Hawkmoths Can Actually See Humidity
May 30, 2012

Hawkmoths Can Actually See Humidity

Brett Smith for RedOrbit.com

Instead of using visual cues or floral scents, some moths detect increases in humidity around flowers to see if it is worth further inspection, new research led by a University of Arizona entomologist has found.

According to the study published in the Proceedings of the National Academy of Sciences, Martin von Arx of the University of Arizona has discovered a previously unknown sensory channel that is used in plant-animal interactions.

"As creatures who use vision and olfaction, humans think in odors and shape, and color," von Arx said. "We are biased by what we can perceive. “

Researchers in the study found that the white-lined sphinx hawkmoth can detect minuscule differences in humidity when hovering near a flower. The moth then uses this information to determine if there is enough nectar inside to warrant a visit.

This ability is crucial to the survival of the hawkmoth, especially those living in Arizona. These moths seek out the tufted evening primrose at dusk and must quickly determine which flowers carry food and which do not, as hovering in front of the flower while probing it with its long proboscis is one of the most energetically draining modes of flight, von Arx said. The insect also becomes vulnerable to predators such as bats as it plunges its head deep inside the flower to reach its nectar.

Unlike other, often misleading, devices used by flowering plants to attract pollinators such as flower size, shape or color; the humidity evaporating from the flower's nectar provides what the researchers call an ℠honest´ signal. Interestingly, scent is independent of nectar, which is odorless in most plants.

"The metabolic cost of hovering in hawkmoths is more than 100 times that of a moth at rest," said Goggy Davidowitz, the study´s co-author and a UA professor. "This is the most costly mode of locomotion ever measured. An individual hawkmoth may spend 5-10 seconds evaluating whether a flower has nectar, multiply that by hundreds of flowers visited a night, and the moth is expending a huge amount of energy searching for nectar that may not be there. The energy saved by avoiding such behavior can go into making more eggs. For a moth that lives only about a week, that is a very big deal."

After finding that humidity just above the opening flower was slightly higher than ambient levels, the research team put artificial flowers in a large cage. While none of the artificial flowers had nectar, the moths would preferentially hover and extend their proboscis into those that the scientists had artificially elevated their humidity levels. The hawkmoths were able to sense if the humidity near a flower was elevated as little as 4 percent above ambient humidity in the flight cage.

These findings help to explain how ℠honesty´ factors into the complex symbiotic relationship that exists between the moths and the evening primrose, according to von Arx.

"If you're one of only a few flowers and there are lots of pollinators out there, you don't have to be honest about how much nectar you have because they'll visit anyway," von Arx said.

"But if you want the attention of just a few, you really have to go all out. So by saying, ℠Hey, come here, I have lots of nectar,' you're giving a faithful signal about an actual benefit that the pollinators can perceive and evaluate."