Phylum Arthropoda – Animals with Jointed Legs

Why they are descendants of annelids?

Arthropods share with annelids the characteristic of having the body divided into similar segments. In Arthropoda nevertheless, the division is not metameric and organs are not repeated in the different segments. Each somite generally is provided with a pair of jointed appendages. However, this arrangement is often modified with both parts and appendages specialized for different functions in different environments. However, in all type of habitat, the jointed appendages offer an efficient way of locomotion, offence, and defence and also assist in reproduction.

Grade of Organization

Arthropods have characteristics of higher forms such as bilateral symmetry, triploblastic, coelomic cavity and organ systems and have actually reached the peak of invertebrate evolution.

Chitin skeleton

The skeleton is external, i.e., exoskeleton. It is the type of an external covering, the cuticle which is light in weight; and is formed chiefly of chitin. Chitin is non-living, non-cellular and is secreted by the underlying skin. It is made of polysaccharide. On the outer side of chitin, there is a waxy layer.

In some Arthropods and in specific parts in other Arthropods chitin is soft and flexible, in others it is hard. In general, it is for protection but it likewise functions as lever for the motion of muscles of jointed limbs. The chitin in the jaws is utilized for biting and squashing food. It also forms lens of the compound considers the copulatory organs and organs of defence and offence. It supplies surface for the attachment of muscles which help in mobility.

Ecdysis/Moulting

In the young Arthropods such as insect larvae, the chitinous exoskeleton is shed from time to time to enable the development of the larva. This process of shedding of exoskeleton is called moulting or ecdysis. Simply put the exoskeleton of chitin in the Arthropods is one of the primaries consider the success of Arthropoda as it helps them to adjust to a wide array of an environment.

Habitat

Arthropods have exploited every kind of environment on land and in water. The water species consist of both freshwater and marine. Many of these can fly, therefore visit air occasionally.

Body structure

Arthropods are variable structurally. Some are worm-like centipedes while the others are lying pests with the body divided into unique areas, the head, thorax and abdominal area. The body is covered with waterproof chitinous cuticle secreted by the epidermis.

Body cavity

The coelom is not present as the main body cavity. Rather a haemocoel has actually developed. It is lowered coelom and communicates with blood vascular system.

Digestive system

The digestion system remains in the kind of alimentary canal with two openings, the mouth and anus. It is divided into different parts each carrying out a specific function. The food consists of little plants and animals.

Excretory System

A well developed excretory system comprising of Malpighian tubules is present in arthropods. The nitrogenous wastes are excreted in the form of uric acid crystals.

Nervous system

A highly developed nervous system is present. It includes paired ganglia (simple brain) connected to a ventral nerve cord. A ganglion is present in each section. Nerves occur from these ganglia.

 

Sensory Organs

The sensory organs are usually a pair of compound eyes and antennae.

Circulatory System

The blood circulatory system in arthropods is unique. It is an open circulatory system. The blood flows in the body cavity bathing the tissues of the body. However, there is a primitive heart and the main capillary situated dorsally. Blood is colourless as it is without haemoglobin.

Respiratory system

A lot of arthropods possess a substantial tracheal system formed of air tubes called tracheae for the exchange of gases. Main tubes open to the exterior through paired openings, called spiracles. Aquatic arthropods respire through gills and book lungs.

Movements in Phylum Arthropoda

The arthropods display active and swift movements. They swim, crawl or fly depending upon the habitat they occupy. The organs of locomotion are paired appendages and, in many cases, paired wings likewise.

Reproduction in Phylum Arthropoda

The sexes are different. The testes and ovaries, produce sperm’s, and eggs respectively.

Metamorphosis

Biography of insects is identified by metamorphosis (meta = change + morphe = form). This is an abrupt change of form or structure throughout the life cycle. There are 3 morphologically unique phases in the life cycle, the egg finally becomes larva which is converted into still pupa that finally develops into an adult. In some primitive pests, the metamorphosis is insufficient. The larva looks like an adult and is called nymph or instar. It resides in the very same habitat as an adult.

Classification of Phylum Arthropoda

It is a large group including excellent range among them. A few of its crucial classes are as follows.

 

1.Class Crustacea

These arthropods are water living and have gills for respiration on the dorsal side of the cephalothorax the exoskeleton is in the form of the carapace. In the exoskeleton deposition of salts in addition to chitin makes it firmer. The appendages are modified for capturing food, strolling, swimming, respiration and reproduction. The coelom is reduced and is in the kind of hemocoel. Head has two pairs of antennal appendages, one set of mandibles (jaws) and 2 sets of maxillae. Sexes are mainly separate.

E.g. Daphnia, Cyclops, Crabs, lobsters, prawn, woodlouse, etc.

2.Class Insecta

This is the largest group not just of Arthropoda but of all the animal kingdoms and has great variety. Insects are found everywhere, and lots of reveal social behaviour.

The body in, insects has 3 distinct areas head, thorax and abdominal area. There are a set of antennae and compound eyes on the head. The head is usually vertical to the body and jaws are ventrally placed. The thorax has three sectors in which exist 3 sets of jointed legs and in lots of insects one or two sets of wings.

Abdominal area has differing variety of segments. Brain is formed of merged ganglia and double nerve cord is ventral. Sexes are separate and animals are oviparous. Metamorphosis occurs throughout development.

E.g. dragonfly mosquito, butterflies, moths, wasps, and beetles etc.

3.Class Arachnida

Body has the anterior sections that are merged to form a combined cephalothorax, with a set of appendages called chelicerae with claws, 2 sets as pedipalps and 4 pairs of legs. There are no antennae and no true jaws. The abdomen may be segmented or unsegmented with or without appendages.

Respiration is by gills or special structures called book lungs; excretion is by the Malpighian tubules. Eyes are simple, sexes are different. They are oviparous (ordinary eggs). No true metamorphosis.

E.g. scorpions, spiders, termites and ticks.

4.Class Myriapoda

The body is divided into a large number of sections each having a set of legs. A set of antennae and a set of eyes are present on the head.

E.g. centipedes

and millipedes.

 

Economic Importance

Humans and insects have actually been at war for the exact same food, same location to reside in. Insects attack man, his domestic animals and likewise his crops, triggering a number of diseases. They are not just a health hazard but also trigger financial loss to humans by destroying his crops. Some insects are also helpful to him such as the honey bee or the silkworm. Insects are therefore of fantastic importance to humanity.

1.Harmful Insects (Pests)

Many types of mosquitoes, flies, fleas, lice and bugs transmit disease triggering organisms to human and domestic animals. We know the mosquito of genus Anopheles, the female of which transfers Plasmodium that triggers malaria in man. The Tse-tse fly of African state transmits Trypanosoma, the reason for sleeping sickness and skin diseases. The typical house fly carries illness triggering organisms to infect food and cause cholera, hepatitis and so on. Some types of Trypanosoma cause diseases in cattle, also.

A number of insects lay eggs on fruits and other crops such as sugarcane, maize, cotton and also on vegetables etc. The larvae of these pest’s damage fruit and the crops resulting in financial loss to farmers. The locusts that relocate large numbers from country to country cause damage to standing crops and other plants.

2.Beneficial Insects

The helpful insects are the honey bee that provides human with honey and wax. Similarly, the silkworm provides us with silk. There are some insects that are predaceous on other harmful bugs. Some pests are scavengers and they eat up dead animal and veggie matter. Insect larvae are a source of food for fish.

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