Monday, June 18, 2012

Hawkmoths See Humidity

Hawk moths of the Sphingidae family tend to feed nocturnally, a strategy which may protect them from daytime predators but which makes finding nectar sources more challenging.  They have the unique ability among nectar feeding insects, shared with hover flies, of being able to hover in place for a few seconds before heading off to the next flower.  Most hawk moths are nocturnal or crepuscular feeders (i.e. active feeders in twilight at dawn and dusk).

Hyles lineata by Chris Barnhart
The nocturnal hunt for nectar sources can be chancy if the moth spends more energy in the search than it gets from its food sources.  Hovering flight is expensive in fuel consumption as the hummingbird moth wings hover at 85 beats per second.  You can imagine that each stop to check out a flower for a few seconds is like idling your car at multiple stop lights while your gas gauge says empty.  Since they only live as adults for a week, efficient shopping is critical.

This raises the question of how they find their food sources.  They tend as a group to prefer long tubular white flowers and are "suckers" (brazen pun intended) for sweet smelling flowers with rich nectar that they can find at a distance. 

A new study reported in redorbit.com suggests that they improve their odds by sensing minute differences in the humidity of flowers.  The moisture around the opening of the blossom relates directly to the amount of nectar available and drops off if an insect has depleted the nectar pool.  The researchers at the University of Arizona further demonstrated that the white lined sphinx moth (Hyles lineata) would preferentially hover and try to feed on artificial flowers if the humidity of the "blossom" was raised as little as 4% from the ambient air, the gradient found on their normal nectar sources in nature.
“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.”
“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.”
Snowberry Clearwing- Roy Thompson
While most hawk moths feed at night, clearwing moths such as our common snowberry clearwing can be seen hovering all day long, hovering like a hummingbird but colored like a bee. Most of the scales which produce wing color are shed early after emerging from the pupal stage by its rapid wing beats.

Hawk moths have the longest tongues in the insect world, up to 14 inches long.  Darwin described a deep star orchid of Madagascar with a tubular flower that was 11 inches long with only an inch of nectar at the bottom.  He faced ridicule when he suggested that a hawk moth must exist with a long tongue which specialized in pollinating the orchid.  Years after his death he was vindicated.  This BBC video demonstrates this form of mutualism by a day feeding hummingbird moth.

More information on hawkmoths in general is at  Wikipedia and this US Forest Service site

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