This is the largest phylum of Animalia which includes mainly insects.
Some of the more well-known arthropods include insects, crustaceans, spiders, scorpion and centipede as well as the fossil trilobites.
Over two-thirds of all named species on earth are arthropods (Figure 4.12).
Arthropods are mainly terrestrial but marine and freshwater species are also well known.
They range in size from microscopic plankton to life-forms that are a few meter long.
Arthropoda varies tremendously in their habitats, life histories, and dietary preferences.
Body
They have organ-system level of organisation.
They are bilaterally symmetrical, triploblastic, segmented and coelomate animals.
Arthropod bodies are divided into segments.
However, a number of segments are sometimes fused to form integrated body parts known as tagmata.
This process of fusion is called tagmosis.
The head, thorax, and abdomen are examples of tagmata.
The body consists of head, thorax and abdomen.
In insects, the anterior portion of the heart is extended into a tube that is called an aorta which directs the blood forward as it goes out into the body cavity.
The body of arthropods is covered by chitinous exoskeleton.
Exoskeleton serves as protection and provides places for muscle attachment.
Arthropods must molt because their exoskeletons don’t grow with them.
They have jointed appendages (arthros-joint, poda-appendages).
However, in most species some appendages have been modified to form other structures, such as mouthparts, antennae, or reproductive organs.
Arthropod appendages may be either biramous (branched) or uniramous (unbranched).
Respiratory organs are gills, book gills, book lungs or tracheal system.
The tracheal respiratory system consists of external openings called spiracles that are linked to a system of branched tubules which allow respiratory gases to reach internal tissues.
Circulatory system is of open type.
Arthropods primary internal cavity is known as hemocoel, which accommodates their internal organs, and through which their haemolymph - analogue of blood - circulates; they have open circulatory systems.
Arthropods have a well-developed, mesodermal, solid nerve cord, ventral and well-developed sense organs.
Arthropods are characterized by a brain as well as a nerve ring around the area of the pharynx, in the oral cavity.
A double nerve cord extends backwards along the ventral surface of the body, and each body segment is associated with its own ganglion, or mass of nerve cells.
Sensory organs like antennae, eyes (compound and simple), statocysts or balancing organs are present.
Excretion takes place through malpighian tubules.
They are mostly dioecious.
Fertilisation is usually internal.
They are mostly oviparous.
Development may be direct or indirect.
Examples: Economically important insects – Apis (Honeybee), Bombyx (Silkworm), Laccifer (Lac insect) Vectors – Anopheles, Culex and Aedes (Mosquitoes) Gregarious pest – Locusta (Locust) Living fossil – Limulus (King crab).
