Axel Michelsen

Plants as transmission channels for insect vibrational songs

SummaryThe vibrational songs of several species of cydnid bugs and ‘small cicadas’ (leafhoppers and planthoppers) living on various types of plants are recorded by means of laser vibrometry. The recorded vibrational songs are analysed with respect to amplitude, frequency spectrum and structure in the time domain (Figs. 2–5).The emission of vibrational songs from singing insects on plants is simulated. A small magnet is glued to the surface of the plant and moved by means of an electromagnet about one cm away (Fig. 1). The vibrations are recorded by means of laser vibrometry. The propagation velocity of the vibrations increases with the square root of frequency, i.e. in the way expected for bending waves.The mechanical properties of plants ranging from soft bean plants to stiff reeds and maples are measured. The results are used for calculating the theoretical propagation velocities of bending waves. The measured and the calculated values are rather close (Table 1). Although the mechanical properties of the plants studied vary widely, the propagation velocities at a certain frequency are of the same order of magnitude (Table 1).In all the plants studied, only little vibrational energy is lost by friction at frequencies below some kHz. Communication by means of bending waves is possible over distances of some meters. The bending waves are reflected with little loss of energy both from the root and from the top of the plant. The vibration signals may therefore travel up and down the plant several times before decaying completely (Fig. 7). The vibration at a certain spot on the plant depends not only on the distance to and nature of the emitter, but also on the modes of vibration of the plant. The amplitude of vibration does not decrease monotonically with distance from the emitter (Fig. 6).These filtering properties of the plants mean that it is essentially impossible to predict which frequencies in the signals will be amplified or attenuated in the plant at the location of the receiving animal. The vibrational signals recorded from the animals cover wide frequency bandwidths. The signals are therefore well adapted to the filtering properties of the plants, but the signals of the species studied here do not appear to be particularly adapted to specific properties of the host plants.The muscular power needed for communication by means of various types of vibrational signals is calculated. The result of this calculation supports the conclusion that the signals recorded here are carried by means of bending waves.The communication strategies open to small insects are considered. Vibrational signals appear to be an efficient means of communication, but only certain types of signals are suited, because the plants cause a considerable distortion of the signals. One kind of distortion, the dispersive property, may — in theory — be used by the listening animals to obtain information about the direction and distance to the singing animals.

The physiology of the locust ear

SummaryThe sensitivity of three different preparations of the tympanal organ (“isolated”, “operated”, and “intact”, see Fig. 7a–c) has been measured over a wide range of frequencies (Figs. 3 and 6). The sensitivity of the intact ear to low frequency sound depends on the fat content of the animal (Figs. 4 and 5). The effect of diffraction (Fig. 8), the sound absorption in internal tissues (Fig. 10), and the sound transmission through the animal (Fig. 9) have been measured in order to explain the observed sensitivities in the three preparations. The internal tissues seem to act as an acoustic low-pass filter. Therefore, at high frequencies the intact and operated ears are acting almost as pressure receivers (Fig. 2a). The isolated ear is acting as an unbaffled pressure gradient receiver (Fig. 2b) with an “effective distance” of 0.8 mm. A mathematical model for asymmetric sound receivers is presented and used to calculate the force acting to move the tympanum in the operated ear at low frequencies. The driving forces in intact and operated ears are of the same order of magnitude as in a similar pressure receiver (table). The membrane vibrations at high frequencies are heavily damped both by the radiation resistance and by friction in the internal tissues behind the ear. The implications of these results for the understanding of directivity are discussed. Some common methods for determination of threshold are compared.

HOW HONEYBEES PERCEIVE COMMUNICATION DANCES, STUDIED BY MEANS OF A MECHANICAL MODEL

SummaryA mechanical model of a dancing honeybee was used to investigate the role of various components of the wagging dance in the transfer of information to follower bees. The model simulates the dance, carries a scent, and has an acoustic near-field similar to that of live dancers. The movements of the model are controlled by a computer, and selected components of the dance can be manipulated independently of others. The number of bees approaching scented baits at various distances and directions from the hive was observed, both during simulated “normal” dances and dances in which different components provided potentially conflicting information about the location of the food. The results indicate that the wagging run is the “master component” of the dance. The figure-of-eight dance path does not seem to convey information. Both sound and wagging must be present in the dance, but no specific roles were found for these components. Both sound and wagging convey information about distance and direction, and they appear to be largely redundant.

Sound Reception in Different Environments

The aim of this book is to discuss “sensory ecology”, that is the adaptation of sense organs to the properties of the environments. For hearing organs, it is at present only possible to describe such a correlation for hearing in air and water. Some physical parameters are different in these two homogeneous media (see below), and the hearing organs are adapted to the medium in which they are being used (references to the literature on hearing in fish, seals and whales can be found in Schuijf and Hawkins, 1976;, Mohl and Ronald, 1975; and Payne and Webb, 1971). The vast majority of hearing animals, however, live in terrestrial environments, and very little is known about the acoustical properties of these environments.

The physiology of the locust ear: I. Frequency sensitivity of single cells in the isolated ear

SummaryThe sensory responses of single receptor cells in the isolated ear of the locustSchistocerca gregaria were measured under controlled acoustical conditions. The four anatomical groups (Fig. 1) differ as to frequency sensitivity (Fig. 11). Although the isolated ear differs much from the intact ear, it may be concluded that fairly accurate information about sound frequency reaches the CNS. The responses of most units showed a maximum sensitivity at two (Figs. 4 and 9) or three (Fig. 8) different frequencies. But several units had only one maximum (Figs. 6 and 7, right).

Sound and vibrational signals in the dance language of the honeybee, Apis mellifera

SummarySound and vibrational signals exchanged by honeybees during the performance of wagging dances were simultaneously recorded by means of a microphone and a laser vibrometer. Previous descriptions of the 280-Hz sounds emitted by the dancing bee were confirmed, and no vibrational (substrate-borne) component could be detected. In contrast, the 320-Hz “begging signals” (emitted by bees following a dancer and used as a request for food samples from the dancer) do vibrate the comb with peak-peak displacement amplitudes up to 1.5 μm. Artificially-generated comb vibrations of sufficient amplitude cause bees standing on the comb to “freeze”. The threshold for obtaining a detectable freezing response was measured for frequencies between 100 Hz and 3 kHz. At 320 Hz it is just below the amplitude of the natural begging signals. Thus it seems likely that these signals are received by the bees as vibrations of the comb. The propagation velocity of waves, damping, and mechanical input impedance of honeybee combs were studied. These results, combined with the observed amplitudes of the begging signals, support the assumption that the begging signals are generated with the flight muscles. The begging signal propagates as a bending wave. The attenuation of the begging signal with distance is relatively small, so the amplitude of the signal probably needs to be carefully adjusted in order to restrict the range of the communication.

The physiology of the locust ear: II. Frequency discrimination based upon resonances in the tympanum

Summary1.The expected resonance frequencies of the tympanal membrane have been calculated from its dimensions, mass, and compliance. The thin part of the tympanal membrane may vibrate independently of the entire tympanum. Thus, there are at least two sets of resonances (Fig. 8).2.The two sets of vibrations have been observed by means of laser holography (Figs. 13–15) and measured with a capacitance electrode (Figs. 16–18). The position and amplitude of the vibration patterns, the phase relationships, and the niteraction of the two sets of vibration have been studied. The results are compared with the frequency sensitivity of the four groups of receptor cells.3.The groups of receptor cells are attached to four specialized areas on the tympanum (Fig. 6). The vibrations of these areas of attachment are a maximum at the frequencies of maximum sensitivity in the receptor cells (Figs. 16 and 17). Thus, the frequency discrimination seems to be a purely physical phenomenon, based partly on the presence of the tympanal resonances, and partly on the different positions of the receptor cells on the tympanal membrane.4.The two sets of vibrations have different spatial positions on the tympanum. The centre of the entire-membrane-vibrations is situated in one end of the membrane (Fig. 15), whereas that of the thin-membrane-vibrations is almost at the centre of the tympanum (Fig. 14). The positions of the centers of vibration are, however, not constant (Figs. 13 and 14). Different modes may have somewhat different centre positions, and these positions may change with frequency because of interactions between the two sets of resonances. Therefore, receptor cells attached to different areas on the membrane may pick up different modes of vibration. Also, the receptor cells may almost fail to respond to some modes, if their area of attachment is at a nodal circle of these modes at resonance.

The acoustic near field of a dancing honeybee

SummaryThe acoustic near field close to honeybees performing the wagging dance was investigated with pairs of small, matched microphones placed in various positions around the dancing bees. The dance ‘sounds’ are produced by the wings, which act as an asymmetrical dipole emitter. Close to the abdomen, the ‘sound’ pressures in the air spaces above and below the plane of the wings are totally out of phase. A zone of very intense acoustical short-circuiting exists close to the edges of the wings, where pressure gradients of about 1 Pa/mm are observed in the dorso-ventral direction (perpendicular to the plane of the wings). The pressure gradients drive air movements with velocity amplitudes up to about 1 m/s. The pressure gradients are much smaller in directions radially away from the bee and decrease rapidly with increasing distance from the wings. The ‘sound’ pressure detected by a stationary probe at one side of the bee is strongly modulated at 12–13 Hz as a result of the bees side-to-side wagging. Surprisingly little ‘sound’ is found near the dancers head. The positions of the follower bees reflect the properties of the acoustic field: The follower bees place their antennae in the zone of maximum acoustical short-circuiting where the air particle movements are most intense. These observations suggest 1) how follower bees can avoid mixing up the messages carried by the dance ‘sounds’ when two or more bees are dancing only a few cm apart and 2) how the followers might extract information about a dancers spatial orientation from the acoustic near field she produces. The observations also provide clues regarding the nature of the putative ‘sound’ receivers.

Biophysics of the ensiferan ear

Summary1.Laser vibrometry is used for observing vibrations of tympanal membranes. With this technique, the velocity and phase lag of vibrations can be measured from areas of 5–100 μm in diameter. The resolution of the apparatus is 10 Ångströms (vibration amplitude) in ‘real time’ broad-band operation (up to 100 kHz), but with averaging or selective filtering it is possible to perform more sensitive measurements.2.At ‘low’ frequencies (1–3 kHz) the sound acting upon the back of the bushcricket tympanum has almost the same numerical pressure as that acting on the front. The ear, therefore, is an ideal pressure gradient receiver.3.At ‘high’ frequencies (above 10 kHz) the sound pressure acting on the back of the tympanum is somewhat larger than that acting on the front. The membrane vibrates in its basic mode (like a piston) in the entire frequency range investigated (1–40 kHz).4.These properties are for ears with open ‘hearing trumpets’ (i.e., the horn-shaped trachea conducting sound to the back of the tympanal membranes). When the opening of the hearing trumpet is closed by blocking it with wax, the ear becomes a pressure receiver (at least at ‘low’ frequencies). It is now more sensitive to ‘low’ frequencies and less sensitive to ‘high’ frequencies.5.The vibrations of a membrane backed by a horn-shaped structure are considered theoretically. The expected variation in amplitude and phase angle of the driving force is calculated for systems in which sound is conducted through the horn-shaped structure with different amounts of gain.

Strategies for Acoustic Communication in Complex Environments

When performing his pioneering studies on the reactions of moths and other nocturnal insects to hunting bats, Kenneth D. Roeder had to get acquainted with acoustics and especially with ultrasound. He did this very well, but he never became interested in acoustics as a scientific subject. He was much more interested in the behavioural significance of the ultrasonic bat cries. His interests were always centered around the behaving animal in its natural environment, and he was well aware of the limitations of laboratory studies of the behaviour (Roeder 1970). A substantial part of Roeder’s impact on the development of neuroethology was due to his repeated demonstration that, although it is necessary to study many details in the laboratory, studies of behaviour should start and end in the field.