Big Time for Small Singers Use and Benefit of Laser Vibrometry in Insect Research

Biology Big Time for Small Singers Use and benefit of laser vibrometry in insect research A planthopper (Hyalesthes obsoletus). This species is a vector of a plant-disease in vineyards and therefore of scientific and economic interest. Cicadas, leafhoppers, planthoppers and spittlebugs (also called Auchenorrhyncha) are a very diverse group of animals within the insect world. Nevertheless, relatively little is known about their behavior and phylogeny. Vibrational signals play an important role in species recognition and mating behavior. Here, the use of laser vibrometer is extremely helpful. 30 | Biology Cicadas and their songs are THE PROBLEM Species differentiation is difficult familiar to many people who live for many groups of Auchen- or vacation in the USA, Mediterra- In contrast to cicada songs, orrhyncha, and until now the nean or tropics. The “true bugs” which are easily heard unaided identification is mainly based on (Hemiptera), which not only by humans, leafhoppers and anatomical characteristics. How- include Auchenorrhyncha but also planthoppers use substrate-born ever these are often very small, bugs, aphids and whiteflies, are a signals. They produce vibrations so other features are needed for very successful and diverse insect (100 - 3,000 Hz) that can only be identification. Aside from molec- order. To date, about 42,000 registered on the plant the animal ular methods, it turns out that species of Auchenorrhyncha have is sitting on. Substrate-born bioacoustics is a helpful solution. been described worldwide. Most signals were ignored for a long of these species are leafhoppers time although they are wide- EXPERIMENTAL SETUP or planthoppers (cover picture) spread among insects. The songs, with a body length of less than often sounding like drums, are In the past, vibration signals were 5 mm. They play a significant produced by so-called tympal or- recorded using very simple meth- role as primary consumers and gans, plates on the abdomen, set ods. The vibrations were made au- consequently as a food source into vibration by muscles. Duets dible by touching the host plant for other animals. Some leaf- can often be observed between with a gramophone stylus. More hoppers and planthoppers are males and females. Because this recently, piezoelectric transducers pests, for example in vineyards communication is mainly used were used. The signals could then or rice fields, and are therefore to find mates, it is believed that be recorded on magnetic tape or of economic importance. the songs are species specific and later on computers. The biggest therefore a barrier to other species disadvantage of these methods (biological species concept). was that the pickup system had ► Figure 1: Experimental setup for recording planthopper songs. The vibrations are registered with a PDV-100 laser vibrometer either directly on the animal or on its host plant. | 31 Biology to either be in direct contact with OUTLOOK the host plant, or at least so close to the animal that it could possi- Additional questions can be bly disturb its natural behavior. addressed with laser vibrometer Laser vibrometers offer incompa- measurements. It is unclear for rable advantages by measuring example if only structure-borne vibrations directly on the animal vibrational signals are important or the plant without direct con- or, at least in close-up range, if tact (Figure 1). In addition, the airborne sound also plays a role in measurement recording locations planthopper communication. Our are more precisely defined and initial experiments have shown therefore more easily reproduced. that a feeble airborne sound can be measured, but the signal is weaker and noisier (Figure 2). The flow of the vibrational sound into the host plant and its vibrational behavior can be studied thanks to the scanning Figure 3: Deflection shape of a plant laser vibrometer (Figure 3). leaf (nettle) induced by a artificial Important considerations are the signal (170 Hz) and the corresponding position of the insect during the frequency spectrum. diffusion and the characteristics of the plant material as well as the relation between the generated frequency and resonance of the plant. Further studies will reveal possible evolutionary adaptations of insects to their host plants, which will lead to a better Authors understanding of speciation Dr. Roland Mühlethaler, Dr. Andreas Wessel, processes in Auchenorrhyncha. ■ Prof. Dr. Hannelore Hoch [email protected] Museum für Naturkunde Leibniz Institute for Evolution and Biodiversity Science of Humboldt-University Berlin www.naturkundemuseum-berlin.de Figure 2: Laser vibrometer and micro- Photo phone measurements compared. Above: The clear vibrational signal Title: E. Wachmann, Berlin. from a male planthopper (Hyalesthes Figure 1: S. Grube/V. Hartung, Berlin. obsoletus). Below: Simultaneously recorded Acknowledgements airborne sound close to the animal. We are very grateful to Dr Reinhard Behrendt and Samy Monsched from Polytec for their intellectual and technical support. 32 |.

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