Phylum Arthropoda is the largest and most diverse phylum in the Animal Kingdom, comprising more than two-thirds of all known animal species. The members of this phylum are characterised by the presence of jointed appendages, a segmented body, and a hard chitinous exoskeleton that provides protection and support. This phylum encompasses a diverse range of organisms, including insects, spiders, scorpions, crabs, prawns, and many other invertebrates, making it the most successful group among invertebrates.
General Characteristics of Phylum Arthropoda
- Arthropods are multicellular, triploblastic, bilaterally symmetrical, and coelomate animals.
- They possess an organ-system level of organisation, which means different organs work together to perform specific physiological functions.
- The body is metamerically segmented and usually divided into distinct regions such as head, thorax, and abdomen or cephalothorax and abdomen.
- The body is covered externally by a thick, tough, non-living chitinous exoskeleton that provides protection, support, and prevents water loss.
- The exoskeleton is periodically shed and replaced through a process known as moulting or ecdysis to allow growth.
- Jointed appendages are present and serve various functions such as locomotion, feeding, defence, sensation, and reproduction.
- The true coelom is greatly reduced and replaced by a blood-filled cavity called a haemocoel.
- The digestive system is complete with a mouth and anus.
- The circulatory system is of the open type, in which blood flows through the haemocoel rather than remaining confined within blood vessels.
- The blood is known as haemolymph and generally lacks respiratory pigments.
- Respiration occurs through gills, tracheae, book lungs, or book gills, depending upon the habitat and type of organism.
- Excretion takes place through Malpighian tubules, green glands, coxal glands, or antennal glands.
- The nervous system consists of a brain, circumpharyngeal connectives, and a double ventral nerve cord with ganglia.
- Sensory organs are highly developed and include compound eyes, simple eyes, antennae, statocysts, and sensory hairs.
- Sexes are generally separate, and sexual dimorphism is commonly observed.
- Fertilisation is usually internal.
- Most arthropods are oviparous and lay eggs.
- Development may be direct or indirect and often involves larval stages and metamorphosis.
Body Organisation of Arthropods
The body of arthropods is segmented and specialised into functional regions called tagmata.
- Head: The head bears sensory organs such as eyes and antennae, as well as mouthparts used for feeding.
- Thorax: The thorax is primarily responsible for locomotion and usually bears legs and wings in insects.
- Abdomen: The abdomen contains digestive, excretory, reproductive, and respiratory organs.
Exoskeleton of Arthropoda
One of the most characteristic features of arthropods is the presence of a chitinous exoskeleton. The exoskeleton is composed primarily of chitin, a tough polysaccharide, along with proteins and sometimes calcium salts. It acts as an external skeleton providing support, protection, and attachment sites for muscles. Functions of the exoskeleton include:
- Protection against predators and injuries.
- Prevention of excessive water loss.
- Maintenance of body shape.
- Support for muscle attachment.
- Facilitation of movement.
Digestive System of Arthropoda
The digestive tract is adapted to suit various feeding habits. Arthropods may be herbivorous, carnivorous, omnivorous, parasitic, scavenging, or detritivorous. Digestion is mainly extracellular, and nutrients are absorbed through the intestinal walls. Arthropods possess a complete digestive system consisting of:
- Mouth
- Foregut (Stomodaeum)
- Midgut (Mesenteron)
- Hindgut (Proctodaeum)
- Anus
Circulatory System of Arthropoda
Arthropods possess an open circulatory system.
- Heart: A dorsal tubular heart pumps blood through arteries into body spaces.
- Haemolymph: The circulatory fluid is called haemolymph.
- Haemocoel: Blood flows into large spaces known as sinuses or haemocoel rather than remaining confined within blood vessels.
Respiratory System of Arthropoda
Different groups of arthropods possess different respiratory organs.
- Gills: Aquatic crustaceans such as crabs, lobsters, and prawns respire through gills.
- Tracheal System: Insects possess an extensive network of air-filled tubes called tracheae. Air enters through external openings called spiracles and reaches body tissues directly.
- Book Lungs: Spiders and scorpions possess book lungs consisting of numerous thin lamellae arranged like pages of a book.
- Book Gills: Horseshoe crabs possess book gills for respiration in aquatic environments.
Excretory System of Arthropoda
Excretion varies among different arthropod groups.
- Malpighian Tubules: Found in insects, centipedes, millipedes, and many arachnids. These tubules absorb nitrogenous wastes from the haemolymph and discharge them into the intestine.
- Green Glands: Present in crustaceans such as prawns and crabs.
- Coxal Glands: Present in some arachnids.
Nervous System of Arthropoda
The nervous system is highly developed.
- Brain: The brain consists of fused ganglia located dorsally in the head region.
- Circumpharyngeal Connectives: These connect the brain with the ventral nerve cord.
- Ventral Nerve Cord: A double ventral nerve cord with segmental ganglia extends throughout the body.
Sense Organs of Arthropoda
Arthropods possess highly developed sensory structures.
- Compound Eyes: Compound eyes consist of numerous visual units called ommatidia and provide mosaic vision.
- Simple Eyes: Many arthropods possess simple eyes known as ocelli.
- Antennae: Antennae function as organs of touch, smell, and balance.
- Sensory Hairs: These detect vibrations, pressure, and chemical stimuli.
- Statocysts: Statocysts help maintain body equilibrium.
Economic Importance of Arthropods
Beneficial Arthropods
- Honey bees produce honey and wax.
- Silkworms produce silk.
- Crustaceans serve as food.
- Insects help in pollination.
- Many arthropods aid in biological pest control.
Harmful Arthropods
- Mosquitoes transmit diseases such as malaria and dengue.
- Houseflies spread pathogens.
- Locusts destroy crops.
- Ticks and mites act as parasites.