Arthropods: Characteristics and Classes

Arthropods

• The largest group of invertebrates, comprising 85% of all animal species.
• Characterized by a tough skin covering, known as a cuticle, which is periodically shed through a process called ecdysis.
• Possess jointed appendages, such as legs and antennae, and an exoskeleton that provides protection and support.

Classes of Arthropods

• Insecta: includes houseflies, characterized by three pairs of legs, two pairs of wings, and respiration through tracheae.
• Crustacea: includes cyclops, crabs, and lobsters.
• Arachnida: includes ticks, spiders, mites, and scorpions.
• Diplopoda: includes millipedes.
• Chilopoda: includes centipedes.

Insects

• Body divided into three parts: head, thorax, and abdomen.
• Respiration occurs through tracheae.
• Largest group of arthropods, with over 1 million described species.
• Characteristics contributing to their success:
  • Short life cycle
  • Adaptability and capacity for flight
  • Small size, requiring minimal food and shelter
  • Variation in lifestyle
  • Light, strong, water-proof skeleton
  • Large reproductive ability
  • Adaptation of appendages
  • Sensory sophistication

Subclasses of Insects

• Apterygota: wingless, with no metamorphosis.
• Pterygota: winged or secondary wingless, with metamorphosis.
• Exopterygota: wings develop externally, with gradual metamorphosis.
• Endopterygota: wings develop internally, with complete metamorphosis.

Medically Important Insects

• Order Diptera: includes mosquitoes, blackflies, and houseflies.
• Other orders of medical importance: Anoplura (lice), Odonata (dragonflies), Dictyoptera (cockroaches), Orthoptera (locusts), Coleoptera (beetles), Hymenoptera (bees, ants), and Lepidoptera (moths and butterflies).

Growth and Metamorphosis

• Growth occurs in discrete stages, with the exoskeleton being shed and replaced through a process called ecdysis.
• The process of transformation from egg to adult is called metamorphosis.
• Incomplete metamorphosis (hemimetabolism): characterized by gradual changes from egg to adult, with a series of instars (stages) between egg and adult.
• Complete metamorphosis (holometabolism): characterized by distinct stages of egg, larva, pupa, and adult, with significant changes between each stage.

Insect Head

• Fused segments, with a series of hexagonal lens forming the compound eyes.
• Simple eyes (ocelli) may be present, but are not capable of perceiving movement.
• Presence of antennae and palps (sensory organs).
• Mouthparts: upper lip (labrum), lower lip (labium), mandibles, maxillae, and hypopharynx.

Insect Thorax

• Divided into prothorax, mesothorax, and metathorax.
• Three pairs of legs, with segments including coxa, trochanter, femur, tibia, tarsus, and claws.
• Wings are outgrowths of the terga, with distinct lines of thickening called veins.
• Wing venation is important for classification.

Respiratory System

• Direct diffusion through spiracles, which are connected to internal tubes called tracheae.
• Tracheae branch repeatedly to form fine tubes called tracheoles, which end in the cells.
• No oxygen-carrying chemicals are present in insects.

Nervous System

• Three pairs of ganglia in the head are fused to form a brain, which coordinates movement and stimuli.
• Ventral nerve cord arises from the brain and reaches body parts.
• Basic sense organ is the neuron, which has a sense cell at its base and axons.
• Modifications arise for mechanoreceptors, chemoreceptors, photoreceptors, and humidity and temperature reception.

Alimentary Canal

• Great diversity due to types of food eaten.
• Mouthparts vary according to type of food eaten.
• Three main regions: foregut, midgut, and hindgut.
• Foregut includes mouth, buccal cavity, pharynx, oesophagus, crop, and proventriculus.
• Midgut is where most digestion occurs.
• Hindgut includes ileum, rectum, and anus, with malpighian tubes and rectal papillae aiding in digestion and absorption.