Diptera - Small insects

This refers to the order Diptera, one of the largest orders of Insecta, and includes insects closely related to humans, such as mosquitoes, horseflies, and flies. Nearly 100,000 species are known worldwide, but many unknown species remain. This genus is characterized by its well-developed, membranous forewings, while its reduced hindwings form a bulging, rod-like haltere. It is the most highly developed group of insects, and belongs to the macropterous group along with caddisflies, butterflies, moths, and fleas.

They are small to medium-sized, with body lengths ranging from 1 millimeter to 5 centimeters, and their integument is softer than that of bees and is often equipped with hairs, bristles, spines, and sometimes scales. The head is connected to a large thorax by a thin neck, and compound eyes often occupy most of it, and in males both eyes often join on the back. Development occurs by complete metamorphosis, and the eggs are usually long oval or banana-shaped. The larvae are generally mosquito- or maggot-shaped and legless, and are often cylindrical to spindle-shaped, although some are ovoid or circular. The head is well developed in prolate horns such as mosquitoes, but degenerates in other species, and is not visible from the outside in higher species such as houseflies. The larval stage lasts from 3 to 8 instars, during which they molt to become pupae, and there are two types: cyclorhaphytes, in which the puparium splits vertically when they emerge, and in which the outer skin of the third instar larva hardens to become an egg-shaped to beer barrel-shaped puparium (called an enpupa), which splits horizontally along the segments when they emerge.

Adults are active during the day, visiting flowers to suck nectar or gathering around fermented or decaying matter, but there are also species that prey on other insects, such as the robber fly, and species that suck blood, such as mosquitoes, black flies, horse flies, and stable flies. These blood-sucking species are also important sanitary pests, along with flies that gather around filth and food, as they are vectors of disease. The larvae are both aquatic and terrestrial, and some live in mud or filth, some burrow into the tissues of plants, such as leaves and stems, some parasitize other insects or animals, and some are free-living, feeding on plants, small animals, and decaying matter, while others are omnivorous. Thus, while there are many pests in agriculture and forestry, there are also species that are beneficial as natural enemies of pests.

Dipterans are generally divided into three suborders and contain around 100 families:

(1) Iliocene horns (long horns): These have long, multi-segmented antennae. These include fungus gnats, crane flies, mosquitoes, black flies, etc.

(2) Brachyhorns: The third segment of the antenna is large, and the others are small and few in number. The head of the larva is incomplete. These include black soldier flies, horseflies, robber flies, and small-headed flies.

(3) Cyclorrha: The antennae are three-segmented and have one stinging hair. The larvae are headless, complete the third instar, and the pupa is a coccinellid. These include hoverflies, fruit flies, Drosophila, house flies, and flesh flies.

[Takehiko Nakane]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

昆虫綱のもっとも大きな目の一つである双翅目Dipteraのことで、カ、アブ、ハエなど人類に関係の深い昆虫を含んでいる。世界中で10万近い種類が知られるが、なお多くの未知種が残されている。この類の特徴は、前ばねがよく発達し膜状であるのに、後ろばねが縮小して先の膨れた棒状の平均棍(へいきんこん)になっていることで、昆虫としてもっとも高度に発達した一群であって、系統的にはトビケラ、チョウ、ガ、ノミとともに長翅群系に属する。

　体長は1ミリメートルから5センチメートル前後で、小形から中形のものが多く、外皮はハチなどより軟弱で、毛や剛毛、棘(とげ)、ときには鱗片(りんぺん)を備えることが多い。頭は細い頸(くび)で大きな胸部に連なり、複眼がその大部分を占めることが多く、雄では両眼が背部で相接することも多い。発生は完全変態で、卵は長い楕円(だえん)形かバナナ形が普通である。幼虫は一般にボウフラ形かウジ形で肢(あし)がなく、円筒形から紡錘形が多いが、卵形から円形のものもある。頭はカなど長角類では発達するが、ほかの類では退化し、イエバエなど高等な類では外から見えない。幼虫期は3～8齢で、蛹(さなぎ)になるとき脱皮し、羽化に際し蛹皮(ようひ)が縦に裂けるものと、3齢幼虫の外皮が硬化して卵形からビヤ樽(だる)形の蛹殻となり（囲蛹という）、羽化のとき環節に沿って横に裂ける環縫類とがある。

　成虫は昼間に活動し、花にきて蜜(みつ)を吸ったり、発酵したものや腐敗したものなどに集まるものが多いが、ムシヒキアブなどのように他の虫を捕食するものや、カ、ブユ、ウシアブ、サシバエなど吸血性のものなどがある。これら吸血性の種は同時に病原の媒介者として、汚物や食物に集まるハエ類とともに重要な衛生害虫である。幼虫は水生も陸生もあり、泥中や汚物中にすむもの、植物の葉や茎などの組織に潜入するもの、ほかの昆虫や動物に寄生するもの、自由生活のものでは植物・小動物・腐敗物を食べるもの、雑食のものがある。したがって、農林業上の害虫も多い反面、害虫の天敵として有益な種類もある。

　双翅類は普通次の3亜目に分けられ、100前後の科を含んでいる。

(1)糸角（長角）類　触角が多節で細長い。キノコバエ、ガガンボ、カ、ブユなど。

(2)短角類　触角第3節は大きく、ほかは小さく少数。幼虫の頭は不完全。ミズアブ、アブ、ツリアブ、ムシヒキアブ、コガシラアブなど。

(3)環縫類　触角は3節で刺毛が1本ある。幼虫は無頭、3齢で終わり、蛹は囲蛹。ハナアブ、ミバエ、ショウジョウバエ、イエバエ、ニクバエなどが含まれる。

［中根猛彦］

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