Sea skaters H. matsumurai and A. shiranui are among the few insects inhabiting the sea. For the past several decades they have become rare in most localities of Japan and have now been designated as threatened II (VU) and threatened I (CR+EN) by Ministry of the Environment. To understand their strategy adapting to the sea, and to develop conservation tactics, it is essential to know their life histories in the field. As the first year-round field study of any sea skater, we investigated their lifecycles in Kujukushima, Kyushu. They occurred sympatrically in coves along the jagged coast and repeated 2 or 3 generations a year and overwintered in the egg stage. At low tide, they formed dense mixed-species aggregations along the shore, while at high tide, they became sparser. The eggs of H. matsumurai were laid on the rocks of the shore and the duration of submergence under water was estimated to range from ca.1.4% to 44.8% of total length of egg stage. These eggs were entirely covered with hard shell. This is presumably as an adaptation, functioning to protect the eggs from desiccation and solar radiation especially during the long overwintering period on the rocks.