Why are flies so hard to swat?
2021-01-14 21:46

The house fly is known for its evasive maneuvers.


(Image: © Suman Acharya/Alamy Stock Photo)

图片:©Suman Acharya / Alamy Stock Photo)

A fly buzzes past your head and lands nearby; you snatch a flyswatter or roll up a magazine and approach cautiously — and you strike! 


But no matter how quick you are, the fly is almost always faster, and it usually manages to evade your wallop and escape unharmed. (Is it trying to annoy you?!)


Flies have many adaptations that lend them heightened speed, maneuverability and perception, making them very, very good at detecting and evading even the swiftest swats. And new evidence shows that flies' modified hind wings play an important part in launching them into a speedy takeoff — often just in the nick of time.


House flies (Musca domestica) belong to the order Diptera, or true flies. Diptera flies possess modified hind wings that have evolved into sticklike structures with a knob at the end, called halteres. Their vibrations help the insects stabilize their bodies while in flight, by sensing body rotations and transmitting information to the wings.   


Flies in the Diptera subgroup Calyptratae, which includes house flies, also vibrate their halteres while walking, but scientists didn't know why. In a study published online Jan. 13, 2021 in the journal Proceedings of the Royal Society B: Biological Sciences, researchers investigated Calyptratae flies to see if haltere oscillation affected their transition into the air, directing additional sensory input to help coordinate movements in the wing and leg muscles.  


Using high-speed cameras to capture tethered and free laboratory-reared flies during takeoff, the scientists recorded footage at speeds up to 3,000 frames per second. They found that Calyptratae flies launched themselves around five times faster than other flies; their takeoffs required an average of about 0.007 seconds (7 milliseconds) and just one wingbeat. 


"None of the Calyptratae had a takeoff duration longer than 14 milliseconds [0.014 seconds]," the researchers reported. By comparison, non-Calyptratae flies' takeoffs lasted about 0.039 seconds (39 milliseconds) and required about four wingbeats, according to the study. 


Next, the researchers removed the halteres, which all Diptera flies have. Calyptratae flies lacking these knobby structures took a lot longer to become airborne, but takeoff time wasn't affected in non-Calyptratae flies without halteres. Stability during takeoff also suffered with haltere removal, but only in Calyptratae flies. For example, the Calyptratae insects known as blow flies that attempted escape takeoffs without their halteres "always resulted in a crash landing," the scientists reported.


"Haltere use thus allows for greater speed and stability during fast escapes — but only in the Calyptratae clade," the scientists wrote in the study. 


In the blink of an eye


Halteres aren't the only secret weapon in a fly's evasive arsenal; once a fly is airborne, it can execute maneuvers that would be the envy of a fighter jet pilot. Fruit flies can change course in under 1/100th of a second — about 50 times faster than an eye can blink, Live Science previously reported. In experiments, perfectly timed wing flaps generated enough force to rapidly propel the flies away from a predator while in mid-air. 


"These flies roll up to 90 degrees — some are almost upside down — to maximize their force, and escape," Florian Muijres, who studied the biomechanics of flight at the University of Washington in Seattle, and is now at Wageningen University & Research in the Netherlands, told Live Science in 2014. 

“为了最大限度地发挥力量,这些苍蝇会向上卷起90度,有些甚至是上下颠倒,”西雅图华盛顿大学(University of Washington)研究飞行生物力学的弗洛里安·缪伊雷斯(Florian Muijres)在2014年接受《生活科学》(Live Science)采访时表示。

Flies also have exceptional vision, which helps them plan their jumps away from a threat. About 200 milliseconds before takeoff, fruit flies use visual input warning of looming danger to adjust their posture and pinpoint the direction that will launch them to safety, scientists wrote in 2008 in the journal Current Biology


In fact, their enhanced perception juggles up to six times more visual input in one second than humans can, the BBC reported in 2017.


Animal brains perceive the passage of time by processing images at speeds known as the "flicker fusion rate," a term describing how many images flash into their brains per second. Roger Hardie, a professor emeritus of cellular neuroscience at the University of Cambridge in England, implanted electrodes into the photoreceptors of flies' eyes to measure their flicker fusion rate, calculating it to be 400 times per second; the average flicker fusion rate for humans is about 60, according to the BBC. This means that movement you perceive as "normal" moves like slow-motion to a fly.

动物的大脑通过处理图像的速度来感知时间的流逝,这种速度被称为“闪烁融合速度”,这个术语描述了每秒有多少图像闪现到它们的大脑中。罗杰·哈迪(Roger Hardie)是英国剑桥大学(University of Cambridge)细胞神经科学的荣誉退休教授,他将电极植入苍蝇眼睛的光感受器,以测量它们的闪烁融合率,计算出其为每秒400次;据BBC报道,人类的平均闪烁融合率约为60。这意味着你认为“正常”的动作对苍蝇来说就像慢动作。

With all these built-in advantages, it's no wonder that the fly you're trying to swat can escape. However, one approach that might improve your chances is aiming your swat at a spot where the fly is likely to go, rather than where it's resting, Michael Dickinson of the California Institute of Technology in Pasadena told The Independent in 2011.

有了所有这些内在的优势,难怪你试图拍打的苍蝇能逃脱。然而,帕萨迪纳市加州理工学院的迈克尔·迪金森(Michael Dickinson)在2011年告诉《独立报》(the Independent),有一种方法可能会提高你拍苍蝇的几率,那就是瞄准苍蝇可能会飞到的地方,而不是它休息的地方。

"It is best not to swat the fly's starting position," Dickinson said. "Aim a bit forward of that to anticipate where the fly is going to jump."


Originally published on Live Science.


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