Hidden diversity in the spotlight : integrative taxonomy of European cicada groups (Cicadidae) with emphasis on acoustics

Cicadas (Cicadidae) are among the most popular insect groups owing to their calling songs. However, they are scientifically relatively poorly known, even in Europe; and the cicada diversity is underestimated crucially, as it was based only on a morphological approach. The songs often evolve more rapidly than the morphology. They are an important component of specific-mate recognition systems and, therefore, excellent markers of the early stages of reproductive isolation among populations. Recently, acoustic characters have been used extensively to reveal hidden taxonomic diversity.
In this thesis the Cicadetta montana species complex served as a model group, where morphological traits are missing or weak but high carrier frequency song patterns (10 kHz to 20 kHz) provide a remarkable, recently discovered and taxonomically relevant richness. We aimed to elaborate a modern and complete systematics for the Italian and Swiss taxa of the Cicadetta montana species complex. The elaboration was based on extensive field work (e.g. 27 weeks in Italy) and the analysis of numerous data sets dealing with acoustics, morphology, genetics (basically COI and COII) and ecology. Moreover, the first national synthesis of the entire Cicadidae (sensu Moulds), beyond the model group, is presented for Switzerland.
As a result of these studies six species as well as two subspecies for Italy and four species for Switzerland are reported newly within the Cicadetta montana species complex. Two species and two subspecies are new to science. We profited from the remarkable complexity of the songs and introduced relative power differences as a character to the taxonomy of cicadas. The importance of perch temperature measurements for temporal characters, when delimiting species, was also demonstrated.
However, the song patterns did not suffice to explain the diversity within the C. montana species complex. This group is intricate and has an unusual evolution, especially in the – herewith identified – song groups brevipennis and cerdaniensis. Hybridisation among taxa of these two song groups is obviously present to different degrees. The cerdaniensis and the brevipennis song groups are characterised by two clearly different basic song patterns and three well supported, but interdigitated mtDNA clades. The acoustic song patterns are similar in some taxa with relevantly different haplotypes, but clearly different in some taxa with closely related or identical haplotypes. The closest relative of each of the four cerdaniensis group species is a brevipennis group taxon. A colouration character supports the molecular differences and opposes the acoustic groups. Only by making use of an integrative approach and searching for “separately evolving metapopulation lineages”, were we able to find surprising results: Mitochondrial molecular clades are most probably diagnostic for glacial refugia in the Pleistocene (central and southern Apenninian, Iberian, Balkan) and as a result of inter-group introgression not for species.
Cicadas in Italy and Switzerland, as well as in many other European countries, occur predominantly in ecotone and woodland habitats. Closely related Cicadetta montana complex species prefer different vertical niches in the vegetation structure (treetops, bushes, herb layer); some are slightly more mesophilic than others. Eight of the ten Swiss cicada species are part of the Red List, which will soon be issued for the first time. In Italy, grassland species (owing to agriculture) and mountainous species (owing to changing species’ interactions induced by global warming) have the highest risk to become extinct.