2018-08-17 09:47 供稿單位:新航道
The first clear evidence of a sense of magnetism in insects
《昆蟲能感知磁力的個確鑿證據》 摘自The Economist June 23rd 2018
Bogong moths are not as glamorous as monarch butterflies. Their name means “brown” in Dhudhuroa, a now-extinct language once spoken in eastern Australia, where they live. And that is what they are—in contradistinction to a monarch’s glorious orange and black. But drab though they may be, bogongs surely match monarchs in migratory tenacity.
Monarchs, famously, fly across much of North America, starting or ending their journeys in one of a few groves of trees in central Mexico. An adult monarch, though, migrates only once. During their lifetimes, Bogong moths that survive to do so will make a pair of 1,000km journeys. One is from their winter birthing grounds in sun-scorched Queensland and New South Wales to a small number of cool caves in the mountains of Victoria where they will spend the summer months resting. The other is back again.
How they find their way to and from these caves is a mystery. But it is less mysterious in light of work by David Dreyer and Eric Warrant at the University of Lund, in Sweden, published this week in Current Biology. Dr. Dreyer and Dr. Warrant suggest that bogongs use a combination of magnetic compasses and topography.
關于它們是如何找到飛往這些洞穴的仍然是個謎。但是從瑞典隆德大學David Dreyer和Eric Warrant發表在本周《當代生物學》期刊上的文章來看，就不那么神秘了。Dreyer和Warrant博士認為布岡夜蛾是結合使用了磁針羅盤以及地形兩個方法。
Several types of animals, including birds, turtles and fish, are known to sense and navigate by Earth’s magnetic field, but evidence for such powers in migratory insects has been tenuous. A lone experiment has suggested monarchs may be able to detect magnetism—but, if so, that is probably just a back-up mechanism. Abundant other evidence suggests monarchs navigate mainly by the sun. For a night-flying moth, though, that is not an option.
To explore any magnetic sense bogongs might possess Dr. Dreyer and Dr. Warrant used light traps to capture hundreds of the moths during their migrations over the course of two seasons. They and their colleagues then glued stalks to the moths’ backs and, using those stalks, tethered the insects inside a flight simulator in which they were free to “fly” in any direction they wished—though, of course, they could not actually move. The simulator was surrounded by a pair of magnetic coils that cancelled out Earth’s field and replaced it with one of similar strength that could be turned through 120°. Because experiments like these, conducted on birds such as pigeons, have revealed that those animals do use Earth’s magnetic field for navigation, the researchers hoped their set up would to do the same with the moths.
It did not. Unlike birds, the moths either failed to react to the movement of the field or reacted in an unpredictable manner. Unwilling to give up their hypothesis, though, Dr. Dreyer and Dr. Warrant wondered if they had simplified their apparatus too much. Many nocturnal insects have exquisite night vision, so the two researchers thought that perhaps the absence of visual cues within their flight simulator, which had been intentional, might actually have confounded their experiment.
To test this, they lined the simulator’s interior with white felt and introduced a triangular black “mountain” above a black horizon as a landmark. During the experiment they started by keeping their magnetic field in alignment with Earth’s and then, after five minutes, began moving it. The moths continued to travel in the same direction with respect to the field for about three minutes after the field began moving but then, presumably as they realised that their visual cue and their magnetic cue were now in conflict with one another, they became disoriented.
This finding suggests that the moths do indeed depend on a magnetic sense to navigate during their long journey, but that they cannot, as it were, fly on instruments alone. They have rather to have sight of visual markers as well. The test of this will be in the next series of experiments Dr. Dreyer and Dr. Warrant are planning, which will move the “mountain” and the magnetic field simultaneously. That, they hope will fool the insects into thinking they are flying home.