Blowflies keep their cool with drool

Dangle, slurp, repeat may help these insects keep their brains from overheating

If this blowfly starts overheating, it has a trick for reducing its temperature. It involves drool.

Muhammad Mahdi Karim/GFDL

SAN FRANCISCO, Calif. — Blowflies don’t sweat. However, they have raised cooling by drooling to a high art.

In hot times, these sturdy, big-eyed flies repeatedly release — and then retract — a drop of cooling saliva. Denis Andrade reported the trick January 4. He was speaking at the annual meeting of the Society for Integrative and Comparative Biology. He studies ecology and evolution at the Universidade Estadual Paulista. That’s in Rio Claro, Brazil.

The fly’s technique isn’t sweating. That is what keeps people cool. Sweat evaporates taking a tiny amount of body heat with it. Blowfly droplets put the cooling power of their natural fluids to use in a different way, explains Andrade.

The saliva hangs on a fly’s mouthparts. There, the droplet starts to lose some of its heat to the air. When the droplet has cooled a bit, the fly then slurps it back in, Andrade and colleagues showed.

X-ray images showed the sucked-up droplet in the fly’s throatlike passage near the animal’s brain. The same droplet seemed to be released, cooled, drawn back in and then drooled again. The insect repeated this several times in a row.

This may prevent dangerous overheating, Andrade proposes. After several droolings, the fly’s body temp can drop up to 4 degrees Celsius (7 degrees Fahrenheit) below that of the surrounding air.

Andrade had never seen a report of this saliva droplet in-and-out technique before he and a colleague noticed it. They had been watching blowfly temperatures for other reasons. But in 2012, Chloé Lahondère and a colleague described something similar — in mosquitoes.  

These blood suckers let their body temperature rise and fall with what’s around them. But they can get a heat rush when drinking from warm-blooded mammals, like us. So while drinking, the insects release a blood-tinged drop of urine from their rear ends. That droplet sheds some of their body heat. There’s some fluid movement within the droplet, says Lahondère. (She’s now at Virginia Tech in Blacksburg.) However, she can’t say whether any of the liquid gets recaptured by the body the way fly drool is.

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A mosquito has a reverse version of the blowfly trick. In a 2012 study, researchers showed how this Anopheles stephensi mosquito drinks mammal-hot blood and grows cooler (paler yellow) at the rear. That’s after the insect releases a drop of bloody urine that cools (blue) before falling and being replaced.C. Lahondère and C.R. Lazzari/Current Biology 2012

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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