invertebrates

Although much research has been done to describe the degradation of sound signals propagating in natural habitats, the directional cues of sound have so far been neglected. This paper describes a first approach to quantifying the degradation of directional cues in sound propagating parallel to the ground in a grassland habitat of orthopteran insects. A matched pair of probe microphones measured the sound amplitude and phase close to the ears of grasshopper carcasses for 12 evenly spaced directions of sound incidence. The degradation was found to increase with frequency and distance from the sound source and to decrease with distance from the ground. The acoustical data were used to predict how well animals with different auditory systems can determine the direction of the sender. At one position in the habitat, the predictions were compared with the pattern of phonotactic responses of live grasshoppers. Amplitude cues appear to degrade much faster with distance than phase cues. Animals exploiting phase cues may therefore maintain a reasonable directional hearing when the amplitude cues no longer make sense. The pressure-difference-receiver type of ears responds to phase differences, and these ears may be particularly suited to overcoming the degradation of directional cues. This suggests that the possession of such ears may be an adaptation not only to small body size (relative to wavelength), but also to the acoustic properties of the habitat

1. Phonotaxis was investigated in two cicada species: Cystosoma saundersii and Cyclochila australasiae. Females were placed on a stick within a flight cage and presented with artificially generated calling songs. These model calling songs had a range of carrier frequencies, but their temporal parameters were similar to those of the natural calling song. They were broadcast at intensities 30 to 40 dB above the physiological threshold for each frequency.
2. Phonotaxis of female Cystosoma saundersii was restricted to a 45 minute period just after sunset, and was highly directional. Between 60 and 70% of flights made during trials in which a model calling song was broadcast were directed towards the loudspeaker at both frequencies tested.
3. Phonotaxis of female Cyclochila australasiae occurred throughout the evening, and showed no directional preference toward the loudspeaker. The mean number of flights per trial period was significantly greater in trials during which a model calling song was broadcast than in control trials during which no model calling song was broadcast. There was no significant difference in the mean number of flights per trial with different carrier frequencies.
4. In female cicadas, acoustic signals of the males are preferentially graded by the tuned auditory system; phonotactic decisions are then made on the basis of relative intensity without active discrimination between frequencies.

1. Experimental and theoretical studies on the acoustics of the singing burrow of the mole cricket Gryllotalpa australis are reported.
2. The burrow typically consists of a bulb about 26 mm long and 20 mm in diameter, connected through a constriction of diameter about 10 mm to a flaring horn with length about 40 mm and equivalent mouth diameter about 34 mm. The mouth geometry of the burrow differs from one species to another, and the aperture may be either single, double or even multiple. The end of the bulb opposite the horn connects to a narrow exit tunnel of diameter about 8 mm and length up to 1 m. The singing cricket positions itself close to the constriction between the bulb and the horn and produces a song with a frequency around 2.5 kHz.
3. Measurements of sound pressure within the burrow when it is excited by an external sound source at the song frequency show a pressure minimum at the constriction and an amplitude and phase distribution that is consistent with resonance of the burrow at its second modal frequency. The burrow is approximately three-quarters of a wavelength long at this frequency. The same result is obtained when the burrow is excited by a dipole source located near the constriction.
4. Non-parametric model calculations confirm this conclusion and also give broad agreement with the measured response curves over a frequency range from about 1.5 to 5 kHz. The calculated curves indicate an additional resonance at about 1.2 kHz associated with the first mode of the burrow - the Helmholtz or Klipsch resonance - which is apparently not utilized by the insect. This detail is consistent with earlier measurements, and is also supported by measured responses reported here that show an increase in sound pressure with decreasing frequency below 2 kHz as predicted by the model.
5. The measured performance of the burrow is broadly consistent with the model. According to the model, the burrow geometry is close to optimal for maximal sound power radiation

Electromyograms (EMGs) of the timbal muscles were recorded during the calling songs and 'protest songs' (also referred to as alarm signals) in five species of cicadas: Cicada barbara lusitanica, Tettigetta josei, Tettigetta argentata, Tibicina quadrisignata and Tympanistalna gastrica.

The timbal muscle contraction rates of all species ranged from 50 to 250 Hz. The basic timbal cycle generating sound during the inward and outward buckling of the timbal was maintained in calling song and protest song for all five species. Comparison of the temporal parameters and the corresponding timbal muscle activity in both sound signals revealed that the timbal muscle period as well as the intertimbal delay showed the same mean values. Major differences between calling song and protest song are apparent in the pattern determining schemes and phrases as well as in the sound amplitude modulation.

The amplitude of the sound pulse during the inward movement of the timbal was positively correlated with the time lag from the timbal muscle activity to the IN sound pulse in the calling songs of Tettigetta josei, Tettigetta argentata and Tympanistalna gastrica, species with a one-to-one correspondence between a single contraction of the timbal muscle and the production of a sound pulse as the timbal buckles inwards. These relationships did not hold for the other two species, where a single contraction of the timbal muscle causes the timbal to buckle inwards in steps to produce a train of sound pulses. This indicates the presence of different mechanisms responsible for sound amplitude modulation. There was no evidence that the period of the timbal muscle contractions correlated with changes in sound pulse amplitude during singing.

Main or calling songs of cicadas Meimuna tavoyana, Platylomia nagarasingna, Platylomia sp. and Purana aft. tigrina from Thailand are described and compared with some previously investigated species. For M. tavoyana a repeated tonal frequency modulated pattern is typical in addition to the broad band buzzing sound. Both closely related chorusing species of Platylomia show broad band acoustic emissions with some degree of frequency band modulation. The song of Purana aft. tigrina from S. Thailand is the most complex, with sharply tuned spectral components (at 2300 and 9400 Hz) and rich amplitude modulation patterns.

1. Chorthippus dorsatus, Ch. dichrous and Ch. loratus are closely related, sympatric grasshopper species (subfam. Gomphocerinae) with few morphological differences.

2. Songs of males contain two elements: pulsed syllables produced during synchronous movements of the hindlegs (part A) and ongoing noise produced during alternating movements of the hindlegs (part B). Part A predominates in songs of Ch. loratus and part B in songs of Ch. dichrous. In Ch. dorsatus both parts contribute nearly equally to the songs.

3. Ch. dichrous and Ch. loratus are Eastern species, while Ch. dorsatus occurs all over Europe including Spain and Italy. The songs of different populations of Ch. dorsatus in Europe are compared.

4. Female stridulation of the three species is similar to male stridulation.

5. Females prefer - expressed by their response songs - the conspecific signals over heterospecific ones. Discrimination, however, is not perfect.

6. Tested with artificial song models, females of the different species differ clearly in their response functions for element numbers in part A while Ch. dorsatus and Ch. dichrous females show nearly identical responses to varied durations of part B. Individuals of a Greek population with male song characteristics intermediate between  Ch. dorsatus and Ch. dichrous show intermediate female response functions as well.

The method usually used to identify different sounds or divisions of sounds is to compare sonograms visually. There have been some attempts to reduce the subjectivity and increase the repeatability of this approach, for example by tracing the sonograms onto paper and examining the areas of overlap and mismatch, the use of multi-variate statistics and digitizing tablets. Digital recording of sounds has allowed sounds to be input directly into computers which can be used to display sounds and facilitate measurement. To date there has been little attempt at their use or analysis. We outline a series of programs which have been developed to compare statistically any unit of a sound with a pre-recorded library of similar units. The creation of such a library allows the rapid and objective categorization of large numbers of sounds. These programs have been used to analyse songs recorded from wrens Troglodytes troglodytes and house crickets Acheta domesticus. Potential applications of this software to the field of bioacoustic investigation are discussed.

The stimulatory organs of male and female Mutillid wasps are similar and occur between their 3rd and 4th abdominal termites. The three investigated species, Smicromyrme rufipes, Dasylabris kozlovi and Mutilla marginata, possess files with different 'ripple' distances that are species specific (2.5 to 5.4 micrometres). The sonogram of the distress calls from female M. marginata shows the ripple frequency with low amplitude rising in the first 20 milliseconds from 1.2 to 1.5 kHz and then falling back in the following 20 milliseconds. The main sound energy occurs in the 2nd and 3rd harmonics.

In Malaysia, males of the noctuid moth Amyna natalis were observed producing a continuous ultrasonic song of high intensity (about 102 dB SPL measured at a distance of 10cm). The frequency spectrum of the sound impulses had its peak between 60 and 80 kHz. During song production the animals were perching on plants and moving their wings up and down quickly. Simultaneously, by twisting the wings it seems likely that a male-specific “bubble'' in the forewing functions as a tymbal, resulting in sound production.

We found a new acoustic signal in Drosophila simulans (si) and D. melanogaster (me). It is a ‘rejection signal’ (RS) produced by adult males and young males and females in response to the courting behaviour of mature males who emit ‘pulse songs’ (i.e. love song: LS). It occurs most frequently in si, less in adults me except if the interacting males belong to different chemical morphs (i.e. temperate or equatorial population). There are no differences in the LS characteristics directed to various sexes and ages. The RSs produced by adult males or by young animals do not differ significantly either. They are emitted by neither virgin nor fecundated adult me females but a few times by virgin adult si females. The RS (like the LS) is a multipulse signal but intervals between pulses are about twice those of LS, around 90 ms for si and 80 ms for me. They are very irregular, as is the distribution of energy along the bandwidth mainly between 300 and 800 Hz for si and 200 and 600 Hz for me. The sound level of the RS is from 10 to 20 dB less than the LS. The RS seems to be linked to the ‘flicking’ behaviour produced by both wings, while the LS always corresponds to the so-called ‘wing vibration’.