Treehoppers are insects related to leafhoppers and cicadas, usually small in body size, and feeding on plant sap. More importantly, they are immensely fascinating creatures!
One reason is their anatomy: parts of the middle body (thorax) are extended into a ‘helmet’, which can be much larger than the rest of the body and occur in a dazzling variety of shapes. Much about this helmet remains a puzzle. For one thing, what is its function? Does each kind of shape have a distinct function? It is unclear which evolutionary mechanisms are responsible for the origin and maintenance of the astonishing diversity of this structure, which is not found in any other insect group. Furthermore, experts cannot agree on its gross anatomy. Whilst some model organisms are now mapped to the last cell, the treehopper helmet complicates even its broadest anatomical identification: is it an extension of the top of the thorax or is it a fused pair of wing-like appendages? The latter hypothesis received much attention a few years ago. It suggests that treehoppers may have found a way of ‘releasing’ their genetic potential for developing winged appendages on the first thoracic segment, a potential that has been inhibited in other winged insects (insect wings are always on the second and third segments). More recent work, however, supports previous accounts according to which the helmet is an extension of the upper exoskeleton. Either way, treehopper helmets transform 19th century-style concerns about gross anatomy into 21st century questions about gene regulatory networks for development, the nature of homology, and the origin of evolutionary novelties. Treehoppers are not the only insects with extraordinary protrusions. Exciting insights into the evolutionary developmental biology of such ‘extreme’ traits arise from the study of the horns of dung beetles. For more information consult the websites of Douglas Emlen and Armin Moczek.
The wing-appendage hypothesis is articulated by Prud’homme et al. (2011) and commented by Moczek (2011). Based on careful anatomical work, the hypothesis was subsequently rejected by Mikó et al. (2012) and Yoshizawa (2012). Their findings are in line with previous work, including my own (1998).
Treehoppers are also remarkable for their communicative abilities. They employ a widespread but comparatively little understood form of communication: vibrations of the substrate on which they sit. Treehoppers use highly specific bursts of vibrations for several purposes, e.g. for attracting mating partners, for advertising new feeding sites to group members, and for alerting mothers to the presence of predators. Rex Cocroft’s website provides information about vibrational communication in treehoppers. Cocroft has joined up with philosopher Claire Horisk to argue that treehopper communication can shed more general light on the contentious nature of animal communication; see Horisk & Cocroft (2013).