Vision and echolocation

Moth do not like to fly in total darkness. We have found that from our early experiments when we had thoroughly hidden every shining LED of our equipment. In those conditions it was not easy to record any clicks: a moth could either fly silently or cease its flight soon after the experiment onset. But, provided with even a very low background illumination, moths were able to maintain steady flight for tens of minutes being tethered to our shift sensor.

From these observations we had supposed that there must be some interrelation between the visual and the echolocational systems. To address this question in experiment we have developed an original 'virtual reality device' adapted for our moths.

An image of approaching obstacle - that was a vertical bar, dark on a lighter background, imitating a trunk of a tree moving towards a moth - was projected to the flat screen before the moth. To project an image with a very high flicker rate we have used an oscilloscope which was able to give us 700 fps. The image (a signal to drive the oscilloscope) was generated by a computer equipped with custom-made video-adapter. The light intensity on the screen was set to imitate a full moon night (0.2 - 0.5 lux).

Such a visual stimulation caused a remarkable increase of click frequency, especially when the "tree" had already disappeared from the screen, while the motor responses were rather weak (Lapshin, 1996).

An example of behavior recording

Notice the pronounced maneuvre and the probing clicks rate increase when the 'tree' was approaching close.

The distribution of probing clicks in response to the visual stimulation.

The virtual 'tree' was moving straight to face (the F histogram), turned to the left (L) or to the right (R). Each histogram summarizes the data from 21 stimulus presentations. The increase in clicks rate is significant.

To understand why noctuid moths need echolocation along with their high-sensitive vision we have measured the latent periods of response (the delays between the stimulus appearance and the response onset) to either visual or echo stimulation. We had to recognize the maneuvre as an universal reponse in both cases.

The image below clearly answers the question why the echolocation is of great importance to noctuids. Even when the vision is sensitive enough to detect nearby objects in dim light the information processing delay often might be too long to avoid collision, especially when flying among dense vegetation. At the same time the part of an auditory system that detects and analyses the echo acts much faster. It is fast enough to start the maneuvre at the very next wingbeat which is the best a flying moth could achieve.

Distributions of motor response latencies in response to visual (green) and echo (gray) stimulation.

To make the visual stimulation a constantly shining green LED which was turned off for 1.5s. To measure the response to an echo we analysed the tracks obtained in 'carousel setup'.


Lapshin D.N. Influence of vision stimuli upon the variation of acoustic emission in moth (Amphipyra perflua: Noctuidae) // Sensornye Systemy, 1996, V.10. No3, P.79-87 (in Russian). Translation: Effect of visual stimuli on the dynamics of acoustic emissions by noctuid moths (Amphipyra perflua:Noctuidae) // Sensory Systems. 1996. V.10. No3. P.207-214.