Mimesis is one of the most successful survival strategies in animal evolution. As the most abundant species of organisms on earth, insects have evolved rich and diverse mimetic taxa, such as dead leaf butterflies that simulate dead leaves, orchid praying mantises that simulate orchids, and bamboo sygropods that simulate branches. However, the origin and evolution of insect mimicry are still unclear, and mimesis research has become a hot spot and difficulty in biological research. Recently, Wei Xinli, a project researcher in the Wei Jiangchun Research Group of the Institute of Microbiology of the Chinese Academy of Sciences, and Wang Yongjie's team from Capital Normal University jointly published a research paper lichen mimesis in mid-Mesozoic lacewings at eLife, which discovered the oldest mimetic lichen insects 165 million years ago and revealed the self-protection mechanism of the earliest insect mimesis lichens.
Lichens are symbiotes of fungi and algae or cyanobacteria, and were first considered to belong to plants due to their plant-like leaf bodies; subsequent studies found that their shape was mainly controlled by fungi as the main body, so they were classified into the fungal world. Lichens are usually epiphytic on surfaces such as bark, rock and soil, and are widely distributed from the polar to the tropics, occupying 7% of the Earth's land surface. Lichens can secrete substances such as lichen acid to corrode rocks and promote the formation of surface soil layers, known as "pioneer organisms". In modern ecosystems, lichen acid secreted by lichen makes many animals reluctant to approach lichen, so a complex and special micro-ecological system is formed between lichen and "lichen tolerant animals", and the insects of lichen and mimetic lichen are one of the most representative relationships, such as the "industrial blackening phenomenon" of the birch ruler moth in the 19th century, the industrial revolution caused a large amount of pollution, the demise of lichen on the bark, resulting in the light-colored birch ruler moth camouflaged lichen being more easily preyed by birds, and the number of dark birch ruler moths increased; as the environment improved, After the lichen was recovered in large quantities, the number of light-colored birch ruler moths increased, which is known as "Darwinian evolution in action".
There is still a lack of research on the origin and early evolution of insect mimetic lichens, and the key to solving this problem is to find lichens in older formations and similar insects. The earliest reported fossils of suspected lichen were found in the Early Devonian strata about 400 million years ago, but their taxonomic location has been questionable. Scholars who study living lichens believe that real large lichens appeared no earlier than 65 million years ago. Around the problem of whether there were more ancient large lichens and insects in mimetic lichens in the older Mesozoic Era, Wang Yongjie's team and Wei Xinli collaborated on this. Using fossil sectioning and scanning electron microscopy technology, the suspected lichen fossil Daohugouthallus ciliiferus collected from the Jurassic Yanliao insect group in Ningcheng County, Inner Mongolia was studied, and key structures such as fungal hyphae and algal cells and their entanglements were found. It shows that D.ciliiferus is a symbiote composed of fungal and algal cells, with conclusive evidence to prove its lichen properties, which is also the earliest fossil of large lichen found so far, which advances the appearance of large lichen by nearly 100 million years, providing new evidence for the study of the early evolution of lichen.
The study found a veined insect in the same formation where the lichen fossil D. ciliiferus appeared: Lichenipolystoechotes. Through comparative analysis, it was found that its wing spots were very similar to the leaf morphology of D.ciliiferus; through random measurement and statistical analysis of the width of each part of the insect's wing spots and the width of the d.ciliiferus fossils, the results showed that the insect had a high degree of similarity with the lichen; a large number of black spots were found on the insect's wing spots, and similar spots were also found on the D.ciliiferus phyllaceous, and this structural detail also increased the similarity between the two. Thyridosmylus paralangii is a class of insects with similar wing spots in the order Phytophthalidae, which live in a relatively humid moss-rich and lichen environment and have very good consistency with the environment; when the Jurassic lichen moss inhabits the environment of D.ciliiferus, the similarity between the two makes the lichen moss form a high degree of consistency with the environment. Thus, the Jurassic lichen spider gained a survival advantage by simulating D.ciliiferus, the first example of the oldest insect simulated lichen ever discovered, contributing to the study of fossil insect mimetic lichens, demonstrating that insects and lichens had formed a complex interplay relationship 165 million years ago.
Fang Hui, a doctoral student at the School of Life Sciences of Capital Normal University, is the first author of the paper, Wei Xinli, a project researcher of the Wei Jiangchun Research Group of the Institute of Microbiology, Wang Yongjie, an associate professor of Capital Normal University, and Liu Jiaxi, a professor of Capital Normal University, are the co-corresponding authors of the paper. The research work has been funded by the Innovation Team Project of the Ministry of Education, the National Natural Science Foundation of China, the Beijing Natural Science Foundation project and the First Normal University Interdisciplinary Research Institute.
Figure 1. Section scanning electron microstructures of Daohugouthallus ciliiferus. A. Lichenial cortex structure, B-H. fungal hyphae winding or joining green algae cells
Figure 2: Photographs and line drawings of lichen insect fossils
Figure 3:Photographs of lichen fossils and statistical results of insect wing spots and lichen leaf bodies
Figure 4: Ecological restoration of lichen moths