A white-lined sphinx moth visits an evening primrose flower. At this distance (one tongue length) from the flower, the moth can detect and evaluate a transient relative humidity gradient produced when these flowers contain nectar, without having to land and probe them.
People assume that a flower’s scent, color and shape attract insects to settle on a flower to sip nectar and, thereby, pollinate the plant. But new research shows that a more relevant sensory apparatus may help some insects hunt for a flower’s sweet juice: hygroreceptors that sense humidity.
A study of hawkmoths that hover and sip nectar from tufted evening primroses has shown for the first time that some pollinators can detect a mere 4 percent rise in humidity, which stems from evaporation of nectar directly above a flower.
"We were intrigued by the question; since flower scent, shape and color don’t directly provide information about nectar amount, could there be a more accurate signal?" said Martin von Arx, a postdoctoral entomology fellow at the University of Arizona and the lead author of the paper posted online May 29 in the Proceedings of the National Academy of Sciences. Von Arx was a postdoctoral researcher in the lab of co-author Robert Raguso, Cornell professor of neurobiology and behavior, where the experiments were conducted.
"We found a new sensory channel that mediates plant-pollinator interactions," von Arx added.
The researchers studied the white-lined sphinx ( Hyles lineata ), the most common species of hawkmoth in the United States, with the tufted evening primrose ( Oenothera cespitosa ), a favorite of the hawkmoth that blooms at night in southwestern deserts.
The researchers first scanned living primroses in sealed containers with sensitive humidity measuring devices and found the humidity was slightly higher just above the open flower, where water vapor rose out of the flower’s nectar tube.
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