What are Arthropods

Arthropoda is the largest phylum with about nine lakh species. They may be aquatic, terrestrial or even parasitic. They have jointed appendages and a chitinous exoskeleton.

This phylum includes several large classes and contains the class Insecta which itself represents a major portion of the animal species in the world. They possess the ability to survive in every habitat.

Arthropoda Characteristics

The body is triploblastic, segmented, and bilaterally symmetrical. The exoskeleton is made of chitin. The body is divided into head, thorax, and abdomen.

Triploblastic is a biological term that refers to organisms with three primary germ layers: ectoderm, mesoderm, and endoderm. The ectoderm is the outermost layer, the mesoderm is the middle layer, and the endoderm is the innermost layer. 

They exhibit organ system level of organization. They respire through the general body surface or trachea. They have a well-developed digestive system. They have an open circulatory system. The coelomic cavity is filled with blood.

Their body has jointed appendages which help in locomotion. They contain sensory organs like hairs, antennae, simple and compound eyes, auditory organs, and statocysts.The head bears a pair of compound eyes.

The terrestrial Arthropods excrete through Malpighian tubules while the aquatic ones excrete through green glands or coaxal glands. They are unisexual and fertilization is either external or internal.

Arthrapoda Classification

Insects

Insects have jointed appendages as arthropods (arthropod means "jointed foot"), an exoskeleton (hard, external cover), segmented body, ventral nervous system, digestive system, open circulatory system, and advanced sensory receptors. The word "jointed appendages" applies to both antennae and legs.

Insects are characterized by having three pairs of jointed legs from other arthropods; an abdomen that is divided into 11 segments and lacks any legs or wings; and a body split into three sections with one pair of antennae on the head (head, thorax, and abdomen). Sometimes, insects also have one or two wing pairs.

Arachnids

All arachnids have eight legs, and unlike insects, they don't have antennae. The bodies of arachnids are divided into two sections, the cephalothorax in front and the abdomen behind.

Sometimes times small arachnids like mites and harvestmen have the two sections fused close together so you can't see the separation.

Crustacean

It has two pairs of antennae. A pair of mandibles which are used for eating food. There are two pairs of maxillae on their heads. Crustaceans are either oviparous or ovoviviparousCrustaceans have two compound eyes, often on stalks.

Myriapods

Myriapods can be easily recognized by their unique tagmosis. All Myriapods have a head and a multi-segmented trunk, with multiple legs running down either side of their body.

Centipedes are easily distinguishable from millipedes by only having one pair of legs per body segment on their trunk.