9.3.7 Phylum Arthropoda
(Gk. Arthron; jointed, pous; foot). Joint-footed animals. (The name was given by Ernst Von
Habit and Habitat:-
May be free-living and parasitic, and found almost in all habitats. Number: The biggest phylum about 75% of the animal kingdom.
Body:
(i) Segmented b (which are attached to one another by thin, flexible cuticle).
(ii) Possess several pairs of jointed limbs. (appendages),
(iii) The body is usually divided into the head, thorax, and abdomen.
(iv) The body is covered with a chitinous cuticle. Coelom: Reduced hemocoel (connected with open blood vascular system).
Digestive system:
Well developed with mouth and anus or cloaca. Nervous system: Consists of paired ganglia which is the simple brain and a central ganglionated nerve cord. Sensory receptors especially eyes are present and also cuticular hairs. The mechanoreceptors e.g. antenna.
Excretory system:
Uric acid and guanine are their nitrogenous waste, removed through unique excretory organs called Malpighian tubules or green glands.
Respiration:
Through the trachea or gills. which is the simple brain and a central ganglionated nerve cord. Sensory receptors especially eyes are present as also cuticular hairs The mechanoreceptors e.g. antenna.
Excretory system:
Uric acid and guanine are their nitrogenous waste, removed through unique excretory organs called Malpighian tubules or green glands. Respiration: Through trachea or gills. racers.
The blood circulatory system is of open type, blood is colorless without hemoglobin but hemocyanin (copper-containing protein) is present. The chitinous exoskeleton is present for the attachment of muscles.
Reproduces:
sexually and all are unisexual. Fertilization is internal and external. Development; Mostly indirect, accompanied by metamorphosis, which is of two types: a) Incomplete (larval stage) is present which is similar to adult and known as nymph e.g. cockroach, b) Complete metamorphosis through egg, larva, pupa, and adult e.g. butterfly.
Ecdysis or molting:
The periodic shedding of the exoskeleton occurs during metamorphosis. Why Arthropods are successful among Animals? They possess jointed feet and, a segmented body, which provides them great mobility. The chitinous exoskeleton protects the body and makes it light. Internal fertilization and development within egg case (cocoon). Appendages perform various functions such as quick movement, defense, and offense and help them to live in a variety of habitats.
Origin of Arthropods:
It seems that both annelids and arthropods have some common
ancestors because both groups possess segmented bodies, the presence of cuticles, and appendages but arthropods evolved several new advanced characters which made them the most successful phylum in the animal kingdom.
Evolutionary Adaptation in Arthropods:
There is great variation in the respiratory system of arthropods. For, marine arthropods respire through gills (prawn), terrestrial form has tracheae (cockroaches), and book lungs (spiders) their blood circulation system is an open type, i.e., no capillaries but possess dorsal contractile heart, arteries, and blood sinuses which are formed by reduced hemocoel.
A well-developed complete digestive system with mouth and anus (cloaca). Mouthparts have modified forms of appendages that are adapted for different methods of feeding.
Excretory organs are paired with excretory glands known as coxal, antennae, or maxillary glands. In many forms, Malpighian tubules act as excretory organs. The central nervous system has a dorsal brain connected by the ring around the esophagus with a double ganglionated ventral nerve cord. Compound eyes and some other receptors (ocelli) are also recorded.
9.3.8 Economic Importance of Arthropods
They are both useful as well as harmful to mankind.
Crustaceans:
Provide food directly and indirectly to humans (such as prawn, crab, lobster, etc. Some are harmful because act as intermediate hosts for human parasites (larvae of nematodes) carried by Cyclops.
Beneficial insects:
Help in pollination (such as ants, butterflies, and bees). Used as food in some parts of the world (such as grasshoppers, and cricket). Scavengers; Eat dead and decaying plants and animals. Commercial substances such as honey, bee's wax, silk, and shellac are produced by the honey bees, silkworms, and lac insects respectively. Destroy useless weeds by feeding upon them. Eat other harmful insects such as dragonflies feed on mosquitoes.
Scientific use:
Several insects are being used for scientific studies, such as cockroaches, fruit flies, grasshoppers, etc.
Harmful insects:
Destroy stored foods and grains. (ants and weevils). Carrier of many parasitic diseases (mosquito; malaria), (Tse tse fly; sleeping sickness). Damage crops, fruit trees, and timber trees: For example grasshoppers, bugs, locusts, beetles, caterpillars, weevils, aphids, etc.
Damage books (silverfish), and household articles (such as white ants destroy furniture). They irritate humans in various ways, such as bees sting, and cause many eye diseases. There are certain blood-sucking insects e.g., Louse.
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