Insect Structure and Function

Insect Body

• Covered by an exoskeleton
  • Provides support and protection to the living tissues
  • Acellular so is secreted by underlying epidermis
  • The outer layer is called the epicuticle which is composed of proteins and covered by a waxy layer
  • The inner layers are the endocuticle and exocuticle which are composed of protein and chitin

• Body is metameric (divided into segments)

• Divided into head, body and abdomen
  • Articular membranes link segments allowing movement

Insect Head

Alan Hadley

Alan Hadley

Blowfly Head Close Up - Martin Pot - Wikimedia Commons

• Capsule of fused plates at the anterior end of the body

• One large pair of compound eyes
  • Honeycomb like corneal facets

• Three simple ocelli
  • Dorsal to compound eyes

• One pair of antennae

Antennae

• Form varies amongst insecta
  • E.g. long and segmented, short and squat etc.

• Hairs sometimes present

• Aristae (bristles) sometimes present

Mouthparts

• Modification depending on feeding method

• Insects which suck up liquified food have an expanded sponge like labellae
  • Cannot penetrate skin
  • Palps are also present which are sensory structures

• Insects which suck blood have long slender mouthparts for piercing skin
  • Hypopharynx
  • Mandibles
  • Labrum
  • Maxillae

• Larval mouthparts are prominent
  • One pair of hooks
  • Cephalo-pharyngeal skeleton
  • Mouthparts help identify larvae

Insect Thorax

• Divided into three segments
  • Prothorax, mesothorax and metathorax

• Each segment has one pair of legs attached

• One or two pairs of wings may be present on the mesothorax and metathorax

Leg

• Leg is attached to the body by coxa

• Trochanter

• Femur

• Tibia

• Tarsus, which is composed of several segments

• Claw

• Usually six-segmented

Wing

Alan Hadley

• Insects usually posess two pairs of wings

• Diptera have a reduced second pair of wings called halteres for balance

• Membranous outgrowth of the integument

• Strengthened by a network of veins comprising breathing tubes (trachea) and blood vessels

• The wing venation can be used for identification
  • Longitudinal veins
  • Cross veins
  • Open cells
  • Closed cells

Insect Abdomen

• Segmented

• Soft

• Appendages present
  • Copulatory claspers
  • Ovipositor
  • External genitalia

Respiratory System

• Branching trachea strengthened by spiral thickenings in the walls

• Trachea communicate with outside via spiracles
  • Spiracles on side of body
  • Chitinous openings
  • Muscular control so can open and close at will
  • Mounted on stigmatic plates
  • Lead to trachea

• Muscular contactions of the body wall produce respiratory movements

• Shape of spiracles and stigmatic plates used for species identification

Alimentary and Excretary System

• The precise shape of the gut varies between arthropods

• The Alimentary canal is divided into fore, mid and hind gut

• Foregut:
  • Oesophagus
  • Crop for temporary food storage
  • Proventriculus
  • Gizzard present in insects which eat solid food
    • Muscular wall and teeth on inner surface

• Midgut:
  • Stores food
  • Secretes enzymes for digestion
  • Outlet for malpighian tubules (equivalent of the mammalian kidney)

• Hindgut:
  • Water resorption

Circulatory System

• Heart situated dorsally
  • Valves divide heart into compartments
  • Valves only let blood flow forwards

• Aorta

• Branching blood vessels

• Haemocoele
  • General body cavity
  • Equivalent to the capillary circulation in mammals
  • Contains haemolymph

• Ostia (openings) in the blood vessel walls allow return of blood to the heart

Nervous System

• Small brain above the oesophagus

• Chain of fused ganglia running along the floor of the abdomen and thorax
  • Nerves are given off from chain

• Complex nervous system
  • Well developed visual senses

Fat Body

• Large structure

• Cells containing fat vacuoles

• Lines the body cavity and internal organs

• Equivalent to the visceral and parietal peritoneum in mammals

• Food reservoir during hibernation or starvation periods

Reproductive System

• Most insects have seperate sexes

• Reproductive organs are analogous to mammals

• Spermatheca present in females
  • Accessory female sex organ
  • Recepticle for spermatozoa
  • Sperm remains viable for most of the female's life cycle