Phylum Arthropoda (Jointed Appendages Animals)

MAIN CHARACTERS

• Arthropoda is the largest Phylum of the animal kingdom including 10, 00000 species of different types of animals.
• The word Arthropods is derived from Greek Arthos – Jointed and Podos – Foot.

HABIT AND HABITAT
Arthropodes have undergone an adaptive radiation for aerial, aquatic, terrestrial and parasitic environment. They are widely distributed in each and every place of the world.
NATURE
Arthropoda are “bilaterally symmetrical,” metamerically segmented metazoa.
EXTERNAL FEATURES

• Their body is covered by an exo-skeleton of “chitin” and protein.
• They possess paired jointed appendages.
• Their metamers are not alike but are specialized and their number is generally fixed.
• The head is well developed.

INTERNAL FEATURES

• Musculature is not continues but comprises separates striped muscles.
• The coelomic space in Arthropods is occupied by the blood vascular system and is thus called “Haemocoel.”
• Digestive tract is complete; mouth and anus lie at the opposite end of the body.
• Circulatory system is open with dorsal heart and arteries but without capillaries.
• Respiration through general body surface, by gills in aquatic forms, trachea or book lungs in terrestrial forms.
• Excretion by “Malpighian tubules” or Coelomoducts.
• Sexes are generally separate and sexual dimorphism is often exhibited by several forms.
• Fertilization is internal.
• Development is usually indirect through the larval stage.
• Nervous system of arthropods is quite similar to that of annelids and consists of dorsal anterior brain and a double ventral nerve cord.

CLASSIFICATION OF ARTHROPODA
Phylum Arthropoda is divided into following five classes:
1. CLASS MEROSTOMATA

• Almost all members of the class Merostomata are extinct. The only living merostomes, the king Crabs have survived.
• The animals are horse-shoe shaped.
• The long spike like tail that extends, posteriorly is used in locomotion. It is called “Telson.”
• They feed on mollusks, worms and other invertebrates that they find on the ocean floor.
• King Crabs a hors-shoe crabs have a tough “Carapace” jointed to a smaller abdomen.
• E.g:Limulus Polyphemus (King Crab).

2. CLASS ARACHNIDA

• This class includes spiders, scorpions, mites, ticks and many other terrestrial arthropods.
• The Arachnid body consists of a cephalothorax and abdomen.
• Cephalothorax is comprised of fused head and thorax.
• Arachnids have six pairs of jointed appendages.
• Most Archnids are carnivorous and prey upon insects and other small arthropods.
• Respiration in archnids takes place either by trachea or book lungs or by both.
• They are mainly terrestrial arthropods.
• They have no antenna.
• Cephalothorax is non-segmented.

E.g: Scorpions, Ticks & Mites, Spiders
3. CLASS CRUSTACEA

• They live both in marine and fresh waters.
• A few are terrestrial.
• Crustaceans are unique among arthropods in possessing two pairs of antenna.
• They always have one pair of mandibles and two pairs of maxillae around the mouth.
• Mandibles are usually adapted for biting and chewing. Maxillae are used for holding the food.
• Their body is divided into three distinct parts, i.e. the head, thorax and abdomen.
• Respiration usually takes place through gills associated with appendages.
• The sexes are usually separate and the reproduction is sexual.
• The thoracic and abdominal appendages may be variously modified for walking, swimming, feeding, respiration or as accessory reproductive structures.

E.g: Sacculina (Parasitic Crustacean), Astacus (Cray-fish), Prawns, Shrimps, Lobsters and Crabs etc.
4. CLASS MYRIAPODA

• All the animals are terrestrial.
• Their body is divided into a head and an elongated trunk with many segments.
• Each segment bears one or two pairs of legs.
• They are carnivorous /herbivorous.
• Eyes may present or absent.

E.g: Millipedes and Centipedes etc.
5. CLASS INSECTA (HEXAPODA)

• Insecta is the largest class of the animal kingdom.

HABIT AND HABITAT

• In their adaptive radiation, approximately a 8,50,000 species of insecta have occupied all types of terrestrial habitat.
• Some live in fresh water, however one small group is marine.

NATURE AND ADAPTATIONS

• The great success of insects can be attributed partly to the development of flight in them.
• Flight has provided them the great capacity of dispersal, access to food sources, and favourable habitat and escape from enemies.
• Corresponding to their number of species, there exists a huge variation in their structural and biological adaptations.

EXTERNAL FEATURES

• All insects have their body divided into three well-defined regions i.e. the head, thorax and abdomen.
• There is always a pair of antenna on the head.
• The thorax always consists of three segments:

(a) Prothorax
(b) Mesothorax
(c) Metathorax

• Each thoracic segment bears a pair of legs.
• Head consists of six fused segments and a pair of compound eyes and mouth parts.
• Abdomen comprises 7-11 segments and devoid of appendages.

MOUTH PARTS
The feeding appendages consists of three pairs:
(a) Mandibles
(b) First Pair of Maxilla
(c) Second Pair of Maxilla

• The second pair of maxillae have fused together to form the “LABIUM,” or lower lip
• The upper lip is formed by the projections head and is called the “LABRUM.’

Types:- The mouth appendages have been greatly modified to form five basic types of pattern:
(i) Biting
(ii) Chewing
(iii) Piercing
(iv) Sucking
(v) Siphoning or Sponging
INTERNAL FEATURES

• Heart is elongated, tubular and divided into chambers situated in the abdomen.
• Excretion takes place through “Malpighian tubules.”
• Liver is absent but salivary glands are usually present.
• Respiration is by “TRACHEA”. External gills may be present as accessory respiratory organs in some aquatic insects.

REPRODUCTION
Reproduction is sexual in most insects. However it takes place parthenogenetically i.e. eggs developing without being fertilized by sperms in a number of insects e.g: Aphids and Termites etc.
METAMORPHOSIS

• The development of insects after hatching from egg into adult stage involves considerable growth and in some cases drastic morphological changes.
• The entire post-hatching development is termed as “Metamorphosis.”

(A) INCOMPLETE METAMORPHOSIS

• In some insects the immature form that hatch from the egg are essentially similar in shape to their adults, but are smaller in size, lack wings and reproductive organs
• They attain adult characters after some growth period. This type of metamorphosis is called “Incomplete Metamorphosis.”
• Three stages are Egg → Nymph → Adult.

For example Cockroach, Grasshopper, Bugs etc.
(B) COMPLETE METAMORPHOSIS

• In this type the animal shows following stages during its complete development: Egg → Larva → Pupa → Adult.

For example Mosquito, Butter fly, House fly etc.
ECONOMIC IMPORTANCE OF INSECTS
Insects are of very great importance to man.
BENEFICIAL INSECTS
1. Apis, the honey bees produce honey and also give wax.
2. Insects bring about the cross-pollination.
3. Bombyx and Eupterote are silk-moths and produce silk.
4. The larvae of Lucilla and Pharmia are used in wound healing of bones.
5. Some insects feed upon and destroy harmful insects.
6. Some insects are Scavengers
HARMFUL INSECTS
1. Many types of mosquitoes, flies, fleas, lice and bugs transmit diseases to man and animals.
2. Human food is spoiled by cockroaches, ants and flies.
3. Tinea and Teniola are cloth-moths and destroy cloths.
4. Tenebrio is mealworm. They eat meal, flour and grains.
5. Lepisma destroy the books.
6. Termites destroy books and wood.
7. Many insects injurious to crops e.g. Tree hoppers, Leaf hoppers, Aphids, White flies and bugs.