Arctia caja
The third type of display is triggered by some form of disturbance, generally directed from the front of the moth, but usually unaccompanied by a touch. The moth walks forward, flapping its wings at short intervals (this closely resembles the wing-waving displays of Blest, 1964), somewhat reminiscent of a butterfly sunning itself, the wing movements being accomp- anied by a distinctly audible rattle, which sometimes continues momentarily after the wing flapping has ceased. The patagia are raised (figs. 4-7) and the defensive fluid secreted during this display. The wing flapping not only seems to reveal the maximum amount of red colouring but also serves to disseminate the odour of the defensive secretion. It is problematical (see below) whether or not it is directly concerned with the sound production, and after this display the moth settles down to rest again and does not attempt to fly.
The rather worn specimen from which the recordings were made (fig. 8) was caught in a light trap at Elsfield, Oxford; on opening the trap it responded with the sound-accompanied display. After the recording shown in figure 8 was made, the moth was photographed displaying (figs. 4-7). No attempt to fly was seen during or immediately after this display, and the moth walked a distance of 9 feet, wing- flapping and rattling continuously, before settling down again to rest.
The recordings were made with a Telefunken AKG 19C tape-recorder, with the microphone held about 10 cm. from the moth; tape speed was 7+ i.p.s. Oscillograms were made from the recordings on a Tektronix Type 502A. The stridulation is a rattling noise, and analysis shows that it consists of repeated bursts of sound which vary in duration from 6 to Q sec. and occur with variable frequency. Each of these bursts of sound is made up of two, three or four pulses (fig. 9,each pulse lasting perhaps 3/lOO sec
Such pulses could be made by a tymbal mechanism, but it is unlikely that they could be produced by a frictional mechanism; it is more probable, on account of their duration, that they are generated by movements of the thorax produced by contractions of the wing muscles or of other thoracic muscles. This suggestion is supported by the observation, reported above, that the sound has occasionally been heard after the wing-flapping display has ceased.
The apparatus available did not allow the recording of ultrasonic frequencies, and the possibility of emission of this nature during the display must not be over- looked.
The audible noise is of relatively low intensity, and mammalian and avian predators would have to get close to the moth to hear it; but in such conditions the sound could nevertheless act as a reinforcing stimulus in a protective display. [1]