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The Arms Race Between Moths and Bats

Some moths, notably from the family Noctuidae, have true tympanic ears, located on the thorax just below the second pair of wings. These ears form one of the simplest sensory systems ever found, yet they enable moths to evade bats quite well.

The moth ear consists of two sensory cells coupled to a tympanic membrane. Each cell has a different threshold - the more sensitive one is called A1, and the less sensitive is A2. They are both especially sensitive to frequencies between 23-50 kHz, which is exactly the range used by bat sonar. While bats are unable to track moths at ranges greater than about 10 feet, the moths can hear the bats' squealing from about 100-200 feet away. This is a necessary advantage, since bats fly much faster than moths.

Since the ears are placed directly below the anterior wings, the flapping of the wings has a profound effect on the signal. Moths are able to interpret this effect in order to judge the height of the threat. The effect of the wings seems to give the moths an increased dimension of depth in their hearing, since when a bat is approaching a moth from the left hand side its squeals will be almost completely blocked by wingbeats on the moth's right side. If the bat approaches from directly behind the moth, all receptors will fire at the same rate. And when the bat is above the moth, the moth's receptor activity fluctuates in synchrony with its own wingbeats. These simple variations in receptor activity activate quick pre-programmed evasive actions in the moth:

  • Any moth detecting a bat at long range simply turns away and flies away at maximum speed. Remember that the moths' detection range is far greater than the bats', so the moth simply wants to make sure it stays out of the bats' range.
  • If a bat is detected at close range above the moth, the moth goes into spinning maneuvers, usually followed by a fast dive.
  • At close range on the same level as the bat, the moth flies away in the horizontal plane, turning as much as possible. Moths can, of course, turn much faster than bats, although their velocity is much lower.
  • Finally, if the moth encounters a bat below it, it flies immediately upwards.

The difference in sensitivity between cells A1 and A2 is what tells the moth the range of the bat. A1 is sensitive to low intensities of sound, and is saturated at higher intensities. A2, on the other hand, only begins to fire at middle to high intensities. The ranges overlap slightly, giving the moth a very accurate picture of the bat's distance.

The "arms race" between bats and moths is fairly well balanced. Bats are faster than most insects, and their echolocation system gives them an extremely precise picture of their prey, its location and movement, at a relatively limited range. Moths have a much simpler sensory system that simply alerts them to the bat's presence and vector, but sets off a routine of fast maneuvering. Obviously neither group is 100% successful, but both of them are successful enough to survive as species. Some scientists have suggested that the moths' ears actually evolved in response to bat sonar, as moth species began to diversify at about the same time as bat species. The fact that the ears have apparently been lost in (diurnal) butterfly species that evolved later, seems to reinforce this view - active by day, the butterflies have a whole new set of predators to worry about, and different mechanisms for evading and eluding them.


Most of the classic experimentation on this subject was done by K. D. Roeder in 1966-67. Some later studies have found more major neurons connected to the tympanic system, but their functions are still not understood clearly.

    Sources:
  • Matthews & Matthews, "Insect Behavior" (1978)
  • http://soma.npa.uiuc.edu/courses/physl490b/models/moth_startle/moth_startle.html
  • http://www.irysec.vic.edu.au/sci/goneill/butterflyevo.htm

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