By Shen Chunlei, Reporter from China Science Daily
The chirping of cicadas on a summer afternoon can disturb one's peaceful dreams. However, about 100 million years ago, cicadas may not have been able to chirp loudly. The researchers from Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, in cooperation with scholars from multiple countries, have analyzed the anatomical characteristics of fossils and the extant cicada superfamily group. They found that early cicadas may not have been able to produce loud sounds. They also reported the discovery of the earliest known fossils of final instar larvae of the cicada superfamily. The research results were recently published in Nature Communications. Schematic diagram of the ecological scene of cicadas in the Mesozoic forest.Illustration by Yang Dinghua "This is a very interesting study, and I feel a sense of 'star chasing' success," said the first author of the paper, Jiang Hui, to China Science Daily. "Professor David Emery of the University of Sydney in Australia donated the only existing cicada specimens in Australia for my research. Some 'big names' that have often appeared in previous articles have become my collaborators, and together we completed this research."Limited knowledge of early evolutionary history
Cicadas, commonly known as "zhiliao" in China, are a relatively common insect known for their distinctive chirping sounds, often heard during the summer afternoons. Jiang Hui explained that the extant cicada superfamily includes two families - the widely distributed Cicadidae and the extinct Cicadopsidae, which are only found in Australia. The earliest fossils of the cicada superfamily were discovered in Triassic strata dating back 250 million to 200 million years ago. Fossils of the cicada superfamily in the Mesozoic era (including the Triassic, Jurassic, and Cretaceous periods) are relatively abundant, but most of them are wing specimens preserved in rocks, so there is limited understanding in the academic community about the morphological and larval characteristics of the cicada superfamily. Currently, little is known about the early evolutionary history and paleoecological habits of the cicada superfamily in the academic community. Under the guidance of Wang Bo and Zhang Haichun, researchers from the Nanjing Institute of Geology and Paleontology, Jiang Hui used optical microscopy, computed tomography imaging technology, etc. to summarize and analyze the fossil records of the cicada superfamily in the Mesozoic era. They also reexamined the anatomical structures of the fossils with intact body structures, as well as those of extant Cicadopsidae and Cicadidae. The research found that the cicada superfamily fossils in the Mesozoic era include the extant Cicadidae, Cicadopsidae, and Cicadinae. Some previously classified fossils in the Cicadopsidae family may be more closely related to modern Cicadidae in terms of their phylogenetic relationships. Based on this, the researchers speculate that the two lineages of modern Cicadidae and Cicadopsidae had already diverged in the Middle Jurassic. The classification of insect fossils usually relies on the preserved morphological features. "We conducted a morphospace analysis of the local structures of cicada adults and larvae and found that the highly specialized homologous structures in cicada fossils may contain previously overlooked transitional features," said Jiang Hui. The research results initially clarify the phylogenetic relationships and the early evolutionary history of morphology and ecological habits in the cicada superfamily fossils. They reconstructed the phylogenetic relationships of fossil and extant cicada superfamily groups. Jiang Hui explained, "By conducting more detailed studies on these transitional structural features, we can have a more accurate understanding of the evolution of morphological characteristics, which will help to clarify the macroevolutionary patterns of insects."
Cicadas that do not chirp
The research found that sound signals are an important means for many animals to communicate. "The extant cicada groups can produce the loudest sound among insects, reaching up to 120 decibels. Typically, male cicadas use their well-developed tympanum muscles to flex the tympanum and produce sound, while the abdomen acts as a resonator to amplify the sound," Jiang Hui said. However, although Cicadopsidae has a tympanum and tympanum muscles, they are not well-developed and lack resonators, so they do not chirp but communicate through the transmission of vibration signals. These two completely different signaling mechanisms have led to speculations about the sound-producing structure and behavioral evolution of cicadas. "We first discovered the tympanum structure in the cicada superfamily fossils," Jiang Hui said. The research found that the tympanum structure exists in all the branches of the cicada superfamily and is preserved in both males and females, thereby representing an ancestral characteristic of the cicada superfamily. Through the analysis of fossils, Jiang Hui and others found that cicadas from the mid-Cretaceous period (approximately 100 million years ago) may not have had complex sound-producing and auditory organs, and they may have communicated through vibration signals transmitted in the substrate, similar to modern Cicadopsidae. The research team also discovered fossils of final instar larvae and cicada exuviae from the mid-Cretaceous period, which are the earliest known fossils of final instar larvae of the cicada superfamily. These fossils show similar forelegs to those of modern cicada larvae, with sickle-shaped tibiae and enlarged femora forming grasping structures, indicating their powerful soil excavation and transportation abilities. They may have evolved similar underground living habits to those of modern cicada larvae.
