[B-PT02] Biotic History

[BPT02-03] Micro sensory organs reveal the perception system and ecology of the Cretaceous cockroach

*Ryo Taniguchi¹, Yasuhiro Iba¹, Hiroshi Nishino², Hidehiro Watanabe³, Kosuke Tateishi³ (1.Faculty of Science, Hokkaido University, 2.Research Institute for Electronic Science, Hokkaido University, 3.Department of Earth System Science, Faculty of Science, Fukuoka University)

Keywords:micro sensory organ, cockroach, amber, ecological reconstruction, destructive analysis, Cretaceous

Sensory organs play essential roles for animals in detecting information of the surrounding environment. Insects are the most diverse group in Animalia, and their sensory organs have a notable ability to process complicated information despite the small size restriction. Among insects, cockroaches have maintained the body plan for a long time since they appeared on the earth, and they possess relatively small eyes and well-developed antennae reflecting the nocturnal ecology. Therefore, cockroaches can be considered model organisms on evolution and diversity of the remarkable insect sensors in paleontology. Fossils are the only direct evidence for paleontological research of these organs. Insect organs, however, are hardly fossilized owing to the smallness and fragility. Although they are exceptionally well preserved in amber, existing non-destructive analysis such as X-ray computed tomography does not achieve enough resolution to observe the fossil microstructure. Here, we focused on and visualized micro-sensors of an extinct male cockroach in Cretaceous Myanmar amber (Huablattula hui Qiu et al., 2019), combining destructive and non-destructive methods. Furthermore, we reconstructed the perception system and ecology of the cockroach in high-resolution.

We measured the surface area of the ommatidia and compound eyes based on the macro photographs and 3D-model constructed via X-ray CT. From these data, we estimated the number of ommatidia composing the compound eyes. We applied the thin-sectioning method used in mineralogy to observe antennal sensilla that are sensors of multiple information and too small for non-destructive methods to visualize. In this process, the amber piece containing the right antenna was cut off and ground down to 200 µm thick. We observed the thin section with a confocal laser scanning microscope.

The compound eyes of H. hui are large and round, and they consist of about 6,000 ommatidia. This number is 1.5 times larger than extant nocturnal species whose body sizes are five times as large as H. hui. Many sensilla are well preserved with surface microstructure on the antenna. We were able to classify them based on the morphology and estimate their function: mechanoreception and multiple olfactions. Compared to extant species living in dark environments, these sensilla are small against the body size, and mechanoreceptive sensilla are markedly fewer. These holistic characters of sensors indicate that H. hui relied on light stimulation rather than contact or odors to perceive environmental cues. H. hui was probably the diurnal insect adapted to bright habitats opposite to many modern cockroaches. We also revealed that the abundant sensillum type on the H. hui antenna is similar to extant male mantises. The abundance of particular sensilla in males results from using sex pheromones, and therefore H. hui may have used mantis-like intersexual communication. We show that detailed observation of micro sensory organs with destructive analysis enables reconstructing ecology and behavior of fossil insects in high-resolution.