Watch an insect scuttling along; its legs are almost a blur as they drum against the ground. Each leg's lightning fast movements are controlled by mechanosensors, which detect each footfall, and trigger the neural circuits that control the next footstep. But Reinhold Hustert explains that there is a problem; when ever scientists had calculated the time that it would take for a mechanosensor's nerve signal to travel from the foot to the central nervous system, it was simply too long. Hustert was puzzled by this apparent paradox;he knew that he would have to find a mechanoreceptor(p. 2715) that could send the neural message fast enough to keep the insect scampering.
Insects are covered in thousands of microscopic sensory receptors for detecting the world around them. The receptors range from tiny hair-like structures that fire when something brushes past them, to campaniform sensilla, which are microscopic dome structures that detect when the insect's cuticle deforms as it moves. Hustert realised that the mechanoreceptor responsible for triggering the insect's rapid reflexes must satisfy two conditions. First it must be close to the insect's body, to cut down the distance that the nerve signals travel before reaching the central nervous system (CNS). And secondly, the receptor must have a high conductivity to transmit the signal swiftly. Hustert knew that campaniform sensilla have high conductivity rates, but would he find them close enough to the insect's body?
Markus Höltje began testing locusts' reflexes. He gently probed various sensory organs on the insect's legs, and recording the motoneuron's response in one of the leg's depressor muscles. He also measured the length of time that it took nerve signals to reach the CNS, and found that the signals from most of the leg's receptors took almost 10 ms to travel to the CNS; far too slow. But when he recorded the signal transduction time from the campaniform sensilla near to the body on the leg's trochanter, it was almost 10 times faster, delivering a nerve spike to the CNS in 1 ms. And the superfast signal triggered motoneurons in the locust's depressor muscle.
But how does a mechanoreceptor that is so far up the locust's leg detect the footfall? Hustert realised that as the insect's tarsus touches the ground,a tiny shock wave travels up the cuticle of the insect's leg, but at much faster speeds than any nerve signal travels. Knowing that sound travels through wood at 3500 m s-1, Hustert estimates that the foot's impact generates a pressure wave that travels up the leg in 1 ms, which certainly makes the reflex fast enough. But can the receptor detect the footfall's tiny pressure wave? Höltje tested the delicate sensilla's threshold by gently touching the insect's tarsus, and realised that an impact that was fraction of the locust's body weight was enough to trigger the sensor.
Hustert adds that the campaniform sensilla's extreme sensitivity probably makes them extremely versatile sensory receptors. As well as triggering the insect's running reflex, he suspects that they also use the sensilla to detect gravity, helping the insect's to coordinate their movements, no matter what their orientation, as they roam through rough terrain.
