gryllus pennsylvanicus, probably the most common and certainly the most widespread in North America. They are found from the Atlantic to the Pacific and from Nova Scotia to Mexico.
Crickets have antennae pointing forward from the head and cerci pointing backward from the abdomen. They track moving objects by pointing the closest antenna directly at it and follow it by moving just that antenna, which has both mechanosensory and chemosensory cells used to identify prey items or the sex of an approaching cricket. The cerci bristle with tiny filiform hairs used to detect sound, including the chirping of other crickets.
Field crickets have wings and are polymorphic for wing size. Micropterus crickets have small wings, no wing muscles, and are incapable of flight, while macropterus crickets have long wings, large flight muscles, and are capable of sustained flight. The flight polymorphism is widespread and is maintained by a balance of selective forces. Most macropterus crickets are females, and a gravid female, with an average of 400 eggs, has the potential to fly to an unoccupied site establish a new population. Micropterus insects, spared the energetic expense of developing large wings and wing muscles, generally have higher viability and fecundity.
Male crickets call or chirp to attract to attract females. In the fields, G. pennsylvanicus chirps two or three times per second, and they usually space themselves so that adjacent calling males are 20 to 30 feet apart. They call from the entrance of their burrows, ready to dive for cover if calling attracts a predator rather than a mate. On any night, a single male can attract zero to several females.
Sonograms of field cricket songs showed that songs vary slightly among males. To test the hypothesis that some calls were more attractive than others, calling males were suspended above holes in the ground that would trap females responding to their calls. This clever experiment demonstrated that body size was not important, but somehow the females used the calls to approach older males.
A subsequent study compared solitary males caught in the field with males accompanied by one or more females. In comparison to solitary males, mating males were both older and had lower levels of intestinal parasites. Why did females prefer older males? They had outlived many others, demonstrating greater health and vigor and/or better ability to avoid predators. Their lower parasite loads certainly indicated better health.
Studies of cricket songs were conducted to determine if a male was signaling his age with his call. As males grew older, several components of their songs changed regularly, giving clear indications of the caller’s age.
On a summer night, when cricket calls merge into a pulsed chorus, recall the intrigue behind all that noise. Males are calling to attract mates and females are making decisions, choosing calls that advertise good genes for their eggs.
Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Daily Camera.
April 5, 2016