insects

During the past few years, we used acoustic methods to investigate sound communication and to search for the presence and distribution of singing cicadas (Homoptera:  Cicadoidea) in Slovenia, Croatia, Macedonia and S.E. Asia. For detection of high pitched songs of smaller species, e.g. Cicadetta or Tettigetta the use of a bat detector - in our case Ultra Sound Advice 5-25 with the microphone mounted to a Telinga parabola - proved to be very suitable, as reported already at the IBAC conference in Potsdam. We use it mainly in the heterodyne mode with the frequency selector tuned to the lowest frequency. With such equipment it is possible to detect songs of small singing cicadas even in areas with high traffic noise and at a distance of up to about 50 m. In addition to this Telinga microphones Pro 1II and Pro V Science were used for recording in combination with the DAT recorders Sony TCD-D3, -D7, -D10 and Pioneer D-C88. The latter was used mainly in the HS mode in combination with the special version of the Pro V Science microphone, with one mic capsule sensitive also in the ultrasonic range. Such a system enabled us to make recordings in the frequency range of 20 to 44000 Hz. As a result of such investigations in Slovenia, Croatia and Macedonia in the last few years many new data about songs and distribution of single species were obtained. In addition to this, during this survey we found in Slovenia not only Cicadetta montana (Scopoli) as described by Boulard (1995) but, with the aid of the bat detector, another similar and closely related species with a different song, which still has to be identified. In Macedonia this year we were able to find, record and catch on Galicica mountain yet another unidentified species with a very characteristic song, which is also closely related to Cicadetta montana. Furthermore, it was found that the species Cicadatra hyalinata (Bru1le), considered by some authors to be only a variety of C. atra (Olivier), has a different calling song and is therefore most probably a good species. Most interesting is the discovery of Cicadatra persica (Kolenati) in the Radika Gorge, not previously known from the Balcans and detected for the first time a year before during acoustic scanning of this region. The songs of this species were previously not recorded and analyzed. A detailed description of these interesting cicadas and their songs is in preparation.

In the genus Drosophila, mating is the result of a courtship involving several sensory modalities. A key component is the production of song made by the male. These sounds, directed towards females, are produced by wing vibration. They consist of sine song and intermittent pulse trains varying in length. The pulses are separated by interpulse intervals (lPI), whose mean values are characteristic of each strain. The aim of this work is to compare the development of the species specific song in two related strains Drosophila melanogaster and D. simulans. Is there progressive maturation or a sudden establishment of the song pattern? Is the pattern common at the first stage but divergent afterwards, according to each strain? Males were placed with mature female at six precise stages of maturation (12, 17, 22, 27, 33 hours and mature (4 days)) and the courtship was recorded. In the melanogaster strain, frequencies of pulse and sine are very close to those of the mature stage as early as 12 hours but the rhythm of emission differs. It is only at 27 hours that the mature rhythm appears and consequently that males succeed in mating. In simulans, experiments are under way.

The aim of this work is to amplify the knowledge of acoustic communication in ants. Four myrmicinae species belonging to the genus Messor (i.e. M. capitatus, M. minor, M. structor and M. wasmanni) were tested. Some Messor species have already been the object of a preliminary study only on workers (Schillinger and Baroni Urbani, 1985), but in our work the ultrasonic emission of specimens belonging to the castes of queens, males and workers (minor and major) were recorded. Ultrasonic signals were acquired using a Bruel & Kjaer 2231 with a B&K 4135 transducer (frequency response up to 100 kHz) and a bat detector Ultra Sound Advice S-25. Signals were fed into an amplifier with an anti-aliasing low-pass filter to be digitally recorded and analyzed on a Pc-based Digital Signal Processing Workstation. Sampling frequencies up to 20,0000 s/sec allowed recording up to 87.5 kHz. Ants were held with a pincer and the microphone was kept at a distance of approx. 1 cm. The description and measurements of stridulatory apparata were made by means of S.E.M. (Scanning Electron Microscope Cambridge S 250 TP) analysis on the same specimens used for recordings. In all the individuals investigated, a stridulatory organ occurs in the position regarded as typical of Formicidae: the plectrum on the hind margin of the third abdominal tergite and the file of pars stridens on the pretergite of the fourth. The file is made up of very regular parallel cuticular ridges and, in all the individuals examined, extends for almost the whole length of the pretergite itself, stopping at a very short distance from the anterior margin of the pretergite. The pars stridens shows very sharp margins. Posteriorly, some long bristles occur in proximity of the margins. The hind margin of the third abdominal tergite shows, in its very central part, a thickening connected with the scraper. This thickening makes the scraping action of the tergal margin more effective by conferring rigidity upon this region. For each species it is possible to describe a common general pattern of structure and operation of the organ producing sounds and ultrasonic emission which always have values of maximum frequency higher than 41 kHz. Playback tests are in progress in order to clear up the biological role and significance of the acoustic signalling for the survival of the colony of these species.

New data on the sound produced by Sphingonotus coerulans corsicus Chopard, 1923 and Truxalis nasuta Linneo, 1758 are given. Oscillograms of both sounds are provided for the first time, as well as their physical characteristics and other aspects of the communication between specimens. For both species sound seems to be a territorial and sexual cue; they have not been observed singing when isolated, as other Acrididae, but always when they are close to or in contact with other individuals. The song of Truxalis nasuta consists of isolated echemes composed of 7-29 syllables lasting about 0.234 seconds. The syllable repetition rate is about 2.2 syllables/sec. The main frequency of emission is between 1 and 10 kHz, with the main peak at 5 kHz. For Sphingonotus coerulans corsicus two different songs have been recorded, one for territorial behaviour and other for courtship. The territorial song consists of isolated syllables lasting about 0.350 seconds with the main frequency between 4 and 8 kHz, with a peak at 7 kHz. The courtship song consists of a syllable produced by the movement of ' the hind legs followed by several microsyllables produced by light movements of the hind legs. The syllable lasts about 0.670 seconds. Sometimes, after the sounds referred to above, the specimens produce a new syllable, always shorter than the first, and with more microsyllables, but always fewer than before. The frequency of this song lies between 3 and 5 kHz, with the main peak at 3 kHz. This communication has been partially supported by the D.G.I.C.Y.T. grant number P889-0448 of the Spanish Government.

The genus Megatrupes Zunino 1984 is up to now represented by two species, M. cavicollis mates 1887) and M. fisheri (Howden 1987), both distributed between the western Sierra Madre and the Sistema Volcànico Transversal, in Mexico. The aim of our research is to analyze the sound produced by M. cavicollis through two distinct apparatuses - thoraco-elytral and coxo-abdominal. The individuals examined were collected during the summer of 1987 at the Reserva de la Biosfera "La Michilia'', Durango, Mexico. Signals were recorded, acquired and analyzed through the program Signalize 3.12. Each emission consists of a disyllabic chirp, with two distinct sub-units separated by a pause. This is a standard situation among the Geotrupinae. Seven variables concerning duration and frequencies of the chiro have been analyzed. The sexual dimorphism, as regards the morpho-anatomy, does not appear in the acoustic context. Both apparatuses contribute to the sound production. Nevertheless, the coxo-abdominal apparatus seems to be more effective than the thoraco-elytral. Individuals deprived of the elytra emitted stridulations very similar to those usually produced by individuals not experimentally constrained, whereas individuals deprived of their hind legs emitted very thin, unrecognizable sounds. Although statistical analyses of frequencies and duration show significant differences among individuals, males and females do not appear to be separated into two distinguishable clusters.

Hearing in moths has evolved to enable them to detect and evade echolocating bats. Thus most moths are silent. Among Arctiidae click production with tymbal organs on the metathoracic episternites is fairly common, but in most species this is also part of the interaction with bats. However, a few moth species produce sound and use their hearing for intraspecific communication. Generally, the noctuid species described so far produce sound by some kind of stridulatory mechanism, often involving the wings and legs. Here we describe quite another mechanism of sound production in the male noctuid moth Pseudoips fagana (Fabricius) of the subfamily Chloephorinae, involving a ventral "tymbal'' organ centrally located on the ventral part of the basal abdominal segment. P. fagana, the green silver line, is common in northern Europe. The sound production has been detected while the moths my around the tree-tops in the dusk. The clicks sound like electric sparks to the human ear and may be heard at several meters distance. We recorded clicks from males in stationary flight in the lab. The moths would only click while flying in place. We elicited the clicks by very intense ultrasonic sound pulses. The moths produce short series of clicks each lasting around 0.3 to 0.4 ms with maximum sound energy around 30 kHz. The sound pressure level was intense, 119 do SPL at 2 cm. The sound producing organ is buried deep in a groove, but may be observed if the moth is placed ventral side up and the abdomen is bent dorsally. The hearing of P. fagana was measured by recording extracellularly from the auditory nerve. Both males and females were most sensitive around 30 kHz, with a threshold of about 35 do SPL, thus matched to the spectrum of the sounds. There are no behavioural observations on these moths, but we believe it most likely that the sounds are part of the sexual display. Research supported by the Danish National Research Foundation.

It is well known that the singing cicadas in S.E. Asia show very high biodiversity, with many species inhabiting the same ecosystem. They have to cope with the problem of sending their acoustic messages in a very noisy environment, since many insects, birds, frogs and other animals are emitting sounds at the same place and the same period of the year. Therefore the cicadas have developed their special strategies to cope with the jamming problem. One peculiarity is the evolution of songs with characteristic rhythmic patterns and in many cases even with a high degree of frequency modulation. The second adaptation to the extreme biodiversity is the fixed time of singing. Many cicada species are acoustically active only during a certain species specific time period of the day. Such a time window is usually 30 to 60 minutes long. Many species are singing only in the evening or only in the morning hours. Nevertheless, the dawn-dusk species are an exception in the Malaysian peninsula. Such time sharing between different species is probably not the only reason for "dawn'' "dusk'' or even "midnight cicadas''. This could also be an adaptation to avoid or minimize predation by birds or some other insectivorous animals. It should not be overlooked that many other cicada species there are not limited by a specific time window. Anyway, in the high biodiversity of S.E. Asia it is not difficult to find exceptions or special cases, and this is true also for the singing of cicadas. Therefore, the repertoire of the sound emissions of the S.E. Asian cicadas is challenging for every sound recordist. With suitable equipment nowadays it is not very difficult to make good documentary recordings there, despite high humidity. The main problem is still to see, identify or even catch the singing animals in the dense vegetation, and to find out the ethological context of the recorded sound emission.

The tymbal of Cyclochilca australasiae has a posterior plate, with anterior alternating short and long tymbal ribs joined together by sheets of resilin. The tymbal muscle acts on the tymbal plate causing the ribs to buckle inwards in sequence. As it buckles, each rib deforms from a convex to a V-shaped profile. The maximum vibration during the inwards buckling occurs at the V at the middle of the long ribs; the ribs vibrate in this region when driven by sound. After the tymbal has buckled outwards, the whole tymbal rib surface vibrates at around 6 kHz. Inward-going tymbal clicks produce a sound frequency of 4 to 4.5 kHz, which is a dominant frequency of the insect's song, and outward-going clicks produce 5.5 to 6.5 kHz. The resonant frequency of the tymbal changed from 5.5 kHz when buckled out and to 4.3 kHz when buckled in. The tymbal appears to be the primary resonator determining dominant frequency in the insect's song and provides a pressure drive inside the insect's abdomen. The abdomen acts as an acoustic load to the tymbals, resonant at the same frequency, and providing acoustic matching between the tymbals and the surrounding air.

Tympanate moths can hear echolocating bats and take evasive action to avoid capture. A model is described to calculate the distances at which the A1 and A.2 cells of noctuid moths would detect foraging bats. The model is constructed using empirical data of the moths' tympanic response, call intensity of echolocating bats, and echo target strengths of insects. With no excess atmospheric attenuation, the distances at which moths, detect bats are very large, up to 250 m, but adding even small values of attenuation reduces the A1 cell detection distances to below 15 m. As more attenuation is added to the model the A1 detection distances are reduced, but the A2 cell distances remain relatively stable. Reduction of source level appears to be the best mechanism by which a bat can detect the moth before the moth detects the bat. This is more successful if the bat also uses a high frequency call. It is proposed that FM bats are not able to increase the frequency of their calls beyond certain limits imposed by atmospheric attenuation. This limits the options for reducing acoustic apparency to either very short duration calls or ones of low intensity. Bats using CF calls are probably not so restricted to the lower frequency ranges and so may exploit the higher ranges to reduce their call apparency.

A major influence on the evolution of insect song patterns is thought to be their importance as species recognition signals. This predicts that female preference should be species specific, i.e. females should prefer the song of their own species but not discriminate among common differences between males of their own species. This could result either through selection for species recognition during speciation or coevolution. An alternative view is that female preference may have evolved by sexual selection, with female preferences functioning to allow discrimination between potential mates of the same species. Recent work on frog acoustics has suggested yet another possibility is that preference and male trait can be substantially mismatched. Most work in insect  bioacoustics has assumed that the species recognition function predominates. Here I describe work in progress on the song of the bushcricket Ephippiger ephippiger. This species shows unusual variability in song pattern. Female preferences change between geographic variants of male song, implying coevolution has occurred. The geographic variation occurs largely independently of the presence of similar species. Examination of the structural components of song which influence preference has implied that subtle aspects may be involved. Female preference is strongly influenced by changes which occur commonly between males within natural populations. These observations suggest sexual selection has had a greater impact on the evolution of song preference than species recognition. I shall discuss if this might be true generally.