When a cricket is airborne, it is on the alert for the ultrasonic calls of predatory bats. However many species of insects, including crickets, have both flying long-winged forms and flightless short-winged forms. Insects that are preyed on by bats are more likely to hear ultrasound, but since short-winged crickets can't fly, would their ability to hear ultrasound be affected because they are no longer preyed on by bats? Gerald Pollack and Ruben Martins, McGill University Canada studied the behaviour and the nervous system of long-winged and short-winged crickets (Gryllus texensis) to find out (p. 3160).
To measure the crickets' behavioural responses to cricket song and ultrasound, the team attached crickets to a stick with wax and put them in an air stream to induce flight. They played sounds of different intensities that mimicked a cricket's song or ultrasonic pulses out of one of two speakers either side of each cricket. As a cricket turns towards or away from a sound, it moves its abdomen and the team monitored these movements by measuring the abdomen's shadow cast on a photocell.
They found that all of the crickets tried to steer away from the ultrasound pulses and towards the cricket song, regardless of whether they were short-winged or long-winged. The threshold for the response to cricket song was the same in both types; however, the long-winged crickets had a threshold 8 dB lower to the ultrasound than the short-winged. This showed that long-winged crickets are more sensitive to ultrasound.
The team then made recordings from the nervous system, to find out how the auditory neurons responded to sound signals. As crickets age, their flight muscles degrade, in a process called histolysis. So they divided their crickets into three groups: short-winged, long-winged with intact muscles and long-winged with histolysed muscles. They recorded from a neuron called ON1, which is sensitive to a wide range of frequencies and receives inputs from one ear while inhibiting inputs from neurons transmitting signals from the other ear. They found that the threshold for ON1's response was similar in all the groups at 5.2 kHz, the frequency of cricket song. At ultrasonic frequencies, though, the response threshold of ON1 was lower in all long-winged crickets than in short-winged ones. This shows that ON1's response depends on the crickets' wing types: long versus short.
The response of a second neuron, AN2, which responds better to higher frequencies and triggers steering away from ultrasonic sounds, was also different between the groups. The sensitivity of AN2, however, depended on flight ability. While the threshold for the steering response was similar for cricket song, the thresholds for ultrasonic frequencies were higher for short-winged and long-winged histolysed muscle groups, neither of which can fly. This implies that the sensitivity of AN2 changes as the muscles break down.
Although the team's results clearly show that flightless crickets are less sensitive to ultrasound, the mechanism is unclear. They suspect that rather than short-winged crickets losing the ability to hear ultrasound, long-winged crickets are gaining increased ultrasound sensitivity as part of the suite of physiological changes that occur in preparation for flight, bringing with it an increased likelihood of being preyed on by ultrasonic-clicking bats.
- © The Company of Biologists Limited 2007