Insect Anatomy and Morphology

• Entomology is the scientific study of insects and a branch of zoology.
• It encompasses all facets of insects, including their physiology, anatomy, behavior, ecology, and evolution.
• Insect morphology concerns both the internal and external structures of insects.
• It is fundamental to classifying, identifying, and understanding insect functions.

The Insect Body Plan

• Arthropoda is the phylum that includes insects, defined by an exoskeleton, segmented body, and jointed appendages.
• The insect body consists of the head, thorax, and abdomen.

Head

• Sensory perception and food acquisition are functions of the insect head.
• The insect head is made up of several fused segments to form a head capsule.
• Key features of the head:
  • Compound eyes offer a wide field of view and motion detection.
  • Ocelli are simple eyes that detect the intensity of light.
  • Antennae are sensory appendages that detect odors, vibrations, and other stimuli.
  • Mouthparts are extremely diverse and adapted to different feeding strategies like chewing, piercing, and sucking.

Thorax

• Bearing the wings and legs, the thorax is the center for insect locomotion.
• It consists of the metathorax, mesothorax, and prothorax.
• Each segment of the thorax has a pair of legs.
• The insect leg is made up of the coxa, trochanter, femur, tibia, and tarsus.
• Claws for grasping are located at the tip of the tarsus, which is further split into tarsomeres.
• When present, wings are attached to the mesothorax and metathorax.
• Veins support insect wings, which are exoskeleton outgrowths.
• Taxonomic identification relies on wing venation patterns.

Abdomen

• The digestive, excretory, and reproductive systems are found in the abdomen.
• Although some segments may be fused or reduced, it usually has 11 segments.
• The abdomen's external structures:
  • Spiracles are openings used for gas exchange within the respiratory system.
  • Cerci are sensory appendages located on the posterior end.
  • Genital structures are modified for oviposition and mating.

Exoskeleton

• Providing support and protection, the cuticle or insect exoskeleton is a rigid external covering.
• It is made of proteins and chitin, which is a polysaccharide.
• The epidermis, a single layer of cells beneath the cuticle, secretes the exoskeleton.
• The exoskeleton consists of:
  • Epicuticle: A thin, waxy outer layer to prevent water loss.
  • Procuticle: The exoskeleton's main body, consisting of:
    • Exocuticle: The pigmented, hardened outer layer.
    • Endocuticle: The flexible inner layer.
• Insects must shed their exoskeleton by molting or ecdysis in order to grow.

Muscular System

• Like vertebrate skeletal muscles, insect muscles are striated.
• Muscles attached to the exoskeleton enable movement.
• Insect muscles include:
  • Skeletal muscles control the movement of wings, legs, and other appendages.
  • Visceral muscles control the internal organs.

Digestive System

• Extending from the anus to the mouth, the insect digestive system is a complete gut.
• It is divided into:
  • Foregut (Stomodeum): Includes the mouth, pharynx, esophagus, crop (for food storage), and proventriculus (for grinding food).
  • Midgut (Mesenteron): The primary site of digestion and absorption, with digestive enzymes secreted from the midgut epithelium.
  • Hindgut (Proctodeum): Involved in water absorption and waste elimination, consisting of the ileum, colon, and rectum.
• Malpighian tubules are excretory organs that remove waste products from the hemolymph and deposits them into the hindgut.

Circulatory System

• Open circulatory systems are present in insects.
• The insect blood, known as hemolymph, circulates freely within the body cavity or hemocoel.
• The dorsal vessel that pumps hemolymph toward the head is the heart.
• Hemolymph functions in nutrient transport, waste removal, and immune defense.
• Hemolymph is not used for oxygen transport.

Respiratory System

• Gas exchange in insects occurs via a tracheal system.
• The body has a network of branching tubes called tracheae.
• The outside is connected to the tracheae via spiracles.
• Oxygen directly diffuses to the tissues, and carbon dioxide diffuses out.
• Ventilation may be aided by air sacs, which are dilated sections of the trachea.

Nervous System

• The insect nervous system consists of a brain, a ventral nerve cord, and ganglia.
• The brain is located in the head and is composed of three main regions:
  • Protocerebrum: Processes visual information and controls higher-order functions.
  • Deutocerebrum: Processes antennal information.
  • Tritocerebrum: Integrates sensory information and connects to the ventral nerve cord.
• Each segment has a ganglion along the ventral side of the body through which the ventral nerve cord runs.
• Local functions are managed by ganglia, which are clusters of nerve cell bodies.

Reproductive System

• Insects have distinct male and female reproductive organs.
• The male reproductive system consists of:
  • Testes for sperm production.
  • Vas deferens to transport sperm.
  • Seminal vesicle to store sperm.
  • Accessory glands to produce seminal fluid.
  • Aedeagus as the intromittent organ.
• The female reproductive system consists of:
  • Ovaries to produce eggs.
  • Oviducts to transport eggs.
  • Spermatheca to store sperm.
  • Accessory glands to produce eggshell components.
  • Ovipositor for laying eggs.
• Asexual (parthenogenesis) and sexual reproduction are both possible in insects.
• Insect development can be:
  • Hemimetabolous (incomplete metamorphosis): The insect gradually changes through several nymphal stages.
  • Holometabolous (complete metamorphosis): The insect undergoes a dramatic transformation from larva to pupa to adult.