The world of insects is full of vibrations, from wind-driven fluttering to deliberate drumming. Almost everything vibrates on the insect’s scale. In recent years, researchers have studied the wide range of ways that insects use vibrations. Many communicate by leaf-borne vibrations. Even incidental vibrations from crawling and chewing may contain valuable information. But do insects actually respond to vibrational cues before other signals? If they do, can they distinguish useful vibrations from vibrational noise produced by wind and rain?
In a recent paper in the Journal of Comparative Physiology A, Raul Guedes and his colleagues in Joyce Yack’s lab at Carleton University set out to answer these questions using birch caterpillars (Drepana arcuata). Previously, members of the Yack laboratory found that these caterpillars respond to each other with distinct vibrational signals, but they did not know how caterpillars distinguish communication signals from other environmental vibrations.
To find out whether caterpillars were listening in on others’ vibrations, the researchers allowed individual caterpillars to set up shelters on leaves before allowing an intruder to approach. To see whether or not the caterpillars could distinguish between intruders, the residents were exposed to the approaches of other birch caterpillars or predatory bugs. Using a laser vibrometer, Guedes and colleagues measured leaf vibrations near the resident caterpillar over the course of each intruder’s approach. As the intruder closed in, its crawling produced low amplitude vibrations. In response, the residents produced a series of deliberate vibrations, warning the intruders to back off. The speed of the response depended on the type of intruder – the residents responded much more quickly to predators than to other approaching caterpillars. Furthermore, the vibrational warnings often worked: the predators backed off about 80% of the times that the residents responded!
But Guedes and his colleagues wanted to make sure that residents were responding to vibrations from the invader, not visual or chemical cues. To check this, the researchers tried the experiments again – but this time, they made a cut in the leaf between the intruder and the resident, cutting off the resident from vibrations produced by the intruder. Without vibrational cues, resident caterpillars did not respond to the intruder until it crossed the cut in the leaf, much later than when the leaf was uncut. This suggests that leaf-borne vibrations are one of the primary signals used by caterpillars to identify the approach of others.
Next, the authors wanted to know whether birch caterpillars could recognize vibrational cues from intruders against a background of environmental noise. They simulated rain, by spraying water, and wind, using a fan, and repeated the experiments with the approaching caterpillars. In the absence of an intruder, the resident caterpillars did not respond to wind or rain. However, the residents were able to pick up and respond to an intruder’s approach despite the additional environmental vibration.
Although the vibrations produced by different sources largely overlap in frequency and amplitude, caterpillars may be able to isolate and recognize specific components of vibrational signals in order to distinguish between friend and foe. In a world full of vibration, perhaps it’s worth learning even more about caterpillars’ sophisticated senses.