Arthropods, Biology tutorial


Wide-ranging Characteristics of Arthropods:

Arthropods are triploblastic bilaterally proportioned coelomates with metameric segmentation. The major diagnostic trait is having chitineous and at times calcareous exoskeleton. This may be rigid, firm or stretchy. Every segment bears the pair of jointed appendages utilized for locomotion, feeding or as sensory organs. Coelom is decreased. The major body cavity is haemocoel.

The Basic Body Plan:

The arthropod body plan explained as based on segmented body plan of annelids. Earliest arthropods have several similar jointed appendages that perhaps served as gaseous exchange, food accumulation, locomotion and finding stimuli. In modern arthropods, various segments have turn out to be more specialized and greater division of labour has resulted. Appendages have turn out to be fewer and more specialized.

Members of phylum are thought to have specialized the basic structures for is known as principle of adaptive radiation. Mouth parts of insect's for instance basic mouth parts of class insecta are labrum (upper lip, a pair of mandibles, a hypopharynx (floor of mouth), pair of maxillae and labium (fused 2nd pair of mandibles lower lip). By increasing, varying, decreasing and losing some of the structures, insects adapted them for sucking, licking and biting, chewing; and piercing and sucking.

Exoskeleton or Cuticle:

Exoskeleton in arthropods is secreted by epidermis. It is composed of chitin. Chitin is the nitrogenous polysaccharide that strongly looks like cellulose, strengthening material of cell wall of plants. Chitin has high tensile strength. This signifies that it is difficult to split them by bending from both ends. By joining chitin with other chemicals, such as salts, (particularly calcium salts) and proteins exoskeleton can be prepared harder as in crustaceans. The wide range of adaptation in exoskeleton is possible. Hardness and flexibility are significant for limbs and joints. At this point in time one should understand significant benefit of the exoskeleton that we are meeting so widely in animals for first time.

Benefits of exoskeleton:

i) It provides support mainly on land

ii) Forms a basis for attachment of muscles responsible for locomotion

iii) Protects body parts from physical damage

iv) As the extra waxy layer on body, it avoids aridity on land.

v) Presence of elastic protein in exoskeleton improves insects abilities to fly and jump.

vi) It is not heavy. This is significant for those insects which fly.

v) Due to its rigidity and tensile strength, it presents flexibility at joints and between segments

vi) It can be altered to form mouth parts adapted for various kinds of feeding.

Drawbacks of exoskeleton:

There are though two main drawbacks in exoskeletons:

i) It restricts size which can be obtained by the organism. Large animals don't keep exoskeletons.

ii) It limits growth, periodic moulting (ecdysis) is needed for such animals with exoskeleton to grow. This makes arthropods susceptible to predators attack at time of moulting. They should thus look for shelter before they go into process.

Jointed Appendages:

This is diagnostic characteristics of arthropods. The word accurately represents jointed foot. Such jointed appendages are utilized for the variety of functions like locomotion, feeding and as sense organs.


In arthropods and mulluscs, coelom is occupied with another cavity that develops from vascular blood system and is filled with blood. Several major organs are batted in blood. This large volume of blood allows animals to maintain the high metabolic rate. Space filled by haemocoel is so much that there is high risk of loss of blood from injury to animal. Coelom is decreased to excretory organs and reproductive ducts.

Specialization of Body Parts:

There is a greater partition of labour in arthropods than in annelids. There is head, thorax and abdomen. Head houses sense organs antennae, eyes and brain. It is first to come in contact with world as animal moves that are why these fronts end should be so specialized. In annelids, head is not so specialized. Sense organs are not so well enhanced.

Categorization of Phylum Arthropoda:

Arthropods are categorized into 5 classes. These five classes are given below:

i) Super class Crustacean

ii) Class Insecta

iii) Class Chilopoda

iv) Class Diplopoda and

v) Class Arachinda.

Arthropods are placed in different classes stated by their

i) Habitat

ii) Structure of head (whether identified or not)

iii) Number of antennae

iv) Number of pairs of mouth parts

v) Number and kind of eyes (compound or simple)

vi) How appendage are altered for the specific function

vii) Whether or not there are larval forms in life cycle.

viii) Whether gaseous exchange occur through trachea, gills or lungs.

Several deviations in structural adaptations are probable to be affected by habitat. Like that animal adapted to life in water have gills for gaseous exchange. Likewise, aquatic arthropod would barely be expected to expand wings. The range of growth in head region of arthropods comprises fusing together head and thoracic segment to form the cephalothorac or keeping both separate. At times head is not so stated. Arthropods are either terrestrial or aquatic by habitat. They either have pair, 2 pairs or no antennae. Few have no true mouth parts where as others could have 1 or 3 pairs of mouth parts. Every arthropod has eyes.

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