General Characteristics of Arthropoda Class Insecta:  

Insects are mostly terrestrial. They have well-stated head, thorax and abdomen. They have the pair of antennae and generally 3 pairs of mouth parts. They also have the pair of compound eyes and pair of simple eyes. There are 3 pairs of thoracic legs, 1 pair each in the segment. There are 1 or 2 pairs of wings; 1 on 2nd, when they are 2, another on 3rd thoracic segment. Life cycle comprises complete or partial metamorphosis. There is generally larval stage. There is generally no gill in adult in which gaseous exchange is through tracheae. Typical and common cases of insects are bees (Apis), cockroaches (Periplaneta), white butterfly (Pierus). There are numerous others more such as beetles, bugs, wasps, flies, fleas, dragonflies, termites and grasshoppers.

Structure of the Insect:

Typically body of insect is split into 3 parts, head, thorax and the abdomen.

1) The Head:

Head is composed of 6 segments fused together. This head has the parts:

i) A pair of antennae

ii) Mouth parts: labium (lip-jaw)

iii) 2 sets of paired appendages

iv) A pair of lateral compound eyes

v) One or more simple eyes.

2) Thorax:

Thorax is not fused to head as we saw in crustaceans. It is generally related by the narrow neck. It consists of 3 segments each with the pair of walking legs. Wings (when present) are not altered appendages but are novel outgrowths from dorsal side of last two thoracic segments.

3) Abdomen:

Generally there are 11 segments in abdomen. It generally has no locomotive appendages in adult form. Fundamental segmented nature is stated even in nervous system.

Internal Cavity:

As in other arthropods, internal cavity is haemocoel. Heart is ostiate (lying in dorsal portion of abdomen. digestive tube comprise of:

a) The chitin-lined fore gut. This is composed of mouth to which salivary gland discharge. There is also muscular pharynx, oesophagus with dialated crop (serving as the food reservoir and small gizzard.

b) Mid gut that is lined by the folded epithelium. It is generally further elongated by several tubular projections known as pyloric caecae. Cells of mid gut do all task of digesting and absorbing food substances.

c) The chitinlined hind gut is provided with tubes ending blindly at anterior end. These are malphighian tubules. They situated in haemocoel. They are excretory organs. There are rectal glands at the end that reabsorb water from faeces prior to they are passed out.

Water Regulation in Arthropoda:

For these land animals, general needs of cells and tissues for water, salts and oxygen should also be met and kept in the constant state. This is due to health and activity of cells relies on stability of internal environment. In terrestrial animals, it is generally not simple to get water in them. It is in its place simpler to lose water if unchecked. To keep the internal state of organism, constant water loss should be continuously checked. The methods in which insects regulate water are:

i) The waxy outer surface prevents evaporation.

ii) Malphighian tubules, lying in blood-filled haemocoel, gather unnecessary chemicals and send them to hind gut for elimination. It at same time reabsorbs surplus water.

iii) Extra water is reabsorbed by rectal gland, in order that only solid matter is excreted. Insects have developed such extensive excretory tubules to cope with problem of water conservation.

Tracheal System in Arthropoda:

This is tubular system in animals that performs function of oxygen supply and carbon dioxide removal. In insects heart and blood are not engaged in tissue respiration. Heart serves mostly to maintain blood in motion. Blood deals out nutrients absorbed from digestive tube and permits effective working of regulatory malpigian tubules.

The trachea is the system of tubes that penetrate body from outside forming the network, with each part conversing with every other part.

Openings to exterior known as spiracle or stigmata, is generally composed of 2 pairs on thorax and eight pairs on abdomen. They lead to main tracheal branches. These converse sideways with one another and also subdivide to form 5 capillary tubes leading to neighboring organs and tissues. The capillary tubes are known as tracheoles and comprise of intracellular tubes of special branching cells situated at ends of tracheal tubes. Intercellular tubes lie near surface or on surface of cells and tissues to bring in oxygen and give out carbon dioxide. Trachea are line with chitin to avoid their collapse. Circulation of air is brought about by muscular respiratory movements of body particularly abdomen and equivalent opening and closing in tracheal system. Respiratory movements in insects are managed by respiratory centers in nerve ganglia inside each body segment. But this kind of respiration is restricted by fact that oxygen reaches tissues by direct diffusion.

Wings of Arthropoda:

Insects are usually winged with exception of very primitive ones. Though like lice, and fleas have secondarily vanished their wings. Simply adult forms have wings. Wings are the novel creation in insects, not like wings of birds, where there are changed limbs. They are generally 2 pairs but at times one may be transformed into the body cover or the organ of equilibrium. Wings are thin folds of skin of upper sides of thorax. They are compressed horizontally to permit limbs to carry on functioning in the normal ways. These flat folds are enclosed by usual chitinous layer of body surface. Wings comprise of two of layers fused back to back apart from where tracheal tubes, blood vessels and nerves separate two membranes. Wing veins are thick walled tracheal tubes that function as the strengthening framework for wings.

Chitin - a Limiting Agent:

The chitinous covering of insect body has the great affect on development. Once it has hardened, body can't expand. This firm chitinous coat should be cast off occasionally to permit growth increase. After such casting off and development increase, another chitinous cover should be formed. This procedure is known as moulting. It should take place every time body volume is bigger. It comprises outer cover of body and lining of fore-gut and hind gut, and tubular lining of trachea system. Only trachioles are not engaged in moulting when moulting occur, insect will have to wait till new wings are created before it can continue its usual ways of life. This is hazardous circumstances for the insect. Body surface is at this time insecure from aridity, not capable to fly away from enemies and to look for food. This type of circumstances is evaded by delaying formation of wings until development has finished. This is possibly why merely adult forms bear wings and only young wingless ones grow and moult. Procedure by which the change in form happens in the life cycle is known as metamorphosis. In more ancient forms, larva looks likes adult forms during growth. Every successive larval form is known as nymph or instar. This kind of growth is known as partial metamorphosis. Metamorphosis is finish when final larval moult makes adult tissues, using components from disintegrating larval tissues.

Insect Societies:

Termites, ants, bees and wasps are insects which have grown characteristic societies. Insect egg is not just yolky and surrounded in protecting case, but should be fertilized inside body of the female by sperm before received in copulation with the male. Once she has received sperm, female grows to be independent reproductive mechanism. She might choose to perform in one of different ways: lay them in numerous ways and places and abandon them. She might lay them and wait until eggs hatch and feed larvae until they fend for themselves. When female and young stay together for a time as the interacting family group, the insect society has been set up. In termite families female sheds off the wings and specializes in egg production.

Some facts are about the formation of insect societies are given as follows:

i) Males are haploid individuals that grow from unfertilized eggs.

ii) Body of egg laying female (the queen) is able to allow or to forbid passage of sperm (from store of spermatozoa acquired earlier) to eggs in process of being laid. This is extremely developed way of sex control.

iii) All other members of society - queen, workers and soldiers are genetically haploid but it is only queen which becomes sexually mature, others are barren.

iv) Nutritional control of growth, involving the offering and maintenance of royal jelly and its contained growth hormone is major factor connecting to production of queens and workers in bees.

Structural Specialization in Arthropoda:

There are several more insects than all other structures of life put together. Their number is the reflection of the great variety. Alteration of mouth parts is one of the significant characteristics of adaptation that make them very successful group. In primitive insect like cockroach that is said to have survived up to 300 million years with small or no change there are labrum (pair of mandibles, upper lip, a hypopharynx joined with salivary glands, pair of maxillae, and labium, or lower lip resulting from fusion of the second pair of maxillae. In cockroach labium and maxillae are utilized to manipulate food that is mixed with saliva by hypopharynx and chewed with mandibles.

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