Mayflies - Kagerou (English spelling)

A general term for insects in the order Ephemeroptera of the class Insecta. They are also called mayflies. Adults have delicate bodies and are very short-lived. Their larvae, along with stoneflies and caddisflies, are representative aquatic insects.

The adult body is long and cylindrical, and the compound eyes are often divided into two parts, one above and one below. The compound eyes of males are larger than those of females. They have three ocelli, and the antennae are short and inconspicuous. The mouthparts are degenerate, and they do not take food. The legs are thin, and the tibiae and tarsus of the male's forelimbs are significantly longer than those of females. There are two pairs of wings (rarely one pair), which are thin and transparent, and shine when exposed to light. The wing veins are arranged in an alternating line, with veins that protrude upward and veins that protrude downward from the membrane surface. When at rest, the wings are raised up on the back with the left and right sides together. There are two or three long tails at the end of the abdomen. The male has a pair of grasping organs at the rear end of the ninth abdominal segment, and is distinguished from the female by the presence of a copulatory organ between them. On the other hand, the larvae are called nymphs, and live in rivers and lakes. Many of the flowing water ones have a flattened body shape dorsally and ventrally, while many of the steady water ones are spindle-shaped. The main characteristics of the larvae are that they have several pairs of tracheal gills of various shapes on both sides of the dorsal side of the abdomen, and well-developed mouthparts. When the larvae mature, they emerge on the water's surface or near the water's edge, becoming subadults, and then molt once more to become adults. The subadult trait is not seen in other insects and is unique to the Ephemeroptera order, and although they have a morphology very similar to that of adults, they are sexually immature and have weak flying ability. They are also slightly larger than adults, grayish in color, with thick and short legs and tails, and opaque wings.

After emerging, adult males form swarms and fly up and down over the water's surface. Eventually, females enter the swarm to mate, and immediately lay eggs in the water after mating. The larval stage lasts an average of one year, with some species lasting up to three years, but the subadult stage lasts for about one day, and the adult stage also lasts for about one day, with some species only surviving for four to five hours. Because of this short adult period, they are used as an analogy for something fleeting, and the name of the taxonomic order comes from the Greek word ephēmeros, which means only one day of life. The name mayfly comes from the way they fly around as adults, which resembles a heat haze, and the English word mayfly comes from the fact that many of them emerge in May or June. There are over 2,000 known species of mayflies in the world, and in Japan there are over 60 species in 21 genera and 10 families, including the Siphronuridae, Baetidae, Heptageniidae, Oligoneuriidae, Leptophlebiidae, Ephemerellidae, Potamanthidae, and Ephemeridae.

[Yamazaki Egane]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

昆虫綱カゲロウ目Ephemeropteraの昆虫の総称。蜉蝣類(ふゆうるい)（目）ともいう。成虫は繊細な体をしており、きわめて短命である。また幼虫はカワゲラ類、トビケラ類とともに水生昆虫の代表である。

　成虫の体は細長い円筒形で、複眼は上下2部に分かれているものが多く、また雄の複眼は、雌より大きい。単眼は3個、触角は短くて目だたない。口器は退化しており、食物をとらない。肢(あし)は細く、雄の前肢(ぜんし)の脛節(けいせつ)と跗節(ふせつ)は雌に比べて著しく長い。はねは2対（まれに1対）あって薄膜状、また透明で、光を受けると輝く。翅脈(しみゃく)は、膜面に対して上に突出している脈と、下に突出する脈とが交互に並ぶ。静止時には、はねを左右あわせて背上に立てる。腹端には2、3本の長い尾をもつ。雄の第9腹節の後端には1対の把握器があり、その中間に交尾器があることで雌と区別される。一方、幼虫はとくに若虫(わかむし)とよばれ、河川の渓流あるいは湖沼に生活する。流水性のものには背腹に平たい体つきのものが多く、静水性のものでは紡錘形になったものが多い。幼虫の大きな特徴は、腹部背面の両側にさまざまな形をした数対の気管えらをもつことと、口器がよく発達していることである。幼虫は成熟すると、水面または水辺で羽化して亜成虫となり、さらにもう1回脱皮して成虫となる。亜成虫という形質は、ほかの昆虫類にはみられず、カゲロウ目に固有のもので、成虫とよく似た形態をもつが、性的には未熟で、飛ぶ力も弱い。また、成虫よりもやや大きく、灰色がかっており、肢や尾は太くて短く、はねは不透明である。

　成虫の雄は、羽化ののちに群れをなし、水面上で上下飛行運動を繰り返す。やがて雌個体がこの群れの中に突入して交尾し、交尾後ただちに水中に産卵する。幼虫期間は平均1年、長いもので3年を経過するが、亜成虫の時期はおよそ1日ぐらいのもので、成虫も1日程度、短い種で4～5時間しか生存できない。このように成虫期間の短いことから、はかないもののたとえに用いられ、分類階級の目の名称もギリシア語のephēmeros（わずか1日の命の意）に由来する。また、カゲロウの名は、成虫になってから飛び交うさまを陽炎(かげろう)になぞらえたもので、英語のmayflyは5～6月に羽化するものが多いところから出た。カゲロウ類の種数は、世界で2000種以上が知られており、日本にはフタオカゲロウ科Siphronuridae、コカゲロウ科Baetidae、ヒラタカゲロウ科Heptageniidae、ヒトリガカゲロウ科Oligoneuriidae、トビイロカゲロウ科Leptophlebiidae、マダラカゲロウ科Ephemerellidae、カワカゲロウ科Potamanthidae、モンカゲロウ科Ephemeridaeなど、10科21属60種以上が知られている。

［山崎柄根］

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