Plants As Transmission Channels for Insect Vibrational Songs
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Behavioral Ecology Behav Ecol Sociobiol (1982) 11:269-281 and Sociobiology 9 Springer-Verlag 1982 Plants as Transmission Channels for Insect Vibrational Songs Axel Michelsen 1, Flemming Fink 1, Matija Gogala 2, and Dieter Traue 3 Institute of Biology, Odense University, DK-5230 Odense M, Denmark 2 Institute of Biology, University E. Kardelj, Ljubljana, Yugoslavia 3 Institut ffir Allgemeine Zoologic, Freie Universit~it, D-1000 Berlin Received May 24, 1982 / Accepted October 18, 1982 Summary. The vibrational songs of several species of vibration of the plant. The amplitude of vibra- of cydnid bugs and 'small cicadas' (leafhoppers tion does not decrease monotonically with distance and planthoppers) living on various types of plants from the emitter (Fig. 6). are recorded by means of laser vibrometry. The These filtering properties of the plants mean recorded vibrational songs are analysed with that it is essentially impossible to predict which respect to amplitude, frequency spectrmn and frequencies in the signals will be amplified or atten- structure in the time domain (Figs. 2-5). uated in the plant at the location of the receiving The emission of vibrational songs from singing animal. The vibrational signals recorded from the insects on plants is simulated. A small magnet is animals cover wide,, frequency bandwidths. The glued to the surface of the plant and moved by signals are therefore well adapted to the filtering means of an electromagnet about one cm away properties of the plants, but the signals of the (Fig. I). The vibrations are recorded by means of species studied here do not appear to be particular- laser vibrometry. The propagation velocity of the ly adapted to specific properties of the host plants. vibrations increases with the square root of fre- The muscular power needed for communica- quency, i.e. in the way expected for bending waves. tion by means of various types of vibrational The mechanical properties of plants ranging signals is calculated. The result of this calculation from soft bean plants to stiff reeds and maples supports the conclusion that the signals recorded are measured. The results are used for calculating here are carried by means of bending waves. the theoretical propagation velocities of bending The communication strategies open to small waves. The measured and the calculated values are insects are considered. Vibrational signals appear rather close (Table 1). Although the mechanical to be an efficient means of communication, but properties of the plants studied vary widely, the only certain types of signals are suited, because propagation velocities at a certain frequency are the plants cause a considerable distortion of the of the same order of magnitude (Table 1). signals. One kind of distortion, the dispersive prop- In all the plants studied, only little vibrational erty, may - in theory - be used by the listening energy is lost by friction at frequencies below some animals to obtain information about the direction kHz. Communication by means of bending waves and distance to the singing animals. is possible over distances of some meters. The bending waves are reflected with little loss of ener- gy both from the root and from the top of the plant. The vibration signals may therefore travel Introduction up and down the plant several times before decay- Airborne sound waves offer an efficient way of ing completely (Fig. 7). The vibration at a certain communication for man and many animals. The spot on the plant depends not only on the distance environment is a complicated acoustic filter, how- to and nature of the emitter, but also on the modes ever, and the sound signals therefore have to be adapted to the particular environment, in which * Dedicated to Dr. F. Ossiannilsson, whose pioneering studies the animals live (reviews: Michelsen 1978; Wiley led to the suggestion that small insects may use plants as and Richards 1978). transmission channels for their songs It is difficult for small animals to use airborne 0340-5443/82/0011/0269/$02.60 270 sounds for communication, except at short dis- Several kinds of waves can propagate in rod- tances. Small sound emitters are very inefficient shaped solid structures: longitudinal, transverse, at low frequencies (see e.g. Michelsen and Nocke torsional, bending, and (in thick rods) surface 1974), and ultrasonic signals are not suited for pen- waves. The longitudinal waves are analogous to etrating an environment dominated by plants (Mi- sound waves, where the particles vibrate in the di- chelsen and Larsen, in preparation). Vibrational rection of wave propagation, but for this and other signals, on the other hand, appear to be a better structure-borne waves the displacement of a choice. For physical reasons, animals living in or certain part of the rod from its equilibrium posi- on water or solid media (soil, wood) may efficiently tion is associated with a tensile stress. The tensile emit vibrational signals at low frequencies, where stresses are here in the direction of wave propaga- little airborne sound can be produced (Markl tion. Pure longitudinal waves do not occur in rods, 1968). because local changes in length are associated with The existence of vibrational communication changes in cross-section. In such quasi-longitudi- has been demonstrated by means of behavioural nal waves the ratio (R) between the transverse and observation in several groups of insects (Markl longitudinal displacement amplitudes is 1973; Gogala et al. 1974; Cokl et al. 1978; Str/ib- ing 1977; Bell 1980). Most of the signals are related R = 2z~Izr/2L (1) to sexual behaviour. The range of communication where rc is 3.14 ..., r is the radius of the rod, 2 L may be limited to a few cm in heavily damped is the longitudinal wavelength, and g is Poisson's media like soil, but ranges of 0.8-2 m have been ratio (which is around 0.2-0.3 for most materials). reported for small insects living on plants (Chvfila The velocity of propagation (%) is determined by et al. 1974; Ichikawa 1976, 1979). The methods Young's modulus of elasticity (E) and the density used in most of the earlier studies have been re- of the material (p) stricted to, for example, observing the behaviour of animals sitting on the same plant or on neigh- bour plants. The vibrational signals have been c L = (2) measured either with vibrational transducers (grammophone stylus, accelerometer) or as weak Transverse waves ('rotational' waves) and tor- airborne sounds with condenser microphones. sional waves are closely related, and both propa- These methods did not allow detailed measure- gate with a velocity which is about 60% of c L. ments or a determination of the kind of vibrational The motion of the material and the direction of waves used by the animals. The exceptions known the tensile stresses are here in a plane perpendicular to us are the pioneering studies of the vibrational to the propagation of the wave. Transverse waves signals of ants buried in the soil (Markl 1968) and may occur in large plates, but no component of of plecopteran signals transmitted through plants the movement is perpendicular to the surface. The (Rupprecht 1968). The results of the latter study same is true for torsional waves travelling in rods are discussed in detail in this article. with a circular or annular cross-section. Compo- Vibrations in the nm range may be measured nents of motion normal to the surface only occur within a broad frequency range by means of laser in beams with non-rotational cross-section. vibrometry (Michelsen and Larsen 1978). The laser Bending waves (' flexural' waves) are sometimes beam does not load the plant mechanically, and termed 'transverse' waves, but this is not the defi- the vibrational signals can therefore be detected nition of transverse waves in the strict sense used without any interference from the measuring sys- in this paper. The propagation of bending waves tem. In this article we report measurements of the is complicated by being dispersive (frequency-de- vibrational songs of several species of small insects pendent). Furthermore, for short pulses of sinusoi- (cydnid bugs and 'small cicadas') on a variety of dal carrier wave (of frequency f) one has to distin- plants with rather different physical properties. We guish between the propagation velocity of the car- further describe experiments and calculations, rier wave (the phase velocity, ca) and that of the which were carried out in order to elucidate the pulse envelope (the group velocity, Ca, which is nature of the vibrational signals and the filtering twice %). % is given by properties of the plants. j- Basic Physics of Waves in Beams cB = Vco (3) The physics of structure-borne vibration is rather complicated. A detailed introduction is given by where I is the (surface) moment of inertia, m' the Cremer et al. (1973). mass per unit length, and co = 2 zcf 271 I is proportional to the fourth power of the beam substrates. In this study we measured the premating songs from radius (r) and m" to the second power of r. males and females both on dry beech leaves and on the stems c B is therefore proportional to the square root of of Galeobdolon vuIgare. r and to the square root of f. The power flow The Vibrational Songs. The vibrations were measured with a is made up of a force contribution and a moment laser vibrometer. The use of this instrument for vibration mea- contribution, which are totally out of phase and surements has been described in detail (Michelsen and Larsen of equal magnitude, so that the power flow is con- 1978). In brief, laser light is focussed at a spot of 30 gm diame- ter on the surface of the plant.