True flies traded hindwings for spinning balance clubs
Flies belong to order Diptera—“two wings”—because hindwings became halteres, gyroscopic stubs that steady flight. A mobile head carries huge compound eyes and mouthparts built to pierce or to lap. They are major pollinators, second only to bees and fellow Hymenoptera, and among insects’ big ecological orders.
More than 150,000 species have been described, from crane flies and hoverflies to horse-flies and midges, and many regions remain barely surveyed. Claws and pads on the feet let them walk on glass. Most have three small simple eyes on top of the head besides the two big compound eyes, which in some species split into upper and lower zones, perhaps for swarming. Motion-sensitive neurons read the streaming visual field to judge yaw, roll and sideways drift.
True flies date to the Middle Triassic, about 240 million years ago, long before flowering plants arrived around 140 million years ago, so the first ones must have fed on something other than nectar. They may have been among the earliest pollinators, and they still rank second after bees and wasps, becoming more important than bees in wet and cold places. Louse flies are among the few that lost their wings again.
The old split relied on antennae: long-bodied Nematocera such as mosquitoes carry many-segmented, often feathery ones, while rounder Brachycera have short stubs. A 2023 mitochondrial study confirmed the nematocerans as paraphyletic and made Anisopodidae the sister of the Brachycera.
Lifestyles are astonishingly varied. Gall midges produce more galls than any other insects, leaf miners tunnel between leaf surfaces, and cave fungus gnat larvae glow. Megaselia scalaris larvae will eat paint and shoe polish, while shore fly and midge larvae survive glaciers, hot springs, sulphur pools and even crude oil. Brachyceran larvae parasitise snails, woodlice, millipedes and mammals. Fruit flies serve as laboratory models, and mosquitoes spread malaria and dengue.
Source: Fly
