A museum specimen of the fine striped sweat bee changes color as it goes from blue-green in dry air (right) to green-copper in humid air (left). Most of the color change occurs in the first 24 hours.
Jorge De La Cruz
When the weather gets muggy, iridescent bees shift their metallic hue.
The insects are a rich blue-green when ambient humidity is low. But as the amount of moisture in the air increases, the bees turn a coppery green, researchers report April 22 in Biology Letters. The reversible mood ring–like effect may be an overlooked phenomenon determining day-to-day color in bees and other insects.
Many bees have a shimmery exoskeleton. Some researchers working with specimens in museum collections have noted it changes color. For instance, the bees appear to dramatically shift color when placed in a high humidity chamber used to make them more flexible for mounting and imaging.
“It can be a bit alarming when you’re not expecting it,” says Madeleine Ostwald, a functional ecologist at Queen Mary University of London.
When Jorge De La Cruz, an undergraduate student at the University of California, Santa Barbara, noticed this effect when curating bee specimens, he, Ostwald and their colleagues decided to investigate further. The researchers exposed two dozen museum specimens of fine-striped sweat bees (Agapostemon subtilior) to high and low humidity conditions for 55 hours each and then took photos of the bees. The researchers also gathered over 1,000 photos of living sweat bees from the citizen science app iNaturalist and noted the humidity at the time and place that the image was taken.
In dry conditions, under 10 percent humidity, the bees were blue-green. But at 95 percent humidity, they turned a lighter, copper green with more moisture. The researchers suspect the change comes about as humidity causes layers in the bees’ exoskeletons to swell. These layers manipulate the way light is reflected, creating the iridescent effect. This generates particular colors based on the light wavelengths that are scattered by the layering. If the layers swelled, the space between them would increase, causing them to reflect longer, redder wavelengths.
“Because we saw redder bees in more humid conditions, this fits with that [scientific explanation],” Ostwald says.
Future work with high-powered microscopes might confirm this idea. The phenomenon may be widespread in insects that similarly produce color with microscopic structures rather than pigments, Ostwald says.
“Color can be really dynamic and respond to the environment in ways we didn’t expect,” she says. “It’s really important to study the color of the living organism in its natural environment, because as soon as we remove them from that context, the color changes.”
