Secrets of wing mechanics in tree crickets

In acoustic animals the physical size of the singer is believed to be encoded in the frequency of their calls, low-pitched calls are usually associated with large males and high-pitched calls with small males. This has been shown in many insects, like field crickets, but the rule seems not to apply to tree crickets. Male tree crickets produce tonal songs at low pitch with frequency varying from 2.3 to 3.7 kHz, using their fore wings. A new study by researchers at the University of Bristol, published in PNAS, has shown how frequency does not always advertise male body size in tree crickets. This study shows that tree crickets evolved wings optimized for effective sound radiation with variable frequency being a side effect rather than an adaptation. The work also shows how song frequency can be decoupled from body size by a small change in morphology, belying the common assumption that insects are obliged to signal honestly their size through song.

HFSP Cross-Disciplinary Fellow Fernando Montealegre-Z and colleagues
authored on Fri, 15 June 2012

Male field crickets attract females by singing loud repetitive songs at night. They rub their wings together, setting them into resonant vibration. The singing wings are broadly oval and nearly concave. Resonance enhances the production of loud and intense sounds, enabling the sound to travel further, reaching distant females. The females use this sound not only to find the male but also to judge the quality of the singer. There are many cues females can use to gauge quality, but one that has received considerable attention is male body size, which is encoded in the pitch of the male calling song. In the field crickets species females tend to prefer large males; it seems that larger males are better at finding and using resources. Large males make lower pitched sounds, and smaller ones produce higher pitched sounds. Therefore females have the chance to listen and gauge the size of their males. Field crickets use an escapement mechanism to couple the vibration of both wings to produce musical sound. Such a mechanism does not allow individual frequency variation, even with temperature changes.

Figure 1: Wing resonance in tree and field crickets, measured using Laser Doppler Vibrometry. A. Male tree cricket singing from a leaf hole (which he uses as a baffle). B. Vibration map of  the wings of a tree cricket male showing the main regions of wing deflection. C. Vibration map of the wings of a field cricket. Note that a large area of the tree cricket wings is used for sound production, while in the field cricket deflection is mostly limited to a small triangular region in both wings. (Data not included in the published research but used here to illustrate the main point). Lower panels in A and B indicate the average spectrum of the vibration.The elongated wings of the tree cricket offer a broader range of frequencies than the more rounded field cricket wings.

On the other side of the spectrum are the tree crickets — tiny, nearly transparent and very gracious creatures (Fig. 1A). These lovely insects also use wing resonance, but, unlike field crickets, their wings are elongated and nearly flat. They can also change the output frequency of their wings according to the ambient temperature. For instance, Oecanthus henryi, an Indian tree cricket used as model organism in this study, changes the pitch of its call from 2.3 kHz when the temperature is 18°C to 3.7 kHz in mild conditions at 27°C. Such temperature dependence raises many questions because, as mentioned before, frequency variation in crickets is unusual. The biomechanics of sound production in tree cricket stridulation is disclosed in this research.

In a collaborative study, scientists from the University of Bristol and the Indian Institute of Science investigated these curious biomechanics. They approached the problem using microscanning laser Doppler vibrometry (LDV), which can pick up vibrations at the nano scale. The researchers found that the pattern of wing vibration in tree crickets was different to that of field crickets. The entire wing surface vibrated instead of just a small part and instead of having a single sharp vibratory peak near song frequency, there were two fused peaks (Fig. 1 BC).

Dr. Natasha Mhatre, a Marie Curie Fellow, and Dr Fernando Montealegre-Z, a former HFSP fellow, both experts in the biomechanics of sound production and hearing in insects at Bristol’s School of Biological Sciences, teamed up for the development of this project. In this study besides LDV, the researchers also applied finite element modelling, a technique borrowed from engineering; and demonstrated that wing geometry is the key for the two peak resonance pattern observed. Digitized wing geometries were manipulated and their resonant properties computed.

Because insects are cold-blooded, their activity is highly influenced by temperature. Hence, when the temperature rises, singing tree crickets rub their wings faster and engage vibrations closer to the higher frequency peak. What this means is that their size is no longer related to their song frequency but to how fast the tree cricket is able to move its wings. This opens up many possibilities for these unique crickets including using song to hide their true size.

Finite element modelling showed that this double peak resonance depended on the elongated shape of the tree cricket wings (Fig. 1B Wing geometries with a shortened aspect ratio (such as those of field crickets, Fig. 1C) favoured a single-mode sharp frequency response. Therefore, these models show how song frequency can be decoupled from body size by a small change in morphology, belying the common assumption that insects are obliged to signal honestly their size through song.The researchers also suggest that tree crickets have evolved such particular large wing geometry to increase the amount of sound they can make. Now, the mystery of what exactly is coded in the pitch of the tree cricket song remains to be unravelled. This study therefore provides new inputs to the conventional views of the cricket mating system.

Enticingly, the techniques of analysis applied here have opened up new ways to investigate acoustic communication systems, not only in terms of sound production but also sound reception. In terms of technology, this study might provide inspiration for miniature sound radiators using evolutionarily honed geometries, to design actuators many times smaller than the sound waves they broadcast.

The study was funded by the UK India Research and Education Initiative (UKIERI), an EU Marie Curie fellowship grant, HFSP Fellowship, and a Biotechnology and Biological Sciences Research Council (BBSRC) grant.

Reference

Changing resonator geometry to boost sound power decouples size and song frequency in a small insect. Mhatre, N., Montealegre-Z, F., Balakrishnan, R. & Robert, D. 2012. Proceedings of the National Academy of Sciences, USA. 109: E1444–E1452: doi/10.1073/pnas.1200192109/

Pubmed link

PNAS link