People are visual beings and we are always so busy that we sometimes find it difficult to really listen to attractive animal sounds, such as or howling. These sounds can reveal many things, like the identity of the “singers”, the purpose of their message, their physiological condition and level of motivation. It is even harder to understand those animals that make sounds we cannot hear with our own ears. Reading this sentence, an educated reader may think of ultrasonic bat calls, but this article will actually discuss a more obscure topic – insects that communicate by vibrating the surface which they are on and were chosen as a subject of research by the Department of Entomology at the National Institute of Biology.
In today’s “crisis”, both moral and economic, many people (especially the worthy decision-making bodies, but that is a topic that should be discussed another time) would assume that recording the vibrating of bugs cannot be very useful. Then why finance such bizarre things? I realise that I am biased but many historical examples show the narrow-mindedness of such reasoning. The following example will try to explain why.
The transmitters, the medium and the receivers
Vibrational communication starts when an animal that transmits signals causes the vibration of a solid substance, usually the surface on which it is standing. The vibrations travel via the substance from the transmitter obeying physical principles and can eventually reach (or not) another animal, which senses the vibrations and responds to them if they are identified as a signal. What this simplistic definition means is that to understand of this communication one must get acquainted with three basic elements: the transmitter, the medium and the receiver. All three elements of the vibrational communication are very special, which provides the first lesson on communication: the three elements are inseparably linked to each other and cannot be explained separately. In contrast, there are so many questions a researcher can ask about each of them that in order to fully understand them, small steps have to be taken. At this point we must mention that insects are not the only creatures that communicate through vibrations but they are convenient for research, making it easier to ask systematic questions and look for answers.
Female calling song of the southern green stink bug, recorded with a laser Doppler vibrometer
Starting with the transmitter, the first general question that arises is the purpose of the communication. Despite an apparently limited system, the answers are quite diverse. Vibrations can be part of the communication between individuals of the same species or can function as a communication bridge between different species. Regarding the purpose of the communication, there are no particular limitations in comparison to other means of communication: mating partners can share information on their identity, position and readiness to mate. Vibrations can be used to send a warning to other group members or to the predator itself, but usually they are just a simple message about their presence and position to mark territory or to keep the group together. More specific are the methods bugs use to create these signals. The simplest method for producing vibrations is drumming, which is enabled by the insect’s hard skeleton, and this is very frequent. On soft surfaces like the green parts of plants, it is more efficient for the animal to rock its body rhythmically with its muscles, causing the surface (leaves, stems, etc.) to vibrate. The true bugs, which are being researched at our department, specialised in this method by developing a particular structure on their abdomen called a “tymbal”. This is the part of the skeleton holding the muscles that allow the abdomen to move up and down. The vibrations are then transferred through the legs to the plant and from there to the receiver.
This brings us to the second element of communication, the medium. Depending on the way it is stimulated, it bends in different ways and its characteristics define what it can actually transmit. The principle is the same as with musical instruments: a firm string will emit a higher tone than a soft one, just as drumming on the bark will make the plant vibrate with a higher tone than drumming on the leaf. In this borderland between biology and physics, it is becoming more and more apparent how harmonised insects have become with their medium through evolution: their signals are easily transmitted through the plant even to a distance of one metre or more. True bugs and other Hemiptera, for instance, emit vibrations with relatively low frequencies since green plants completely filter everything else when the distance from the transmitter is slightly larger. They also sometimes use drumming, but only to communicate over short distances.
As mentioned above, the transmitter often vibrates to communicate its position. But can a simple oscillation even carry this kind of information? From the medium we now pass to the receiver. Different insect species developed different methods to sense the direction of the signal, which is again associated with their small size. True bugs, which are larger, can use the time difference: the amount of time the wave needs to travel from one leg to another is actually very short but long enough for the bug to interpret it as an information about direction, similarly to how we sense the direction of sound. Vibrations are perceived by special sense organs in the leg joints, which react to rhythmic oscillation at the right frequencies, and are then processed by the nervous system that transmits this information to decision-making centres. Signals as a whole and their sequence make up a code that enables recognition. Several species of true bugs and leafhoppers have already been recorded and each of them has its own set of signal sequences that mostly differ in length patterns and speed of repetition.
The above-presented system is the framework for communication, a dynamic process that is an essential part of the life of all living creatures. This topic has been covered in several books so I will keep it short and present only a few details. One of the most interesting findings is that vibrational communication is evolutionarily older than sound communication since it is used by practically all insects groups while sound communication is widely used only by cicadas and orthoptera (and less often by specimen of other groups). The reason for this lies in their size: due to their smallness, insects are a poor source of sound, which makes this means of communication more of an exception than a rule.
In this phase, our research group is also interested in the way vibrational communication combines with other means of communication (for instance, cave crickets use chemical communication through pheromones), and is studying the influence of the limitations imposed by the medium, the diversity of the recognition code, the role of vibration signals in creating new species, etc. The latter is especially interesting in leafhoppers, where we are researching a group of related species that are extremely hard to differentiate according to physical characteristics. A morphologist would identify them as only one species, but a completely different picture appears after recording their sounds. Their songs are so different that the specimen of different species can no longer recognise each other and consequently also do not mate. That is why they set an example for the emergence of species which, judging by the physical similarity, happened only recently. They are also being researched from the ecological and genetic perspective.
In the transmitter–medium–receiver system there is one more element that has not yet been mentioned: unwanted listeners. Some leafhoppers are familiar with the tactic where, using a special type of signal, the male distracts another male’s courting thus gaining some time to look for the female. But what is dangerous for them is that courting can also be sensed by other animals. It has recently been proven that some spiders react to a cicada’s signal, which can be also seen in their natural environment, since spiders often capture males who are more active “singers” than females. (source)
How and why
If you think that special equipment is necessary to study inaudible signals, you are correct. Signals are mostly recorded with an instrument called a laser Doppler vibrometer. This machine transforms the shift of the reflected laser beam due to the vibration of the test surface into a sound recording, which can then be analysed with sound processing computer programmes. In past decades, a gramophone needle was used instead of a laser beam. Despite it being widespread, due to its inaccessibility vibrational communication is still practically unknown, which is why researchers still tackle the basic questions presented above.
It is a rather fascinating system that teaches how closely living creatures are connected to their living and non-living environment. This research helps us understand the language of creatures that are so different from people but still have the same primary objective: to assert their own interests in relation to others.
With a little imagination we come to the more or less direct applicability of this knowledge. For instance, when farmers notice a suspicious bug near their crops and, based on its appearance recognise it as a pest, that bug could actually be a related species that does not feed on cultivated plants (insect species are often created by passing to a new host). By correctly identifying it, they might avoid unnecessary spraying that costs money and poisons the produce. The analysis of vibrational songs is a method that can be used in taxonomy and through it more widely in practice. Even if our particular cicadas are not a pest, this research is important for the development of such instruments. One serious pest is the American grapevine leafhopper that feeds on grapevines and carries a microbial vine disease called flavescence dorée. In cooperation with Italian colleagues, a method for interrupting their communication was developed by emitting the before-mentioned jamming signals. The goal was to chase the leafhoppers from the vine and thus protect it from the disease, which would not happen if we did not first understand the communication between these animals and the process of transmitting signals through plants. (source)
Author: Jernej Polajnar, a post-doctoral biologist working abroad and a geek in his spare time – a Wikipedian, a science-fiction literature enthusiast and a board game strategist. Follow him on Twitter at @JPolajnar.
Extra: A recording of treehoppers: the male and the female trying to mate. The vibrational signals heard in the background are generated by the two animals.
Translated by: Sarah Humar.