Most blood-sucking insects urinate while they feed so they can avoid filling up on fluid and get more nutrients out of their meal.
But some species of mosquito also do what is called preurination — they excrete drops of freshly ingested blood without extracting any of the nourishing blood cells.
The behavior has always confused scientists because “blood is a very precious resource,” said Claudio R. Lazzari, an entomologist at François Rabelais University in Tours, France. “The risk of taking it is very high.”
New research, conducted by Dr. Lazzari and colleagues and published in the journal Current Biology, shows that the preurine may serve to keep the cold-blooded mosquitoes from overheating while they take their blood meal, which can be as warm as 104 degrees Fahrenheit, depending on the host animal.
Roughly one to two minutes after she starts feeding, an Anopheles stephensi mosquito will excrete urine and preurine through the anus, at the end of the abdomen. Sometimes a drop of the fluid will form and cling to the body before falling off; when this happens, some fluid evaporates like sweat and cools the mosquito’s abdomen by almost four degrees.
Mosquitoes also feed on nectar, but they tend not to preurinate when they eat lower-temperature, sugar-based meals.
The mosquito is not the only insect that uses ingested food to regulate its temperature. Aphids excrete honeydew to prevent their abdomens from getting too hot, and some bee species regurgitate a bit of nectar to keep their heads cool while they fly.