SYDNEY: Using high-speed video footage, bioengineers have discovered the key to the evasive manoeuvrability of flies – and found the best strategy for swatting them successfully.
Michael Dickinson has been interviewed hundreds of times about his research on the biomechanics of insect flight. One question has always dogged him: Why are flies so hard to swat?
"Now I can finally answer," said Dickinson, a bioengineer at the California Institute of Technology in Pasadena, USA.
Tiny brain, big escape plan
Using high-speed, digital imaging of fruit flies (Drosophila melanogaster) faced with a looming swatter, Dickinson and graduate student Gwyneth Card determined the secret to a fly's crafty behaviour.
Long before the fly leaps, its tiny 'brain' calculates the location of the impending threat, comes up with an escape plan, and places its legs in an optimal position to hop out of the way in the opposite direction. All of this action takes place within about 100 milliseconds after the fly first spots the swatter.
"This illustrates how rapidly the fly's brain can process sensory information into an appropriate motor response," said Dickinson who this week publishes a paper detailing the research in the U.S. journal Current Biology.
In this study, the swatter was actually a 14-centimetre-diameter black disk, dropping at a 50º angle toward a fly standing at the centre of a small platform.
The researcher's videos show that if the descending swatter comes from in front of the fly, the fly moves its middle legs forward and leans back, then raises and extends its legs to push off backward.
Rear attack
When the threat comes from behind, however, the fly (which has a nearly 360º field of view) moves its middle legs a tiny bit backwards. With a threat from the side, the fly keeps its middle legs stationary, but leans its whole body in the opposite direction before it jumps.
"We also found that when the fly makes planning movements prior to take-off, it takes into account its body position at the time it first sees the threat," Dickinson said.
"When it first notices an approaching threat, a fly's body might be in any sort of posture depending on what it was doing at the time, like grooming, feeding, walking, or courting," he said.
Perfecting pre-flight posture
Yet, the experiments hinted that the fly somehow 'knows' whether it needs to make large or small postural changes to reach the correct pre-flight posture. This means that it must integrate visual information from its eyes with sensory information from its legs, to tell it how to move to get in the optimal pose for take-off.
The results offer new insight into the nervous system of insects, and suggest that within the fly brain there is a map in which the position of the looming threat "is transformed into an appropriate pattern of leg and body motion prior to take off," Dickinson said. "This is a rather sophisticated sensory-to-motor transformation and the search is on to find the place in the brain where this happens."
Handily, the research suggests an optimal method for successfully swatting a fly.
"It is best not to swat at the fly's starting position, but rather to aim a bit forward of that to anticipate where the fly is going to jump when it first sees your swatter," suggested Dickinson.
悉尼发稿：高速摄影机拍摄的镜头，使生物工程学家发现了苍蝇能屡屡逃之夭夭的秘密，同时也找到了成功拍死苍蝇的策略。
Michael Dickinson已经接受了数百次采访，采访内容都和他在生物力学上的一项研究有关：昆虫飞行。有个问题一直困扰着他：为什么很难拍死一只苍蝇。
“现在我终于能回答这个问题了，”美国Pasadena加州理工学院的生物工程学家Dickinson说到。
脑袋瓜虽小，逃生策略却一套套
通过用数码成像的高速摄影机拍摄苍蝇拍拍向果蝇（学名Drosophila melanogaster）的过程，Dickinson和研究生Gwyneth Card发现了苍蝇狡猾的逃生行为背后藏着的秘密。
早在苍蝇临危一跃之前，它的小脑瓜里就已经在盘算着潜在威胁来袭的方位了，它想好逃跑方案，将6条腿调整至最佳位置，然后朝着与苍蝇拍来袭方向相反的方向一溜烟地飞跑了。从苍蝇觉察到苍蝇拍，到这一整套动作全部完成，只需短短100毫秒。
“这说明苍蝇从大脑处理接受到信息，到采取合适行动这一过程是多么地迅速，”Dickinson说到，他这周刚刚在Current Biology上发表了一篇学术论文，详细介绍了他在这一课题上的研究成果。
该项研究中，所谓的苍蝇拍实际上是一个直径14厘米的黑色圆盘，以50º角拍向落在一个小试验台中心的苍蝇。
从研究者录制的视频显示来看，如果苍蝇拍是从苍蝇的前方拍过来，苍蝇便会将中间两条腿前移，同时身体后倾，然后抬起身体，将腿伸展开，便向后飞走了。
后方来袭的危险
然而面对来自后方的危险，苍蝇（苍蝇的视角能达到360º）却是将中间两条腿稍向后移动。对于来自侧方的危险，苍蝇中间两条腿保持不动，但在跃起逃跑前，它整个身体会向另一侧倾斜。
“我们还发现，当苍蝇在完成它逃跑前的一套动作时，还会在觉察危险的第一时间内，考虑其身体的位置，”Dickinson说。
“当苍蝇在第一时间内察到即将降临的危险时，根据苍蝇当时具体在做什么，比如它可能正在清洁身体、进食、漫步或者求爱，因此它的身体可能处在任何一种姿态，”Dickinson说。
完美的预逃跑姿势
然而，试验结果还暗示了苍蝇能通过某种方式“知道”它是需要大幅调整，还是只需微调身体姿态，以达到正确的预逃跑体位。这就说明苍蝇必须整合来自复眼的视觉信息和来自脚下的触觉信息，并根据这些信息，在起飞逃跑前将身体调整到最佳姿势。
试验结果在昆虫神经系统方面提供了一些新的见解，试验结果表明，在苍蝇的大脑中有一张显示有危险来袭方向的地图。“根据这张地图，苍蝇设计出一整套逃跑前的动作来调整它的腿和身体，”Dickinson说。“从感知危险到采取行动，这其中的过程非常复杂，我们也在寻找这一过程发生在苍蝇大脑的哪个部位。”
这项研究就这样找到了一种成功拍死苍蝇的好方法。
“最好不要从苍蝇逃跑的起点处落拍，而要有一定的提前量，即落拍点选在你估计的苍蝇逃跑的方向上靠前一些，”Dickinson向大家建议 |
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