Praying mantids are large and juicy, so it's little wonder that bats find them appetising. But mantids perform a cunning trick to stay out of a predator's clutches; their single ear is tuned to bats' ultrasonic calls, and when they hear a bat sneaking up on them, they plunge into a speedy nosedive. Exactly which characteristic of a bat's echolocation call triggers the escape artists' dramatic dive wasn't clear, until Jeffrey Triblehorn and David Yager timed exactly when praying mantids plummeted in response to different bat attack calls (p. 1867).
Bats emit pulses of echolocation calls during their nightly hunt. Triblehorn explains that they start calling at a low rate, but as they home in on a hapless insect, they switch to a higher calling rate to get more frequent updates on their prey's location. `Sometimes this transition from a low to a high calling rate is gradual, but sometimes it's very fast,' Triblehorn says. He wondered if mantids use the timing of a bat's transition pattern to decide exactly when to take the plunge.
To discover what triggers a mantid's power dive, Triblehorn took a close look at mantids' responses to bat calls. He tethered mantids in a sound chamber and set up a gentle breeze to encourage the insects to fly. He then played the last 1.2 seconds of five bat attack sequences, some with gradual and some with rapid transitions in calling rates, and waited for the insects to dive. To mark the millisecond timing of the mantids' escape response, he projected a laser light beam from the top of the sound chamber, which passed just in front of the tethered insect and contacted a light-detecting photocell on the chamber floor. Mantids' forelegs are neatly tucked away during flight,but when ultrasound calls trigger their escape manoeuvre, their legs shoot forwards and break the light beam, providing Triblehorn with the dive's exact timing.
But there's a problem when you're working on a millisecond scale: there is a time delay between the moment that a mantid's nervous system triggers the escape response and the moment that you record the insect actually performing the dive, called the response latency. Luckily, Triblehorn already knew the duration of the response latency between the trigger point and the mantids'dive. He knew that a particular artificial call rate definitely evokes a dive,and had recorded when mantids performed their dive after the onset of this particular call, giving him the mantids' response latency. Now, he could identify the exact point in the five bat attack calls that triggered the mantids' nervous system, by subtracting this known response latency from the timing of the dives that he had recorded for the mantids listening to these five calls. He found that all the mantids' dives were triggered when the bats called at 20-40 pulses per second, and concluded that this specific calling rate triggers the mantids' decision to dive.
But Triblehorn discovered that mantids' survival chances really depend on when bats hit this calling rate; the timing of bats' calling rate transitions is crucial. When bat calls switch rapidly from low to high calling rates, the mantids' alarm bells only start ringing very late in the call sequence, when the bat's attack is imminent. But a gradual transition appears to provide mantids with a `tip-off', because they perform their life-saving dive much earlier during these attack calls. Clearly, the odds that a bat will catch its dinner improve dramatically with a hasty increase of its pulse repetition rates.