The retransmission method

Before obtaining a proof of noctuids' ability to perceive the echoes of their own probing signals it was very important to determine which kind of behavior we should accept as a response to the echo. Earlier, similar problem arose and was solved in the course of studying responses to artificial stimuli in bats. It appeared that a good indicator of increased attention of a bat to biologically significant echolocation signals could be increased emission of the probing pulses (Grinnell, Griffin, 1958). Such changes were reproducible and could easily be registered.

Earlier we have found out that the maneuvres of a moth were very variable and often spontaneous (appearing without any noticeable stimulation). Moths had no certain sign of response to the echo: they demonstrated either movement to the object which was detected by means of echolocation or a detour manoeuvre, the changes of motivation occurring repeatedly during the experiment in some specimens. That was the other reason to search for a reliable behavioral test for moths.

The method we have finally elaborated was based on a principle of artificial echo retransmission: an echo from a non-existent (virtual) obstacle was retransmitted towards a moth after a certain delay in relation to it's own click. There was a dialog between the moth and the stimulating apparatus (some kind of self-stimulation). What is most important is that such stimulation system did not contain any movable details and thereby guaranteed that the results obtained were independent of the visual and tactile sensitivity of an insect. We have recorded the manouvres of a moth along with its clicks in every experiment but have not used them in further statistical analysis.

After the emission of each stimulating click, its repeated start up from the microphone was blocked during ca. 6 ms by means of a special electronic circuit. Thereby, the effects of direct action of the loudspeaker onto the microphone, and, as a consequence, the possibility to generate signals not related to the moth activity was eliminated.

A photograph of one of the first versions of a retrasmission setup

To record acoustic clicks produced by the moth, a condenser microphone (¼" Brüel & Kjær 4135) was placed on one side of the moth at the distance of 5 cm. After amplification, the signal from the microphone output was led to an electronic switch controlled by a computer. When the circuit was switched on each click of the moth (if it exceeded 80 dB SPL being recalculated to the distance of 2 cm from the insect) started up an analogue generator of click stimuli. A single acoustic stimulation pulse was similar to an expected echo from a plane obstacle (signal duration was ca. 100 microseconds, peak amplitude at the point of reception was equal to 71 dB SPL). During further experiments we have found that that level of sound should be lowered to 36-60 dB SPL to get stable responses from noctuid moths.

The delay of the retransmitted stimuli in relation to the respective probe clicks was equal to 0.48 ms. This interval corresponded to the distance of 8 cm from the insect to the virtual obstacle and consisted of the sum of the time required for the moth's probing click to reach the microphone (5 cm - 0.15 ms), the delay due to signal transformation in the electronic generator (0.03 ms - 1 cm), and the time of stimulation click propagation from the emitter to the moth (10 cm - 0.3 ms) - in total, it was 0.48 ms which corresponded to 16 cm, or 2x8 cm.

It has been found that echo-stimulation evokes significant increase in mean emission rate of own clicks (Lapshin, Vorontsov, 1998).

Here we imply by the 'stimulus' the whole period of readiness to respond (i.e. when the electronic circuit was switched on and the apparatus was ready to produce an echo in response to a moth's click) if it resulted in one acoustic contact, at least (i.e. the linked pair of events: own click of the moth - echo signal). The moment of the first such contact was automatically assumed as an onset of the stimulus time counting.

Later we have divided the stumulus into four parts of 0.2s each divided by non-stimulating intervals of the same length. This was done for better imitation of natural changes of echolocation situation around the flying moth. In fact, moths responded more pronounced to that kind of stimulation program.

As a rule, the stimuli having similar acoustic parameters but not correlated in time with own clicks a moth evoked an opposite response - the suppression of acoustic activity.

Responses to the echo-like stimulation

Moth Blepharita satura

A. An example of response (see previous image for explanation)

B. A histogram summarizing the responses to the echo-like stimulation. The click of a first acoustic contact (time point "0") was attributed to the background activity.

C. The same but the stimuli were driven by external 30 Hz generator and were not correlated to the clicks of a moth.

From our numerous results it could be also concluded that, at present, variations in the rate of acoustic emission should be considered as the most reliable index of the insect behaviour response to echo (Lapshin, Vorontsov, 2005, 2009).

We have used the retransmission method with many different experiment designs including the frequency threshold measurements and the estimation of the temporal resolution of a moth's ear.


Grinnell A.D., Griffin D.R. (1958) Sensitivity of echolocation in bats // Biol. Bull. V.114. No1. P.10-22.

Lapshin D.N., Vorontsov D.D. (1998) Activation of echolocation signal emission by noctuid moths (Noctuidae, Lepidoptera) in response to retranslation of echo-Like stimuli // Doklady Biological Sciences, V.362. 4. P.448-450. Translation: Doklady Akademii Nauk, V.362, 4, P.567-569.

Lapshin D.N., Vorontsov D.D. (2005) Retransmission of echo-like signals: method and results of studies on nocturnal moths (Insecta) // J. General. Biol., V.66, 1, P.75-89. (In Russian)

Lapshin D.N., Vorontsov D.D. (2009) The dependence of behavioral auditory thresholds on the delay of echo-like signals in noctuid moths (Lepidoptera, Noctuidae) // Journal of Integrative Neuroscience, V.8. 1. P. 1-12.