We may spend inordinate amounts of time removing unwanted body hair, but hairy body parts are a matter of life or death if you're a cricket. Perched on the end of a cricket's abdomen is a small pair of hairy appendages called cerci. The cerci's mechanosensory hairs are sensitive to the slight air currents generated by wasps' wings or toads' tongues, alerting the cricket so it can make a timely escape from a predator sneaking up from behind. But nobody has looked to see how structural variability in the cerci's sensory hairs influences crickets' predator detection performance. To find out how morphological variability affects sensitivity to predators, Olivier Dangles and colleagues in Jérôme Casas' team at the University of Tours decided to measure the hairy appendages of crickets living in a range of environments (p. 461).
The neuroethology of crickets' mechanosensory hairs has been studied extensively in the lab. `But what's the point', asks Dangles, `if we don't know how these hairs work in the crickets' natural habitat?' Most researchers assume that the hairs form part of an effective predator escape strategy, but as Casas points out, `we don't even know what crickets' natural predators are!We need to put on our boots and get out there to study crickets in the field.'Dangles explains that the wood cricket Nemobius sylvestris is an ecologically relevant model system because this cricket spends most of its time foraging on the ground, making it vulnerable to wandering and flying predators.
Casas' team had already produced a biomechanical model of the responses of the cerci's sensory hairs to airflow. But to see how the hairs help wood crickets survive in their natural habitat, the team needed to explore how crickets' sensory hairs differ depending on the environment. They could then include this natural variation in their model to predict how morphological variability in the crickets' sensory hairs affects their ability to detect danger.
The team collected wood crickets from five locations all over France,ranging from forests to grassland and scrubland. Watching the animals under a microscope, they carefully measured the number, length and density of the crickets' long cerci hairs. The team then incorporated these micro-anatomical measurements into their biomechanical model. When they tested the model using ecologically relevant sound signals, such as the frequencies generated by a running or flying predator, they found large differences in the sensitivity of different cricket populations to approaching predators. Dangles explains that both the number and length of the cerci's hairs help the cricket escape impending doom; hairier populations are more sensitive to incoming predators,and populations with the largest proportion of long hairs are most sensitive to low frequencies. `This variability in sensitivity between natural populations suggests that different cricket populations have adapted to respond to different types of predator,' says Dangles.
Casas emphasizes the importance of the link between mechanism and function. He says `It's no good knowing all about the neuroethology of sensory perception and then getting stuck because we don't know what it's good for.'It was long overdue, but now crickets' predator responses have been tested in an ecologically relevant setting.