In this study, the microstructure was confirmed by examining the exoskeleton of Cybister japonicus and Hydrophilus acuminatus, which are representative aquatic insects. And the exoskeleton was observed through the microscope. The digestive organs and respiratory organs were observed through anatomy, respectively. Hydrophilus acuminatus had longer midgut and hindgut than Cybister japonicus, and Cybister japonicus had larger crop than Hydrophilus acuminatus. The structural difference of the hind legs revealed that there was a difference between their swimming behavior. Cybister japonicus swing both legs at the same time, while Hydrophilus acuminatus swings alternately on both hind legs. This study is expected to be the basis of ecological studies and researches of aquatic insects by studying the basic structure of aquatic insects.