General Characters of Phylum Arthropoda
Phylum Arthropoda contains more species than any other phylum. Insects like cockroaches, grasshopper, butterflies, and mosquitoes are most common arthropods. They show following characters.
- Jointed foot: They are commonly called Arthropods, Word “arthro” means “joined” and “pod” means “feet”. So they have jointed legs.
- Segmentation: Their body is segmented. Segments are attached with each other by a modified part of cuticle. This cuticle is thin and flexible. So they possess jointed appendages. These appendages are modified to perform special functions.
- Relationship with Annelida: The annelids and arthropods have many common characteristics like segmented body, appendages and cuticle. So it is believed that both annelids and arthropods have common origin.
- Body Form: Arthropods have different body structures. Some are worm like centipedes. Some are flying insects. Their body is divided into head, thorax and abdomen. Their body is covered with waterproof chitinous cuticle. It is secreted by epidermis.
- Coelom: The coelom does not form main body cavity. They have developed heamocoel. It is reduced coelom. It is communicated with blood vascular system.
- Digestive system: Their digestive system is in the form of alimentary canal. It has two openings, mouth and anus. Alimentary canal is divided into different parts. Each part performs a specific function. Their food is small plants and animals.
- Excretory system: A well-developed excretory system is present in Arthropoda. It is composed of Malapighian tubules. The nitrogenous waste of arthropods is solid uric acid.
- Nervous system: They have highly developed nervous system. It consists of paired ganglia (simple brain). These ganglia are connected to a ventral double nerve cord. A ganglion is present in each segment (segmental ganglion). Nerves arise from these ganglia. The sensory organs are a pair of eyes and a pair of antennae.
- Respiratory system: Most arthropods possess an extensive tracheal system for exchange of gases. This tracheal system is composed of air tubes called tracheae. Main tracheae open outside through openings called spiracles. Respiration in aquatic arthropods takes place through gills.
- Blood vascular system: They have a unique blood vascular system. They have open circulatory system. The blood openly flow through the body cavity and bathe the tissue of the body. However, they have a primitive heart and a blood vessel .There blood does not have hemoglobin .So it is colorless.
- Exoskeleton: The skeleton is external i.e. exoskeleton .In this case, cuticle forms outer covering. Cuticle is light in weight and it is chiefly composed of chitin. It provides surface for the attachment of muscles. These muscles are used for locomotion.
- Locomotion: They show active and swift (fast) movements. Their mode of locomotion depends on their habitat. So they can swim, crawl or fly. The organs of locomotion are paired appendages, Insects also have paired wings.
Reproduction and life History of Phylum Arthropoda
The sexes are separate. The testis and ovaries produce sperm and eggs.
The changes which take place during conversion of larva into adult are called metamorphosis. “Meta” mean “change” and “morphe” means “from”. It is the characteristic features of the life histories of the insects. There are three morphologically distinct stages in the life cycle of the Arthropods
- Egg: It develops into larva.
- Larva: It changes into motionless pupa.
- Pupa: It finally develops into adult.
Metamorphosis is incomplete in some primitive insects. The larva resembles with the adult is called nymph or instar. It lives in the same habitat as adult.
Classification of Phylum Arthropoda
Phylum Arthropoda is a large group. It consists of variety of animals. Some of its important classes are as follow:
- They are aquatic. They have gills for respiration.
- The dorsal side of the cephalothorax (head + thorax) has exoskeleton called carapace.
- There is a deposition of salts in the exoskeleton along with chitin. It makes exoskeleton more firms (strong).
- The appendages are modified for capturing of food, walking, swimming, respiration and reproduction.
- Coelom is reduced and it is in the form of heamocoel.
- Head has two pair of antennae, one pair of mandible (jaws) and two pairs of maxillae.
- Sexes are mostly separate.
Examples: Daphnia, Cyclops, Crabs, lobsters, prawn, wood louse.
- This is the largest group of all the animal kingdom. It shows a great variety of animals. Insects are found everywhere. They may show social behavior like aunts. An arthropod having two pairs of wings and three pairs of legs is called insect.
- The body of insects is divided into three distinct regions:
- Head: There is a pair of antennae and compound eyes on the head. The head is usually vertical in position to the body and jaws are ventrally placed.
- Thorax: The thorax has three segments. These three segments have three pairs of jointed legs and two pair of wings.
- Abdomen: It has different numbers of segments.
- Brain is composed of fused ganglia. There is double nerve cord in the ventral side.
- Sexes are separate and animals are oviparous
- Metamorphosis takes place during development.
Examples: Dragon fly, Mosquitoes, butterflies, moths, wasps and beetles etc.
- The anterior segments of the body are fused to form a combined cephalothorax.
- The cephalothorax has following appendages
- A pair of chelicerae with claws.
- Two pairs of pedipalps.
- . Four pairs of legs.
- . There are no antennae and no true jaws.
- Abdomen may be segmented. It may or may not have appendages.
- Respiration takes place by gills, lungs or special structures called book lungs.
- Excretion takes place by Malapighian tubules.
- Eyes are simple.
- Sexes are separate.
- They are oviparous.
- No true metamorphosis.
Examples: Scorpions, spiders, mites and ticks
- Their body ¡s divided into a large number of segments.
- Each segment has a pair of legs.
- A pair of antennae and a pair of eyes is present on head.
Examples: Centipedes and Millipedes.
General Organization of Arthropods
Arthropods show all the characteristic of the higher organisms. They have bilateral symmetry. Arthropods are triploblastic and coelomate. They have well developed organ system. So they have reached the peak (last stage) of the invertebrate evolution. There are two main achievements of the Arthropods. These are chitinous exoskeleton and locomotory mechanism.
1. Chitinous Exoskeleton
Chitin is a non-living and non-cellular compound, composed of protein and carbohydrates. It is secreted by the outer epidermis.
There is a waxy layer on the outer side of the chitin. Chitin is soft and flexible in some arthropod. It is soft and flexible in certain parts of some other Arthropods. It is hard in remaining Arthropods.
Functions of chitin: It performs following functions:
- Generally, it is used for protection.
- It also acts as lever for the movement of muscles of the jointed limbs.
- The chitin in the jaws is used for biting (breaking) and crushing of food.
- It forms the lens of the compound eyes.
- Chitin forms the copulatory (sex) organs.
- It forms the organs of offence and defense.
Moulting or Ecdysis: Chitinous exoskeleton is shed from time to time in young larva. It allows the larva to grow. This process of shedding of exoskeleton is called MouIting or ecdysis.
In short exoskeleton of chitin is one of the primary factors in the success of the Arthropoda. It has helped them to adapt in different habitats.
2. Jointed appendages
The bodies of the Arthropoda, like Annelida, are divided into similar segments. However, the segmentation in the Arthropods is not metameric.
Their organs are not repeated in the different segments. Each segment has a pair of jointed appendages. But in most case, this arrangement is modified. Their segments and appendages become specialized for performing different functions in the different habitats. Joint appendages perform following functions: .
- Joint appendages provide an efficient means of locomotion in all kinds of habitats. These animals can walk, swim or fly.
- They are used as organs of o fence and defense.
- They also help in reproduction.
Economic importance of Phylum Arthropoda
There is a war between man and insects for food and space (place of living). Insects attack on man, his domestic animals and his crops. They cause a large number of diseases in them. They destroy the properties and crops and cause economic losses to man. Some insects are also useful for man like honey bee or silk worm. So insects have great importance for mankind.
1. Harmful Insects
Harms in animals: Many types of mosquitoes, flies, fleas, lice and bugs transmit diseases causing organisms to man and domestic animals. Some of these are:
- Mosquito: Female Anopheles mosquito transmits Plasmodium. Plasmodium causes malaria in man.
- Tse-tse fly: It transmits Trypansoma in African countries. Trypansoma causes sleeping sickness and skin disease in man. Some species of Trypansoma also causes diseases in cattle.
- Common house fly: It transmits disease causing organism to food. This contaminated food causes cholera, hepatitis etc.
Harms in Plants:
- A large number of insects laid eggs on fruits and other commercial crops like sugar-cane, maize (corn), cotton and vegetables. The larvae of these animals damage fruits and crops. It causes economic losses to the farmers.
- The locust moves large number from country to country. They damage the standing crops and other plants.
2. Beneficial Insects
There are following beneficial insects:
- Honey bee provides man honey and wax.
- Silk worm gives us silk.
- Some insects eat the harmful insects.
- Some insects are scavengers. They eat dead animals and decayed vegetables.
- The larvae of insects are source of food for fish.