Temporal range: Early Devonian–Recent
|Classes & Orders|
The subphylum Hexapoda (from the Greek for six legs) constitutes the largest number of species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considered insects). The Collembola (or springtails) are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs. Most other arthropods have more than three pairs of legs.
Hexapods have bodies ranging in length from 0.5 mm to over 300 mm which are divided into an anterior head, thorax, and posterior abdomen. The head is composed of a presegmental acron that usually bears eyes (absent in Protura and Diplura), followed by six segments, all closely fused together, with the following appendages:
- Segment I. None
- Segment II. Antennae (sensory), absent in Protura
- Segment III. None
- Segment IV. Mandibles (crushing jaws)
- Segment V. Maxillae (chewing jaws)
- Segment VI. Labium (lower lip)
The mouth lies between the fourth and fifth segments and is covered by a projection from the sixth, called the labrum (upper lip). In true insects (class Insecta) the mouthparts are exposed or ectognathous, while in other groups they are enveloped or endognathous. Similar appendages are found on the heads of Myriapoda and Crustacea, although these have secondary antennae.
The thorax is composed of three segments, each of which bears a single pair of legs. As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods and with gill on the abdominal segments of some immature aquatic insects. In most insects the second and third thoracic segments also support wings. It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves.
The abdomen consists of eleven segments in all true insects (often reduced in number in many insect species), but in Protura it has twelve, and in Collembola only six (sometimes reduced to only four). The appendages on the abdomen are extremely reduced, restricted to the external genitalia and sometimes a pair of sensory cerci on the last segment.
- Blattodea (cockroaches) - they have a flattened body with modified legs which enable them to move quickly. Approximately, 4,000 species of cockroaches are present.
- Coleoptera (beetles) - they have two pairs of wings, one which thinks, and the other which has membranous chewable mouthparts. They have their front pair of wings modified to form hard wing cases called elytra. Approximately, 400,000 species of beetles are present.
- Dermaptera (earwigs) - they have a biting mouthpart with their posterior pincer being large. Approximately, 1,200 species of earwigs are present.
- Diptera (true flies) - they have a pair of sucking mouthparts and also they have a pair of wings. Approximately, 151,000 species of true flies are present.
- Embioptera (webspinners) -
- Ephemeroptera (mayflies) - they have a pair of triangular front wings, a fan-shaped abdomen with a pair of filaments, and long front legs.
- Hemiptera (true bugs) - they have two pairs of wings, one which thinks, and the other which has membranous parts, just like the beetles. In addition, they also have piecing mouth parts. Approximately, 85,000 species of true bugs are present.
- Hymenoptera (ants, bees, wasps) - they have two pairs of membranous wings. Approximately, 125,000 species of ants, bees, and wasps are present.
- Isoptera (termites) - they are social insects just like hymenoptera, however, most of the termites are wingless. Approximately, 2,000 species of termites are present.
- Lepidoptera (butterflies, moths) - they have two pairs of wings which are covered with scales and long proboscis. Approximately, 120,000 species of butterflies and moths are present.
- Mecoptera (scorpionflies)
- Megaloptera (alderflies) - their hindwings have elongated fan-folded areas.
- Neuroptera (lacewings) - they have four membranous wings which include the forewings, the hindwings, and the chewable mouthparts. The forewings and the hindwings are of the same size.
- Notoptera (rockcrawlers, gladiators)
- Odonata (dragonflies, damselflies) - they have a pair of long and membranous wings with a long and slender body.
- Orthoptera (grasshoppers, crickets) - they have a pair of large hindlegs which enable them to jump and two pairs of wings. In the two, one pair is feathery, while the other being membranous.
- Phasmatodea (walking sticks, timemas) - these species mimic plants.
- Phthiraptera (lice) - they have well-developed claws for grasping and have no wings.
- Plecoptera (stoneflies) - they have two prominent cerci.
- Psocoptera (booklice, barklice)
- Raphidioptera (snakeflies)
- Siphonaptera (fleas) - they have modified legs for jumping and have no wings. Approximately, 2,400 species of fleas are known.
- Strepsiptera (twisted-winged parasites)
- Thysanoptera (thrips) - they have sucking mouthparts and have one mandible on the left side.
- Trichoptera (caddisflies) - they have two pairs of wings. These two pairs of wings are hairy and have chewing mouthparts.
Hexapod evolution and relationships
The myriapods have traditionally been considered the closest relatives of the hexapods, based on morphological similarity. These were then considered subclasses of a subphylum called Uniramia or Atelocerata. New work, however, has called this into question, and it appears the hexapoda's closest relatives may be the crustaceans. The non-insect hexapods have variously been considered a single evolutionary line, typically treated as Class Entognatha (cladogram A), or several lines with different relationships with the Class Insecta. In particular, the Diplura may be more closely related to the Insecta than to the Collembola (spring tails) or the Protura (cladogram B). There is also some evidence suggesting that the hexapod groups may not share a common origin, and in particular that the Collembola belong elsewhere.
Molecular analysis suggests that the hexapods diverged from their sister group, the Anostraca (fairy shrimps), at around the start of the Silurian period - coinciding with the appearance of vascular plants on land.
An incomplete possible insect fossil, Strudiella devonica, has been recovered from the Devonian period. This fossil may help to fill the arthropod gap from 385 million to 325 million years ago.
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