Insecta

PARASITES ARTHROPODS

Classification

Arthropod Classification - copyright nabrown RVC

The phylum arthropoda is divided into several subphylums including Chelicerata (which includes the arachnids), Myriapoda, Hexapoda (which includes the insecta), Crustacea and Trilobitomorpha. These are then divided into many classes. The two of major veterinary importance are the insecta and arachnida classes.

Structure and Function

Insect Body

• Covered by an exoskeleton secreted by underlying epidermis

• Divided into head, body and abdomen

Insect Head

• 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

Wing

• 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

Insect Abdomen

• Segmented

• Soft

• Appendages present
  • Copulatory claspers
  • Ovipositor
  • External genitalia

Respiratory System

• Branching trachea strengthened by spial 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

• Muscular contactions of the body wall produce respiratory movements

• Shape of spiracles and stigmatic plates used for species identification

Alimentary and Excretary System

• Alimentary canal 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

• 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

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

Life cycles

Fleas

Lice

Nuisance flies

Biting Flies

Myiasis Flies

• Oestrus ovis causes infection in nasal cavity
• Hypoderma bovis in myositis
• Cause skin infections, Simuliidae and Ceratopogonidae transmit onchocerciasis, flies involved in cutaneous habronemiasis and stephanofilariasis, protozoal skin infections
• Flea bite hypersensitivity
• Culicoides hypersensitivity