Young resembling parents, but aquatic in habit, breathing dissolved
air by thoracic tracheal gills.
Includes the single family of the Perlidae (Stone-flies), formerly grouped with the Neuroptera.
Order: Isoptera.
Biting mandibles; second maxillae incompletely fused. Fore-wings similar in shape and texture to hind-wings, which do not fold. In most species the majority of individuals are wingless. Short, jointed cerci. Six or eight Malpighian tubes. Young resembling adults; terrestrial throughout life.
Includes two families, formerly reckoned among the Neuroptera—the Embiidae and the Termitidae or “White Ants” (see Termite).
Order: Corrodentia.
Biting mandibles; second maxillae incompletely fused; maxillulae often distinct. Cerci absent. Four Malpighian tubes.
Includes two sub-orders, formerly regarded as Neuroptera:—
1. Copeognatha: Corrodentia with delicate cuticle. Wings usually developed; the fore-wings much larger than the hind-wings. One family, the Psocidae (Book-lice). These minute insects are found amongst old books and furniture.
2. Mallophaga: Parasitic wingless Corrodentia (Bird-lice).
Order: Ephemeroptera.
Jaws vestigial. Fore-wings much larger than hind-wings. Elongate, jointed cerci. Genital ducts paired and entirely mesodermal. Malpighian tubes numerous (40). Aquatic larvae with distinct maxillulae, breathing dissolved air by abdominal tracheal gills. Penultimate instar a flying sub-imago. [Includes the single family of the Ephemeridae or may-flies. See also Neuroptera, in which this order was formerly comprised.]
Order: Odonata.
Biting mandibles. Wings of both pairs closely alike; firm and glassy in texture. Prominent, unjointed cerci, male with genital armature on second abdominal segment. Malpighian tubes numerous (50-60). Aquatic larvae with caudal leaf-gills or with rectal tracheal system.
Includes the three families of dragon-flies. Formerly comprised among the Neuroptera.
Order: Thysanoptera.
Piercing mandibles, retracted within the head-capsule. First maxillae also modified as piercers; maxillae of both pairs with distinct palps. Both pairs of wings similar, narrow and fringed. Four Malpighian tubes. Cerci absent. Ovipositor usually present. Young resembling parents, but penultimate instar passive and enclosed in a filmy pellicle.
Includes three families of Thrips (see Thysanoptera).
Order: Hemiptera.
Mandibles and first maxillae modified as piercers; second maxillae fused to form a jointed, grooved rostrum. Wings usually present. Four Malpighian tubes. Cerci absent. Ovipositor developed.
Includes two sub-orders:—
1. Heteroptera: Rostrum not in contact with haunches of fore-legs. Fore-wings partly coriaceous. Young resembling adults.
Includes the bugs, terrestrial and aquatic.
2. Homoptera: Rostrum in contact with haunches of fore-legs. Fore-wings uniform in texture. Young often larvae. Penultimate instar passive in some cases.
Includes the cicads, aphides and scale-insects (see Hemiptera).
Order: Anoplura.
Piercing jaws modified and reduced, a tubular, protrusible sucking-trunk being developed; mouth with hooks. Wingless, parasitic forms. Cerci absent. Four Malpighian tubes. Young resembling adults.
Includes the family of the Lice (Pediculidae), often reckoned as Hemiptera (q.v.). See also Louse.
Sub-class: Endopterygota.
Hexapoda mostly with wings; the wingless forms clearly degraded or modified. Maxillulae vestigial or absent. No locomotor abdominal appendages (except in certain larvae). Young animals always unlike parents, the wing-rudiments developing beneath the larval cuticle and only appearing in a penultimate pupal instar, which takes no food and is usually passive.
Order: Neuroptera.
Biting mandibles; second maxillae completely fused. Prothorax large and free. Membranous, net-veined wings, those of the two pairs closely alike. Six or eight Malpighian tubes. Cerci absent. Larva campodeiform, usually feeding by suction (exceptionally hypermetamorphic with subsequent eruciform instars). Pupa free.
Includes the alder-flies, ant-lions and lacewing-flies. See Neuroptera.
Order: Coleoptera.
Biting mandibles; second maxillae very intimately fused. Prothorax large and free. Fore-wings modified into firm elytra, beneath which the membranous hind-wings (when present) can be folded. Cerci absent. Four or six Malpighian tubes. Larva campodeiform or eruciform. Pupa free.
Includes the beetles and the parasitic Stylopidae, often regarded as a distinct order (Strepsiptera). (See Coleoptera.)
Order: Mecaptera.
Biting mandibles; first maxillae elongate; second maxillae completely fused. Prothorax small. Two pairs of similar, membranous wings, with predominantly longitudinal neuration. Six Malpighian tubes. Larva eruciform. Pupa free. Cerci present.
Includes the single family of Panorpidae (scorpion-flies), often comprised among the Neuroptera.
Order: Trichoptera.
Mandibles present in pupa, vestigial in imago; maxillae suctorial without specialization; first maxillae with lacinia, galea and palp. Prothorax small. Two pairs of membranous, hair-covered wings, with predominantly longitudinal neuration. Larvae aquatic and eruciform. Pupa free. Six Malpighian tubes. Cerci absent.
Includes the caddis-flies. See Neuroptera, among which these insects were formerly comprised.
Order: Lepidoptera.
Mandibles absent in imago, very exceptionally present in pupa; first maxillae nearly always without laciniae and often without palps, or only with vestigial palps, their galeae elongated and grooved inwardly so as to form a sucking trunk. Prothorax small. Wings with predominantly longitudinal neuration, covered with flattened scales. Fore-wings larger than hind-wings. Cerci absent. Four (rarely 6 or 8) Malpighian tubes. Larvae eruciform, with rarely more than five pairs of abdominal prolegs. Pupa free in the lowest families, in most cases incompletely or completely obtect.
Includes the moths and butterflies. See Lepidoptera.
Order: Diptera.
Mandibles rarely present, adapted for piercing; first maxillae with palps; second maxillae forming with hypopharynx a suctorial proboscis. Prothorax small, intimately united to mesothorax. Fore-wings well developed; hind-wings reduced to stalked knobs (“halteres”). Cerci present but usually reduced. Four Malpighian tubes. Larvae eruciform without thoracic legs, or vermiform without head-capsule. Pupa incompletely obtect or free, and enclosed in the hardened cuticle of the last larval instar (puparium).
Includes the two-winged flies (see Diptera), which may be divided into two sub-orders:—
1. Orthorrhapha: Larva eruciform. Cuticle of pupa or puparium splitting longitudinally down the back, to allow escape of imago.
Comprises the midges, gnats, crane-flies, gad-flies, &c.
2. Cyclorrhapha: Larva vermiform (no head-capsule). Puparium opening by an anterior “lid.”
Comprises the hover-flies, flesh-flies, bot-flies, &c.
Order: Siphonaptera.
Mandibles fused into a piercer; first maxillae developed as piercers; palps of both pairs of maxillae present; hypopharynx wanting. Prothorax large. Wings absent or vestigial. Larva eruciform, limbless.
Includes the fleas.
Order: Hymenoptera.
Biting mandibles; second maxillae incompletely or completely fused; often forming a suctorial proboscis. Prothorax small, and united to mesothorax. First abdominal segment united to metathorax. Wings membranous, fore-wings larger than hind-wings. Ovipositor always well developed, and often modified into a sting. Numerous (20-150) Malpighian tubes (in rare cases, 6-12 only). Larva eruciform, with seven or eight pairs of abdominal prolegs, or entirely legless. Pupa free.
Includes two sub-orders:—
1. Symphyta: Abdomen not basally constricted. Larvae caterpillars with thoracic legs and abdominal prolegs.
Comprises the saw-flies.
2. Apocrita: Abdomen markedly constricted at second segment. Larvae legless grubs.
Comprises gall-flies, ichneumon-flies, ants, wasps, bees. See Hymenoptera.
Geological History
The classification just given has been drawn up with reference to existing insects, but the great majority of the extinct forms that have been discovered can be referred with some confidence to the same orders, and in many cases to recent families. The Hexapoda, being aerial, terrestrial and fresh-water animals, are but occasionally preserved in stratified rocks, and our knowledge of extinct members of the class is therefore fragmentary, while the description, as insects, of various obscure fossils, which are perhaps not even Arthropods, has not tended to the advancement of this branch of zoology. Nevertheless, much progress has been made. Several Silurian fossils have been identified as insects, including a Thysanuran from North America, but upon these considerable doubt has been cast.
