Softly, softly, mosquitoes outwit human victims

Abstract

Stealth and haste are often at the heart of most successful raids, and the forays of famished female mosquitoes are no different. ‘Female mosquitoes need a blood meal to develop their eggs’, explains Florian Muijres, from Wageningen University, The Netherlands, adding that zipping in and out after a quick sip without attracting the attention of the unfortunate victim is essential if the insect is to survive. ‘One critical manoeuvre is the escape take-off after blood-feeding’, says Muijres. The fully loaded insect should push off quickly from the surface of its victim to get away swiftly, but a softer and gentler take-off would be less likely to arouse the attention of a disgruntled host. Knowing that humans are particularly vulnerable to mosquito bites, which can transmit deadly disease, Muijres, Jeroen Spitzen (Wageningen University) and Sofia Chang (University of California, Berkeley, USA) scrutinised the voracious insect's escape technique to find out how it manages to evade scrutiny when fully laden.‘The experimental setup was rather complex’, admits Muijres, describing how Chang had to ensure that the insects were well fed on human blood before take-off trials, in addition to coordinating three cameras trained on the insect's launch pad to capture every aspect of the departure in minute detail. ‘To get everything to work properly required a lot of preparation, precise work and a steady hand’, chuckles Muijres.As most insects simply hurl themselves into the air with a hefty push of the legs, Chang, Muijres and Wouter van Veen were surprised to find that the mosquitoes began beating their wings about 30 ms before the final push-off, using an extraordinarily high wingbeat frequency of ∼600 beats s–1 compared with ∼200 beats s–1 used by other similarly sized insects. And when the team calculated how hard the mosquitoes pushed down on their victim over the course of a take-off, they realised that the insects took advantage of their exceptionally long legs, extending them gently while pushing down slowly over the 30 ms take-off, leaving the unsuspecting injured party none the wiser of the departure. In addition, when Chang and Muijres calculated the wings’ contribution to the take-off, they were impressed that they contributed 60% of the force needed to lift a well-fed mosquito off its victim.Next, the team compared the mosquitoes’ take-off performance with that of more stocky fruit flies to find out whether the mosquitoes had compromised the speed of their evasive action for a light-footed getaway. Calculating the take-off force generated by a sturdy fruit fly, the team was surprised that the take-offs weighed in at almost four times the forces exerted by the nimble mosquitoes; sufficient to attract the attention of a human. However, when the team compared the two insects’ take-off speeds, the mosquitoes were every bit as speedy (∼0.24 m s−1) as their heavy-footed counterparts. Mosquitoes are not penalised by their softly-softly strategy, and are every bit as swift as less subtle insects. And when Bart Biemans compared the size of the mosquito and fruit fly leg muscles, the mosquito leg muscles were smaller, ‘because they need to produce a lower force at push-off’, says Muijres.Having discovered the secret behind the mosquito's stealthy departure, Muijres and his colleagues are keen to find out whether other blood-sucking pests use the same strategy to outwit their victims and whether the irritating insects’ landings are as graceful as their departures.