Arthropods: Characteristics, Classes, and Vital Functions
Arthropods: Characteristics and Classification
Arthropods are characterized by their articulated legs, which influence their speed for food acquisition and predator protection. They are the most numerous species, exhibiting triploblastic acoelomate organization. A key feature is their chitinous exoskeleton, which is shed during development in a process called ecdysis.
Classes within Phylum Arthropoda
Class Insecta
Examples include grasshoppers, flies, butterflies, bees, and mosquitoes. Their bodies are divided into three parts: head, thorax, and abdomen. They possess three pairs of legs (Hexapoda) and most have two pairs of wings (tetrapods). They have two compound eyes and four ocelli. Their mouthparts vary: licking (bees), chopping (mosquitoes), sucking (butterflies), and biting (locusts). They have a complete digestive system, a tracheal respiratory system, an open circulatory system, and Malpighian tubules for excretion. Their nervous system is ganglionic. Reproduction is dioecious, with males releasing sperm into the female’s spermatheca. They are oviparous, and development can be direct (ametabolous), hemimetabolous, or holometabolous.
- Ametabolous: Direct development, e.g., moths.
- Hemimetabolous: Development with nymphs, e.g., locusts and cockroaches.
- Holometabolous: Indirect development with metamorphosis (egg → larva → pupa → adult), e.g., butterflies.
Class Crustacea
Examples include marine shrimp, lobsters, crabs, and freshwater crayfish. They have a hard exoskeleton containing chitin and calcium salts. Their body is divided into a cephalothorax and abdomen. They have five pairs of legs, two pairs of front calipers, and two pairs of antennae. They possess compound eyes. The abdomen, called the telson, is important for egg protection. They have a complete digestive system, gill respiration, an open circulatory system with hemocyanin, and green glands for excretion. Reproduction is dioecious, with development being direct in small crustaceans and indirect in others (egg → larva → adult).
Class Arachnida
Examples include spiders, scorpions, mites, and ticks. They are terrestrial and feed on insects. Their body is divided into a cephalothorax and abdomen, with four pairs of legs. They lack antennae (Acer). They have six pairs of articulated appendages: chelicerae for prey capture and venom injection, and pedipalps for sensory, nutritional, and reproductive functions. The last four pairs of appendages are for locomotion. They have spinneret glands for web production. They exhibit extracorporeal digestion. Respiration is filotracheal, and excretion is via coxal glands and Malpighian tubules. They have an open circulatory system with hemocyanin and a ganglionic nervous system. Reproduction is dioecious, with males using pedipalps to transfer sperm to the female.
Class Myriapoda
Includes Chilopoda (centipedes) and Diplopoda (millipedes). Centipedes have one pair of legs per segment, one pair of antennae, and poison claws. Millipedes have two pairs of legs per segment and are harmless, producing humus. They live in humid places. Their vital functions are similar: a body divided into head and trunk, one pair of antennae, a complete digestive system, tracheal respiration, an open circulatory system with hemolymph, and Malpighian tubules for excretion. They have a ganglionic nervous system.