Arthropods

◊ Phylum Arthropods is the largest phylum in the animal kingdom.                                                                       

◊ They can be found in all habitats, including the Arctic ice, on land, in trees, in freshwater     

   and saltwater, and even underground.

◊ Some of themost familiar arthropods are butterflies, beetles, flies, ants, bees, scorpions

    shrimps, crabs, mites, lobsters, and barnacles.

◊ Arthropods are cold-blooded; their body temperature depends on the temperature of their  

    environment.

Features:

●One of the features separating arthropods from all other animals is the strong external     

    skeleton, called the exoskeleton or cuticle. It is composed of chitin, a nonliving material.        

● The exoskeleton protects internal muscles and blood vessels. It cannot grow along with the                      

    organism so an arthropod grows by shedding its cuticle, or moulting.

● In moulting, the inner layers of the cuticle dissolve and are recycled to make a new larger    

     cuticle. This new cuticle is initially flexible and has many folds.

●An arthropod is very vulnerable during moulting, as it lacks the protection of its cuticle.

    The crayfish moults seasonally and is also capable of regenerating lost appendages.

* The eyes may be simple and consisit of only a few photoreceptors (sensory receptors that   

    detect light energy) or may have thousands of receptors.

* Antennae are touch-sensitive and important at light levels.

* Primitive ears composed of a flexible membrane stretched across an opening in the cuticle.

* Projections or pits scattered around the exoskeleton are sensitive and chemical stimuli   

    important in prey detection.

*It has an open circulatory system, with blood bathing the tissues or sinuses. The system of               

   open space called hemocoel.

*A heart and arteries deliver blood to the body and sinuses.

*Insects have a TRACHEAL SYSTEM, consisting of open tubes connecting to the outside by   

  spiracles, which are openings in the cuticle.

*They do not have a backbone.

 Digestive system

Consisting of three basic parts

1) Foregut – from the mouth to the esophagus and then through any preliminary digestive organs.

2) Midgut - the stomach, where digestion of food is completed

3) Hindgut – from the intestine through to the anus.

 Diversity;

Divided into 4 subphyla

Trilobitomorpha – are now extinct

Chelicerata          – have no antennae, first pair of head appendages are feeding appendages.

                                 Arachnida, the best knownclass in this subphylum includes spiders, scorpions,

                                and so on.

Crustaceans       – have 2 pairs of antennae, three pairs of feeding appendages (for swimming

                                and walking).It includes economically important taxa such as lobster, crab   

                                and shrimp  and tiny crustaceans.

Uniramia           –  characterized by uniramous or unbranched appendages.

                                have one pair of antennae and a hardened head capsule.

                                Include two main divisions

                       i) Myriapods: include centipedes (have 1 pair of walking legs) and millipedes (have

                                                2 pairs of walking legs)

                       ii) Insects: have 3 pairs of legs, used for walking and grasping prey.

                                         Have 1 pair of antennae, 1 pair of compound eyes and 1, 2 or no pairs of

                                         wings.

 Feeding habits:

Insects have mouth parts designed for biting (e.g. scorpionflies, horseflies), for chewing (e.g. beetles, earwigs) and also for piercing and sucking animal or plant juices (e.g. mosquitoes, fleas).

Beneficial Effects:

•Bees and butterflies pollinate many of the world’s plants.

•Lobsters, shrimps and crabs are an important source for humans.

•Antartic krill is the main food supply for whales and penguins.

Harmful Effects:

•Fleas and mosquitoes transmit serious diseases such as malaria and sleeping sickness.

•They compete with humans for food, claiming over half of the harvest.   

By: Gowththamy,Mathura,Rajeetha.