It's been said that mosquitoes are the deadliest animal on the planet. Given their abilities to transmit diseases such as malaria and yellow fever to humans, it's no wonder that many people consider them so. There are, however, a couple of caveats to calling mosquitoes as a group dangerous. First, it's only the female mosquitos that need to feed on blood; and second, not all mosquitoes bite humans. In fact, some mosquitoes specialize in feeding on specific animals such as frogs. Of these specialized blood-feeders, the female pale-footed Uranotaenia (Uranotaenia lowii) has developed a surprising method of finding its food: ‘Studies performed in the field show that pale-footed Uranotaenia hunt frogs by hearing’, explains Hoover Pantoja-Sanchez from Purdue University, USA. But it wasn't clear precisely how the mosquitoes eavesdrop on their food, so Pantoja-Sanchez, with a team of researchers from the USA and Sri Lanka, decided to take a closer look at the insects to find out how the females focus in on their frog feasts.
As mosquitoes use their antenna to hear, the team first turned their attention there. Normally, male and female mosquito antennae look very different from each other, but in these frog-feeding mosquitoes, the antennae of both the males and females are much more similar. This suggested that U. lowii must be using their antennae for something different than other mosquitoes. Usually, mosquitoes use their antennae to listen out for a mate, so Pantoja-Sanchez and colleagues monitored a colony of the insects non-stop for a month, looking for signs that the insects take to the wing in swarms to listen out for prospective mates. But the pale-footed Uranotaenia did not form swarms, and when the team followed the insects’ courtship rituals, the mosquitoes were clearly not listening out for each other. Having ruled out that pale-footed Uranotaenia search for mates by hearing, the team turned their attention to find out how well they hear their favourite food.
After placing individual female mosquitoes in a tube linking two cages, the researchers played frog calls in one cage and no sound in the other, and waited to see which cage the females selected. Half of the female mosquitoes flew toward the cage with the frog-calling speaker, whereas only ∼22% flew into the silent cage. The female mosquitoes could hear the frog calls and fly towards them, so the researchers decided to find out how the mosquitoes were finding appetizing amphibians by hearing.
First, the team checked to see how the mosquitoes' antennae vibrate when listening to sounds ranging from 100 to 1000 Hz, to find out which frequency range their hearing might be tuned to. They found that the males’ antennae barely vibrated when listening to sounds pitched at the same frequency as those produced by the flying females; the males are likely deaf to the female's whining wing beats. However, when they checked the females’ antennae, they vibrated strongly when listening to tones pitched in the range of the calls of barking treefrogs (Dryophytes gratiosus), upon which they dine. Then, the team began the difficult task of measuring the responses of the nerves inside of the mosquitoes’ antennae and found that the females’ antennae are strongly attuned to the frog's calls. In short, they can find their food in a way that other mosquitoes can't – by eavesdropping.
Although many animals, including some insects, can listen in on their prey in order to find them, this hasn't been seen previously in mosquitoes. Maybe this incredible sensory feat will help earn mosquitoes the respect they deserve.
