Terumi Ikawa

A method for reconstructing three-dimensional positions of swarming mosquitoes

We developed a new field method for reconstructing the three-dimensional positions of swarming mosquitoes. This method overcame certain inherent difficulties accompanied by conventional stereoscopic methods and is applicable to three-dimensional measurements of other insect species. Firstly, we constructed a probabilistic model for stereoscopy; if mosquitoes and six reference points with known coordinates were photographed simultaneously from two or more perspectives, then from the positions of images of mosquitoes and the reference points on the photographs, 1) the position of each camera with respect to the reference points is estimated; 2) stereo images which correspond to an identical real mosquito are matched; and 3) the spatial positions of the mosquitoes are determined. We automated the processes 1), 2) and 3), developing computer programs based on our model. We then constructed a portable system for three-dimensional measurements of swarming mosquitoes in the field. Initial data that illustrate the application of our method to studying mosquito swarming were presented.

Distribution of the oceanic insects Halobates (Hemiptera: Gerridae) off the south coast of Japan

Specimens of ocean skaters Halobates were collected off the south coast of Japan in the East China Sea in 1995, and from the Kumano‐nada Sea to the East China Sea in 1998 and 1999. Three species were identified: H. micans, H. germanus and H. sericeus. We found two species co‐occurring in comparable densities in different years, a phenomenon not hitherto reported in other regions of the ocean. We discuss distributions of the three Halobates species with special reference to the influence of the Kuroshio Current, temporal variations of sea‐surface temperature, and monsoonal winds.

Skaters of the seas – comparative ecology of nearshore and pelagic Halobates species (Hemiptera: Gerridae), with special reference to Japanese species

Abstract The insects overwhelm all other organisms of the world in species numbers and diversity. However, there are relatively few insects that inhabit the sea. Most of them are confined to the intertidal zone, with only five species of sea skaters, genus Halobates, having been successful in colonizing the open ocean. We discuss the ecology of both coastal and pelagic Halobates and the closely related genus Asclepios, focusing on their distribution ranges and adaptive strategies to marine environments accompanied by brief discussions of their biology. Updated information on the known localities for three species of Asclepios and some 40 coastal species of Halobates are presented. Many species live in vulnerable coastal habitats exposed to environmental pollution and coastal development. As a case study we trace the historical changes in populations of three Japanese sea skaters, Asclepios shiranui, Halobates matsumurai and H. japonicus, all designated as endangered species. For oceanic Halobates, we present an updated distribution map along with global current systems and sea-surface temperatures, examine interactions between distribution ranges and physical factors at the air–sea interface and discuss spatio-temporal variations in populations of each species. Finally, we infer the life history strategy of oceanic Halobates through theoretical considerations.

Life Histories of Two Endangered Sea Skaters Halobates matsumurai Esaki and Asclepios shiranui (Esaki) (Hemiptera: Gerridae: Halobatinae)

Sea skaters Halobates matsumurai Esaki and Asclepios shiranui (Esaki) are among the few marine insects found in Japan. For the past several decades, they have become rare in most localities and have now been designated as endangered by the government. In order to understand their adaptive strategies to the marine environment and to develop conservation measures, it is essential to know their life histories. We studied their lifecycles in Kujukushima Bay off the north coast of Kyushu (Japan) where they co-occurred in small coves along the jagged coast. They appeared to have more than one generation a year and to overwinter in the egg stage. Eggs of H. matsumurai were laid on natural sandstones and man-made sandstone walls along the shore, mostly above the average sea level. The eggs had very hard shells, presumably adaptive to protect them from desiccation, solar radiation, and wave action, especially during the overwintering period.

Occurrence and density of Halobates micans (Hemiptera: Gerridae) in the eastern South Indian Ocean

Two species of ocean skaters, Halobates germanus and Halobates micans, live in the tropical and subtropical waters of the Indian Ocean. From December 1992 to December 1993, Halobates was intensively sampled in the easternmost region of the South Indian Ocean (13–18.5°S, 114–121E°), from which there have been a small number of records of Halobates. No H. germanus was caught, but a total of 1190 H. micans were collected, with densities estimated at 13 900–28 100 individuals/km2. This suggests that H. micans lives in the study area at high densities comparable to those in the Atlantic and the Pacific Oceans. We also discuss the possible effects of ocean currents and winds on the geographic distributions of the two Halobates species in the eastern South Indian Ocean.

Life History of an Endangered Marine Insect Halovelia septentrionalis Esaki (Hemiptera: Veliidae)

Halovelia septentrionalis Esaki is one of the endangered marine water striders found along the Japanese coast. It is of primary importance to investigate its life history in the field so as to develop conservation measures as well as to understand how this species has adapted to marine environments. We studied its life history in Misaki on the southern part of the Miura Peninsular near Tokyo, ca. 35°N, probably the northernmost locality not only for H. septentrionalis but also for any Halovelia species, most of which are found in warm tropical or subtropical waters. The southern part of the Miura Peninsular has jagged coastlines with deep inner bays. Adults and nymphs were found skating along the rocky shore of inner bays with overhanging vegetation nearby. This species appeared to have at least 2 generations a year and to overwinter in the adult stage. We discussed its overwintering strategy in comparison to those of two other Japanese marine water striders.

A Study of the Distributions of Two Endangered Sea Skaters Halobates matsumurai Esaki and Asclepios shiranui (Esaki) (Hemiptera: Gerridae: Halobatinae) with Special Reference to Their Strategies to Cope with Tidal Currents

There exist surprisingly few marine insects, most of which are confined to the intertidal zone. Halobates is the only genus to have some oceanic species along with some coastal species. Among the coastal sea skaters, that is, genus Halobates and its close relative, genus Asclepios, there are variations in their affinity for the shore. We have studied the distributions of two endangered Japanese sea skaters, Halobates matsumurai and Asclepios shiranui in Kujukushima, where they cooccur in coves. To compare their survival strategies at sea, we especially noted differences in their distributions along the shore during high and low tides. The results show that A. shiranui tended to remain along the shore during low tide. This species appeared to cling to the shore against the ebb current and to stay in the protected coves. By contrast, H. matsumurai tended to leave the shore during low tide. Notably, some adults were found skating outside the coves. The strategy of H. matsumurai appeared to be leaving with the ebb current even beyond the coves, thereby using more resources and enlarging its habitat. Some such coastal Halobates might have acquired the ability to live on the open ocean clearly independent of the shoreline.