Apterygotes — Primitive Wingless Insects
The subclass Apterygota contains the most primitive insects and insect-like hexapods — those that never evolved wings. These ancient lineages provide insight into early insect evolution.
- Bristletails / Order Thysanura: Includes the familiar silverfish (Lepisma saccharina) — primitive, wingless insects with silvery scales and three tail-like cerci. Found in homes worldwide, they feed on starchy materials like paper, book bindings, and wallpaper paste. Can live for up to 8 years and survive for months without food.
- Diplurans (order Diplura): Eyeless, pale soil-dwelling hexapods with two cerci at the abdomen tip. Some species have forceps-like cerci used to capture prey. Most are scavengers in soil and leaf litter.
- Proturans (class Protura): Tiny (under 2mm), eyeless, antenna-less soil organisms. One of the most recently discovered arthropod classes (1907). They grow by adding body segments — unique among adult insects.
- Springtails (order Collembola): Abundant soil organisms (up to 250,000 per m²) possessing a fork-like jumping organ (furcula) that launches them into the air. Now often classified outside Insecta proper. Critical for soil fertility and nutrient cycling.
Order Dermaptera — Earwigs
Approximately 2,000 species of earwigs are distinguished by the prominent pincers (cerci) at their abdomen tip. Despite their name (from the Old English "ear creature"), they do not crawl into ears. Earwigs are among the few non-social insects that show maternal care — mothers guard and clean their eggs and tend the young nymphs. They are nocturnal omnivores that fold their intricately pleated hindwings beneath tiny, leathery forewings.
Order Ephemeroptera — Mayflies
With about 3,000 species, mayflies have the shortest adult lifespan of any insect — some species live only a few hours as adults, just long enough to mate. Their name means "living for a day." Mayfly nymphs, however, can live 1-3 years underwater. They are the only insects that molt after gaining functional wings (a unique subimago stage). Mass mayfly emergences can be so large they show up on weather radar and create slippery hazards on bridges.
Order Embioptera — Webspinners
About 400 species of webspinners produce silk from glands in their enlarged front feet — unique among insects. They spin silken tunnels and galleries over bark, under stones, and in soil. Females are always wingless; males of some species have wings. They live in colonies connected by silk tube networks and can run backward through their tunnels with impressive speed.
Order Phthiraptera — Lice
About 5,000 species of wingless, obligate ectoparasites. Lice have co-evolved with their hosts for millions of years, and their phylogenetic tree mirrors that of their host species — providing valuable data for evolutionary biology.
- Chewing lice (suborders Amblycera and Ischnocera): Feed on feathers, hair, skin debris, and secretions. Bird lice (multiple families) are especially diverse, with different species specializing on different parts of the same bird.
- Rhynchophthirinans (suborder Rhynchophthirina): Elephant and warthog lice — the largest lice, up to 3mm.
- Sucking lice (suborder Anoplura): Blood-feeding parasites of mammals.
- Human louse (Pediculus humanus): Exists as head lice and body lice. Body lice can transmit typhus, trench fever, and relapsing fever — diseases that have altered the course of human history.
- Pubic louse (Phthirus pubis): A separate species adapted to coarse body hair. Now declining in prevalence worldwide.
Order Siphonaptera — Fleas
About 2,500 species of laterally compressed, wingless blood-feeding parasites. Fleas are legendary jumpers — they can leap 150 times their own body length (equivalent to a human jumping over a 75-story building). This is achieved by storing energy in a pad of resilin, a nearly perfect natural rubber. The rat flea (Xenopsylla cheopis) transmitted the Black Death (bubonic plague) that killed an estimated 25-50 million Europeans in the 14th century.
Order Megaloptera — Alderflies & Dobsonflies
- Alderflies (family Sialidae): Small to medium-sized insects with smoky wings, found near streams and ponds. Larvae are aquatic predators.
- Dobsonflies (family Corydalidae): Large insects with impressive mandibles in males (up to 4cm long) used in mating displays and combat. Despite their fearsome appearance, males' mandibles are too weak to bite. Larvae (hellgrammites) are voracious aquatic predators prized as fishing bait.
Order Mecoptera — Scorpionflies
About 600 species named for the males' upturned genitalia resembling a scorpion's tail (completely harmless). Scorpionflies are thought to be among the ancestors of the incredibly successful orders Diptera (flies) and Siphonaptera (fleas). Uniquely, males often offer "nuptial gifts" of dead insects or nutritious salivary secretions to females during courtship.
Order Plecoptera — Stoneflies
About 3,500 species of soft-bodied insects found near clean, cold streams and rivers. Stonefly nymphs are highly sensitive to pollution, making them excellent bioindicators of water quality — their presence signals a healthy aquatic ecosystem. Adults are weak fliers and some winter-emerging species can walk on snow.
Order Psocoptera — Psocids
About 5,500 species commonly known as booklice (indoor species) or barklice (outdoor species). Despite their name, they are not true lice and don't parasitize anything — they feed on microscopic molds, fungi, and organic debris. Indoor species thrive in damp conditions and can damage old books, stored food, and museum collections.
Order Thysanoptera — Thrips
About 6,000 species of tiny insects (1-2mm) with fringed wings — "thysanoptera" means "fringe-winged." Many are agricultural pests that damage plants by scraping and sucking cell contents, causing silvery scarring. They are unusual in having only one mandible (the left — the right is reduced). Some species are predatory and used in biological pest control.
Order Trichoptera — Caddisflies
About 14,500 species of moth-like insects whose larvae are aquatic and famous for building portable cases from silk, sand grains, plant material, or even tiny shells and pebbles. Each species constructs a distinctive case architecture. Like stoneflies, caddisfly larvae are water-quality indicators. French artist Hubert Duprat famously provided caddisfly larvae with gold flakes and gemstones, resulting in exquisite jeweled cases.
Order Strepsiptera — Twisted-Wing Parasites
About 600 species of bizarre endoparasites — females live their entire lives embedded inside other insects (especially bees and wasps), with only their head protruding from the host's abdomen. Males have twisted hindwings and branched eyes, and live only a few hours as free-flying adults. Their extreme parasitic lifestyle has made their evolutionary relationships controversial for decades.
Order Grylloblattodea — Ice Bugs
Only about 34 species of these cold-adapted, wingless insects exist, living in ice caves, beneath snowfields, and at high altitudes. They die at temperatures above 20°C and can only survive near freezing. Also called rock crawlers, they are considered "living fossils" representing a very ancient insect lineage. One of the rarest and least-studied insect orders.
Order Mantophasmatodea — Gladiator Bugs
Discovered in 2002 — the first new insect order identified since 1914. Only about 20 species are known, all from southern Africa. They resemble a cross between a mantis and a stick insect. Predatory and wingless, they live in scrubby vegetation. Their discovery demonstrated that significant insect biodiversity remains to be catalogued even in the 21st century.
Order Diplura — Diplurans
About 800 species of eyeless, soil-dwelling hexapods with two prominent cerci. The largest (Japygidae) have forceps-like cerci used to catch prey, while the more common Campodeidae have long, filiform cerci. They are important members of the soil food web and are considered close relatives of true insects.