Dangle, slurp, repeat may help the insects protect their brains from overheating
Muhammad Mahdi Karim
SAN FRANCISCO — Blowflies don’t sweat, but they have raised cooling by drooling to a high art.
In hot times, sturdy, big-eyed Chrysomya megacephala flies repeatedly release — and then retract — a droplet of saliva, Denis Andrade reported January 4 at the annual meeting of the Society for Integrative and Comparative Biology. This process isn’t sweating. Blowfly droplets put the cooling power of evaporation to use in a different way, said Andrade, who studies ecology and evolution at the Universidade Estadual Paulista in Rio Claro, Brazil.
As saliva hangs on a fly’s mouthparts, the droplet starts to lose some of its heat to the air around it. When the fly droplet has cooled a bit, the fly then slurps it back in, Andrade and colleagues found. Micro-CT scanning showed the retracted droplet in the fly’s throatlike passage near the animal’s brain. The process eased temperatures in the fly’s body by about four degrees Celsius below ambient temps. That may be preventing dangerous overheating, he proposed. The same droplet seemed to be released, cooled, drawn back in and then released again several times in a row.
Andrade had never seen a report of this saliva droplet in-and-out before he and a colleague noticed it while observing blowfly temperatures for other reasons. But in 2012, Chloé Lahondère and a colleague described how Anopheles stephensi mosquitoes that exude a liquid droplet that dangles and cools, but at the other end of the animals.
Mosquitoes, which let their body temperatures float with that of their environment, can get a heat rush when drinking from warm-blooded mammals. While drinking, the insects release a blood-tinged urine droplet, which dissipates some of the heat. There’s some fluid movement within the droplet, says Lahondère, now at Virginia Tech in Blacksburg, but whether any of the liquid gets recaptured by the body the way fly drool is, she can’t say.
When it's not drool
A mosquito has a reverse version of the blowfly cooling trick. In a 2012 study, Chloé Lahondère, now at Virginia Tech in Blacksburg, and a colleague showed how this Anopheles stephensi mosquito drinks mammal-hot blood and grows cooler (paler red-orange) at the rear after releasing a drop of urine that cools (blue) before falling and being replaced.
G. Gomes et al. Stay cool with a drop of drool: evaporative cooling blowfly way. Society for Integrative and Comparative Biology 2018 annual meeting, San Francisco, January 4, 2018.
C. Lahondère and C. Lazzari. Mosquitoes cool down during blood feeding to avoid overheating. Current Biology. Vol. 22, January 10, 2012, p. 40. doi: 10.1016/j.cub.2011.11.029.