Aquatic insects

A general term for insects that live in water. Insects are originally terrestrial and are thought to have secondarily invaded water to survive, but the larvae of relatively primitive winged insects such as mayflies and dragonflies are also aquatic. Some aquatic insects live their entire lives in water, leaving the water only to move or overwinter, while others spend their larval or larval and pupal stages in water and leave the water when they become adults. Body shape and breathing methods vary considerably depending on the species, depending on the method and degree of adaptation to the aquatic living environment. For example, among beetles, diving beetles store air between their epithelium and abdomen, occasionally rising to the surface of the water, and their larvae breathe by exposing the respiratory tube at the end of their tail to the surface, while water beetles mainly live on the water surface, and their larvae have tracheal gills on both sides of their bodies and live in water. Water beetles also have dense fine hairs on the underside of their bodies and the tips of their antennae, which create a thin layer of air through which they breathe, but their legs are thin and not flat and paddle-like like those of diving beetles and water beetles. Water beetles also have dense fine hairs on their ventral surface, but their legs are thin and have well-developed claws, which allow them to attach to stones on the bottom of the water and walk. Other beetle larvae include the larvae of the small water beetle and some water beetle larvae that breathe through tracheal gills. Other insects that are common in water include the hemipterans, whose adults and larvae store air in the fine hairs on the surface of their bodies and live on the bottom of the water or in the sand, the water spinners that swim under the water surface with their ventral side up, water stick insects and water squirrels that have a long breathing tube at the end of their tail and breathe from above the water, the large and ferocious giant water bugs that attack young fish, water bugs in which the female lays eggs on the back of the male, and water bugs (commonly known as balloon bugs) that come to lights on summer nights.

Insects whose adults live on land include mayflies, dragonflies, stoneflies, caddisflies, some neuropterans (such as dragonflies), beetles (such as fireflies and root beetles), and dipterans such as mosquitoes, blackflies, and horseflies. Most breathe through tracheal gills, but some breathe through their skin and have a breathing tube. In the pupae of blackflies, the spiracles extend and become thread-like gills. Water striders and water collembolans are sometimes included in aquatic insects, but they live only on the water surface and should be considered semi-aquatic.

[Takehiko Nakane]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

昆虫のなかで水中で生活するものの総称。昆虫類はもともと系統的には陸生であって、生活のため二次的に水中に侵入したものと考えられるが、比較的原始的な有翅(ゆうし)昆虫であるカゲロウやトンボの幼虫も水生である。水生昆虫には一生の間水中で生活し、移動あるいは越冬のためにのみ水を離れるものと、幼虫の時期または幼虫と蛹(さなぎ)の時期を水中で過ごし、成虫になるときに水を離れるものがある。体形や呼吸の方法などは、水中の生活環境への適応の方法や程度によって類によりかなり違っている。たとえば、甲虫類のなかでもゲンゴロウは上ばねと腹部の間に空気を蓄え、ときどき水面に浮上し、幼虫は尾端の呼吸管を水面に出して呼吸するが、ミズスマシはおもに水面で生活し、幼虫は体の両側に気管鰓(さい)をもち、水中で生活している。また、ガムシは体下面や触角先端部に微毛を密生し、空気の薄い層をつくり、これを通じて呼吸するが、肢(あし)は細くてゲンゴロウ、ミズスマシのように平たく櫂(かい)のようにはならない。ドロムシも腹面に微毛を密生しているが肢は細くてつめが発達し、水底の石などに付着し歩行するのに適している。甲虫ではほかにコガシラミズムシの幼虫やドロムシの一部の幼虫が気管鰓で呼吸している。このほか、水中に多い昆虫には半翅類があり、成・幼虫とも体表の微毛部に空気を蓄え、水底や砂中などにいる円板状のナベブタムシ類、水面下を腹面を上にして泳いでいるマツモムシ類、尾端に長い呼吸管をもち水上から呼吸するミズカマキリやタイコウチの仲間、稚魚などを襲う大形で獰猛(どうもう)なタガメ、雌が雄の背面に卵を産み付けるコオイムシ、夏の夜に灯火にくるコミズムシ（俗にフウセンムシという）の類などがある。

　成虫が陸上で生活するものにはカゲロウ、トンボのほか、カワゲラ、トビケラ、脈翅類の一部（ヘビトンボなど）、甲虫（ホタル、ネクイハムシなど）、そのほかにカ、ブユ、アブなどの双翅類などが含まれ、多くは気管鰓で呼吸するが、皮膚呼吸のもの、呼吸管をもつものもある。ブユの蛹(さなぎ)は気門部が伸びて糸状のえらになる。なお、アメンボ類やミズトビムシなども水生昆虫に含めることがあるが、これらは水面上のみで暮らしており、半水生というべきであろう。

［中根猛彦］

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