Surely most people have heard cicadas in the hottest hours of the summer, but perhaps few have ever seen them. They are essentially tropical insects (Hemiptera, Cicadoidea), but also frequent in the Mediterranean area, and thirteen distinct species are confirmed as occurring in Portugal, although there is still little information about some of the species, especially about their distribution.
Adult cicadas are characterized by the fact that males emit typically intense acoustic signals, sometimes for hours at a time, whose main function is to attract females for reproduction. These sounds, generated by the vibration of a pair of timbrales (membranes of the 1st abdominal segment), are different for each species, so they act as true species identity cards.
Females (silent) are attracted by conspecific males and, if in a given population changes occur that lead to the production of distinct acoustic signals, that population will tend to diverge, becoming isolated from the reproductive point of view of other conspecific populations. In other words, it will have its own evolutionary destiny. Thus, a new species may be born, "that mystery of mysteries" as Darwin called the question of the origin of the species (speciation), is an occurrence that even today we consider complex and that may involve different processes.
The cicadas are phytophagous and have a chopping mouth armor through which they suck the sap from the plants. The eggs are laid by the fertilized females in the tissues of the host plants, during the summer or autumn. For this purpose, they use sabre ovipositors that make incisions in the branches or stems of those plants. Days or weeks later, they hatch and the young, tiny nymphs fall to the ground, where they dig a tunnel with specially adapted legs. They feed by sucking on the root sap, and during underground life they grow and moult their cuticle (usually four times). When they reach the last larval stage, they tunnel to the surface of the soil, climb to any substrate, such as a tree, and there they suffer the last moult, finally becoming the adult we hear if he is a male. The underground larval stage can last from one to several years, namely about three years in Cicada orni, one of the most common cicadas in Portugal and the rest of Europe.
Curiously, there are species whose underground phase lasts 13 or even 17 years, strangely both prime numbers. This is the case of the periodic cicadas of the genus Magicicada, which occur in the USA in specific locations and today already mapped and suddenly appear in millions, announcing themselves with a deafening noise, after "eclipses" of 13 or 17 years. They are reminiscent of the periodicity of comets, are a magnificent show for entomologists and all naturalists, and certainly an unexpected bounty for predators like birds and some reptiles!
(Text José Alberto Quartau/2019; adapted from "Quartau, J.A. 1995. Cicadas these insects almost unknown. Nature Mail, 19: 33-38") |
