Even supposedly well-known insects sometimes cause surprises – as is now the case with the leafhoppers. These small but often colorful sap suckers appear to have an amazingly complex organ for sensing vibration signals, a research team has discovered. The organ consists of pairs of sensors in the front part of the insect’s abdomen, in which sensory cells, fine membranes and reinforced parts of the exoskeleton allow the perception of vibrations. The newly discovered organ sheds new light on the evolution of the cicada’s sensory organs and could offer new approaches for biological pest control.
Cicadas are sap-sucking insects that are known for their sometimes loud chirping, especially in southern climes. Each of the more than 3,000 species of these large leafhoppers has an individual song, which the insects perceive with a special “ear” in their abdomen, the so-called tympanic organ. The membrane-covered organ detects the sound waves with the help of around 2000 sensory cells. This is different with the small cicadas, which also occur in large numbers in our parks and gardens. Despite their often very colorful appearance, the insects, which are often only a few millimeters in size, are less well known and we cannot hear their communication either: they send their messages as vibrations to conspecifics on the same plant.
discovery in the abdomen
Until now, biologists have assumed that the leafhoppers use the simple organs in their legs, which are made up of just a few sensory cells and which almost all insects have anyway, to perceive these signals. But that’s not true, as Sarah Ehlers from the Center for Integrative Biodiversity Research at the Museum of Natural History in Berlin and her colleagues have found. For their study, they examined the anatomy of the rhododendron planthopper (Graphocephala fennahi) using microscopic methods and special staining of nerve structures.
This enabled the research team to track down an organ in the anterior abdomen of the leafhoppers that had apparently been overlooked until now: “We discovered that leafhoppers have a sensory organ in the anterior region of their abdomen that is exceptionally large in relation to such small insects and consists of up to 400 sensory cells exists,” reports Ehlers. This sensory organ is in close proximity to the well-known and well-studied signal-generating organ—making it all the more surprising that it has been overlooked, the team says. The six pairs of these vibration sensors, known as chordotonal organs, are sometimes more than 500 micrometers in size and form a sophisticated system of fine membranes and reinforced parts of the exoskeleton, as 3D models of the sensory organs revealed.
Possible precursor of the tympanic organ
Ehlers and her team suspect that the leafhoppers have developed such a complex system of vibration sensors to be able to better distinguish the signals of their fellow species from natural vibrations. “Small insects that live on plants must be able to distinguish between mechanical waves of different directions and causes such as the weather, predators or conspecifics,” explains the research team. “Perhaps the common leg vibration sensors that all insects have were not sufficient for this and that is why the cicadas have developed more complex mechanosensitive organs for this.”
The discovery of the chordotonal organ also allows conclusions to be drawn about how the transition from evolutionarily older communication via vibration signals, as in the leafhoppers, to communication via sound waves, as in the leafhoppers, took place. Since the sensor organs for both signal types are very similar in position and structure, the scientists suspect that the chordotonal organ represents the original form from which the complicated tympanal organ then developed in the sound-hearing cicadas. At the same time, the newly discovered organ also offers new approaches to biological pest control of cicadas. Because some species of leafhoppers can transmit economically important plant diseases to crops. The sensitive vibration sensors of the insects could now make it possible to prevent the mating of the insects by means of an interference signal and thus curb their spread.
Source: Sarah Ehlers (Museum für Naturkunde – Leibniz Institute for Evolutionary and Biodiversity Research, Berlin) et al., Royal Society Biology Letters, doi: 10.1098/rsbl.2022.0078