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EXTERNAL MORPHOLOGY

Features of Arthropods

All arthropods possess

Important Classes

Five important extant classes of Arthropods are arachnids, chilopods, diplopods, crustaceans and insects.

Arachnids include spiders, scorpions, ticks and mites. They all have

Chilopods, which include centipedes, all have

Diplopods, which include millipedes, all possess

Crustaceans include crabs, prawns, slaters, barnacles etc. Their features are

Insects such as beetles, wasps and cockroaches all have

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The Insect Head

The insect head is a sclerotised capsule of mainly rigid sclerites. It houses the compound eyes, simple eyes (ocelli), antennae and mouthparts.

In most insects there is one pair of large, prominent compound eyes. It is composed of several units called ommatidia. There may be up to 30,000 ommatidia in a compound eye. This type of eye gives less resolution than the vertebrate eye, but it gives acute perception of movement -important in flight.

When present, ocelli (either 2 or 3), detect low light or small changes in light intensity.

Antennae

Insect antennae vary morphologically.

Antennae can detect very low levels of chemicals and are used in insect communication, finding host plants or mates.

The first antennal segment (closest to the head) is called the scape.

The second antennal segment is called the pedicel.

The remainder of the antenna is collectively called the flagellum.

Mouthparts

The 4 mouthparts are the labrum, mandibles, maxillae and labium.

The labrum is a simple fused sclerite, often called the upper lip, and moves longitudinally. It is hinged to the clypeus.

The mandibles, or jaws, are highly sclerotised paired structures that move at right angles to the body. They are used for biting, chewing and severing food.

The maxillae are paired structures that can move at right angles to the body and possess segmented palps.

The labium (often called the lower lip), is a fused structure that moves longitudinally and possesses a pair of segmented palps.

The mouthparts of the cockroach are primitive and unspecialised. Some insects have highly modified mouthparts for specialised feeding.

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The Insect Thorax

The insect thorax is box-like with dorsal, ventral and lateral sclerites. The dorsal sclerites are collectively called the notum or tergum. The ventral sclerites are called the sternum and the lateral sclerites are called the pleuron.

This construction allows attachment and contraction of muscles used in the movement of the wings and legs.

The thorax is further subdivided into 3 segments, the prothorax, mesothorax and metathorax. Each of these segments bears a pair of legs. In addition, the mesothorax may bear a pair of fore wings and the metathorax may bear a pair of hind wings.

Legs

The legs, named from the anterior, are the fore, mid and hind legs. Each leg has several segments:

The coxa articulates with the body.

The trochanter is usually quite small.

The femur is usually the longest and strongest segment.

The tibia is usually long and slender.

The tarsus is collectively composed of  2 to 5  smaller tarsomeres. The last tarsomere usually has a pair of claws and often 2 or 3 tarsal pads.

Unmodified legs are used for walking and are called ambulatory.

Legs modified for running are called cursorial.

Digging legs are called fossorial.

Swimming legs are called natatorial.

Jumping legs are called saltatorial.

Grasping legs are called raptorial.

Wings

Most adult insect possess wings. Some have shortened (brachypterous) wings while others may be wingless (apterous).

Wings have a network of veins which give rigidity and support. Air, nerves and blood also pass through the wing veins.

There are several major longitudinal veins. These are the costa, subcosta, radius, median, cubitus and anal veins.

Primitive wings have many, short cross veins. Such wings are called reticulate.

Grasshoppers and cockroaches have leathery forewings which are termed tegmina (sing. tegmen).

Many plant bugs have forewings that are thickened at the base but membranous distally. These are called hemelytra (sing. hemelytron). Most beetles have very hardened forewings called elytra (sing. elytron).

In flies, (Diptera), the hind wings have become modified to form small balance organs, called halteres.

In more advanced insects the wings are fastened together.

Hamuli are tiny hooks on the anterior margin of the hind wing. These hooks engage a vein on the posterior margin of the forewing.

A frenulum is a bristle on the hind wing of many butterflies and moths. The frenulum fits into a hook, (or retinaculum), on the forewing rather like a safety pin.

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The Insect Abdomen and External Genitalia

The insect abdomen has a tergum (never called a notum) and sternum but has no pleuron since it does not bear legs or wings.

Terminally the abdomen bears the external genitalia. In some female insects there is a very obvious ovipositor for depositing eggs.

A pair of cerci are also present at the end of the abdomen. These have a sensory function.

In some orders there may also be additional terminal appendages.

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