Animal gears (Issus cf. coleoptratus) !
The gear train! Such a fabulous mechanical system consisting of at least two gear touching each other, traditionally associated with human technological and intellectual progress. This simple mechanical machine is used in many areas, ranging from big industrial machines to cars and bikes to your electric egg beater! And its symbolic is strong: you can often represent someone thinking by drawing gears in their heads! The history of the gear is ancient. The oldest gear system found in Europe is probably the Antikythera mechanism, a system of astronomical position prediction of Greek origin with an incredible complexity for its time, dated circa -100 BC. Older gears were found the Han’s dynasty China, dated to the 2nd century BC. Anyway, gears are associated with human genius, and nature does not know how to do them. Well, this is what we thought until 1957.
Indeed, in 1957 scientists discovered the presence of gears in planthopper larvae, specifically in the Fulgoroidea superfamily larvae. And more than just “ceremonial” gears, they are totally functional! The gear train takes the form of serrated edges located at the trochanters (second segment of an insect’s leg, starting from the body attachment) of the hind legs. But what’s the point of having a gear for these insects whose larvae barely exceed the centimetre? In 2013, Cambridge researchers looked into this issue and studied this interesting system more closely. First of all, we should know that planthopper larvae are usually incredible jumpers, capable of making powerful and long jumps (speed of 2 m/s or more within a few milliseconds), like crickets. But in the latter, the jumping legs are located on the sides of the body and are parallel to the axis of the body while in the Fulgoroidea the legs are located under the body, leaving perpendicular to the axis of the body. And in this configuration, if the legs are not exactly synchronized during the jump (e.g. if one of the jumping legs start pushing too early compared to the other) it risks giving a rotation to the intended rectilinear path of the larva, which eventually will end screwed-up. To avoid this unfortunate phenomenon, both legs need to be almost perfectly synchronized. And for Issus coleoptratus (Issidae family) for which the gear train was studied, we talk about synchronization of the order of a few dozens of microseconds! And that’s where the gears come into play. Firstly, before the jump, the larva will arm its system by folding the legs so that just the teeth at the beginning of each gear are in touch and the gear train has room for turning. Then comes the jump. Electric pulse is sent by specialized neurons up to leg muscles. Now one of the legs starts moving to propel the bug. This movement is transmitted to the other leg via the gear almost immediately and allows the two legs to act in a synchronized way. If the electric impulse reaches almost at the same time both leg muscles, the gear doesn’t do much. On the other hand if the electric pulse arrives late to one leg compared to the other leg, then the gear still allows the other leg to synchronize and move even if it has not yet received the electric pulse. As it can be seen from the pictures, teeth on the gears appear black. That’s because they’re sclerotized, hardened. If one or more teeth break, it compromises the gear and the entire system. That would probably explain why the gears are present in larvae and disappear at adult stage: larvae will moult several times before reaching adult stage, so if one or more teeth break during one of the larval stages, they will be fully regenerated in the next molt. On the other hand, once an adult, the bug doesn’t molt anymore, and if the gears deteriorate, the deterioration is definitive.
The 2013 study was therefore carried out on the larvae of Issus coleoptratus (family Issidae), a relatively common species of Fulgoroidea. Identifying a Fulgoroidea larva isn’t the easiest thing ever, when it’s possible… Here, the overall look and the blue feather projecting from the back of the abdomen (whose function is unknown to us) gives us the Issidae family, and different details (number of sensory pits, aspect of the face… ) lead me to genus Issus. We have 6 species of Issus in France, but 3 of these 6 species may actually not exist! They were described centuries ago and since then no mentions of these species can be found: they may be synonymous with other species. And based on the 3 “true species” we have, the larva in photo is probably a larva of Issus coleoptratus.
Last update: August 16, 2021
The 2013 article: Burrows, M.; Sutton, G. (2013). Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect. Science, 341(6151), 1254–1256.