Bats and moths have been engage in acoustic warfare more than 60 million years. The interactions between bats and moths often been termed ‘an arms race’. Moth (order Lepidoptera) and bats (order Chiroptera) are active by night. Most Lepidopterans live for just a few days and must evade predators and find a food and a mate. Most Chiropterans orient in the environment, and capture prey in the dark with the use of an acoustic location.
There are two types of acoustic location: the passive acoustic location involves the detection of sound or vibration created by the insect; and active acoustic location involves the creation of ultrasound in order to produce an echo. Typical FM bats make a series of very short chirps. Most echo locating bats emit calls between 11 and 212 kHz and also between 60 and 140 dB.
Some moths use stealth tactics to reduce their conspicuousness to echo locating bats:
(i) Over 14 families of moths have ears (tympanic organ) that are adapted to detect the ultrasonic echolocation calls of bats,
(ii) On hearing a bat, these moths can cease all flight movement and drop to the ground, or make a variety of violent evasive maneuvers.
(iii) Some species of noctuid moths produce a sequence of broadband ultrasonic click. The click signal confuse the bat and jam its echolocation calls allowing the moth to escape,
(iv) The luna moths (Actias luna) generate an acoustic diversion with spinning hind wing tails to deflect echo locating bat attacks away from their body,
(v) As was previously stated, the wings of moths have scale coverage. The coverage reduces the potential of the reflected ultrasound signal from insects and decreases the noise radiation from flying moths.
Therefore the flapping flight of most moth species is silent and not audible to man and, more important, to their predators: bats and owls. In other words, the detection of the flying moths by the acoustic location is difficult for the predators.
As a rule, each of the insect’s wings consists of a thin membrane supported by a system of veins. But, recent works show that sound is very well reflected from a membranous wing. The many-plumed moths (Alucitidae) are a small family of Lepidoptera (Fig. 1). The Alucitidae wings evolved from membranous wings. As they evolved, the wing structure was modified: the membrane was reduced; fore and hind wings were divided into twelve plumes, consisting of rigid spines, from which radiated flexible micro bristles, creating a structure similar to a feather. The wing veins of Alucita hexadactyla are about 7 μm in diameter; the micro bristles are 50 μm long and .5 μm in diameter with 0.5 μm of interdistance, correspondingly, the density is 1000 micro bristles per millimeter. Thus, the wing structure of a twenty-plume moth consists of a system of micro elements: veins and micro bristles form a multislotted wing.
Oldest entomological journal in North America “Entomological News” (was established in 1890) published article “Stealth moths: the multi-plumed wings of the moth Alucita hexadactyla may decrease the intensity of their echo to simulated bat echolocation cries” by Igor S. Kovalev (Kinneret College, Israel). He researched the influence of the multi-plumed wings of the moth Alucita hexadactyla on the reflected ultrasonic signals. Initially, attention was directed to this problem by studies indicating that bats cannot detect thin wire.
Two different objects were used (Fig. 2). The first object was the pair of multiplumed wings of a twenty-plume moth (Fig. 2a). The principal concept of this study was to qualitatively determine the effect of the multi-plumed wings of the moth Alucita hexadactyla on the reflected ultrasonic signals. Therefore, the second object was a pair of flat plate wings of the moth Pediasia contaminella (Crambidae) (Fig. 2b). The wing area of the second object was similar to the wing area of the first object. The ultrasound properties of the twenty-plume moth wings were compared with those of the Pediasia contaminella wings. Two types of ultrasonic sensors were used: a reflection sensor and a through-beam sensor.
These studies showed that, on the one hand, the flat plate membranous wing strongly reflected the transmit pulse to the reflection sensor. The target echo was strong. On the other hand, the twenty-plume moth wings weakly reflected the emitter pulse to the reflection sensor. This experiment showed that the structure of the many-plumed wings of the moth Alucita hexadactyla reduces the intensity of the reflected bat-like ultrasonic signal by a diffraction of the signal around the micro elements of the wing structure. The multi-plumed wing of the moth Alucita hexadactyla is biology’s version of acoustic grating.
This finding expands our knowledge of antipredator deflection tactics, and reveals more detail about evolutionary ‘arms race’ between bats and moths. However, Kovalev said:” every white has its black and every sweet its sour: on the one hand, the multi-plumed wings of the moth Alucita hexadactyla are transparent to the bat’s ultrasonic squeaks, on the other hand, these wings increase the wing form drag. Nevertheless, this stealth tactic of the twenty-plume moth to reduce their conspicuousness to echo locating bats by the wing structure could be used to make stealth aircraft.”
Written by Igor S. Kovalev