Summary
Highlights
Veterinary entomology and ecology is a branch of parasitology that studies small external parasites like insects (flies, lice, fleas, bugs) and acarines (ticks, mites) that affect animals. These organisms are important because they either directly cause diseases in animals or transmit diseases as vectors. All these organisms belong to the phylum Arthropoda, meaning 'jointed legs'.
Arthropods have bilaterally symmetrical, segmented bodies, compound eyes, and a hard chitinous exoskeleton. They possess jointed legs, an open body cavity called a hemocoel filled with hemolymph, a dorsal blood vessel, and a ventral nerve cord. The exoskeleton is crucial for protection and lines parts of the digestive system.
The exoskeleton is a hard outer covering made of chitin, protecting internal organs and lining parts of the digestive tract. It consists of chitinous plates called sclerites (tergum, sternum, pleuron) connected by flexible sutures. Arthropods shed their exoskeleton to grow, a process called ecdysis or molting.
Two classes are particularly important in veterinary science: Insecta (flies, lice, fleas, bugs) and Arachnida (ticks, mites).
An insect's body is divided into three parts: head, thorax, and abdomen. The head contains sensory organs and mouthparts, including a pair of antennae, compound eyes, simple eyes (ocelli), and various mouthparts like labrum, labium, mandibles, and maxillae. Antennae are key sensory organs for touch, smell, and sound.
Insect mouthparts are adapted to their feeding habits, such as lapping/sponging (housefly), piercing/sucking (mosquitoes, lice), cutting/sponging (horseflies), and chewing (some lice). Mouthparts can also be classified by their direction: prognathous (forward), hypognathous (downward), or opisthognathous (backward).
The thorax, the middle part, has three segments (prothorax, mesothorax, metathorax), each with a pair of legs. Most insects have two pairs of wings attached to the mesothorax and metathorax, though some flies have reduced hindwings called halteres. The abdomen, the last part, is soft and segmented, containing reproductive structures like the aedeagus (males) and ovipositor (females).
Insect legs consist of coxa, trochanter, femur, tibia, and tarsus, ending with claws and pulvilli for grip. The main internal cavity is the hemocoel, filled with hemolymph. The digestive system has three parts: foregut (stomodaeum), midgut (mesenteron), and hindgut (proctodaeum). Malpighian tubules are special excretory organs.
Arthropods have an open circulatory system, where the dorsal, tubular heart pumps hemolymph directly into the hemocoel to bathe organs. Respiration varies; many insects and mites breathe through spiracles, while aquatic stages use gills. Scorpions and spiders have book lungs. Insects and ticks primarily use a tracheal system for direct air transport.
The nervous system includes a dorsal brain in the head and a double ventral nerve cord with segmental ganglia controlling movement. Malpighian tubules are the primary excretory organs in insects, releasing waste into the hindgut. Arachnids also use coxal glands for excretion.
Arthropods have well-developed reproductive systems. Males have testes, vas deferens, seminal vesicles, and an ejaculatory duct opening into the aedeagus. Females have ovaries, oviducts, a common duct with a spermatheca for sperm storage, and an ovipositor. Accessory glands secrete fluids for lubrication and egg attachment.
Reproduction can be oviparous (egg-laying), larviparous (depositing larvae), or pupiparous (depositing pupae). Metamorphosis is the change in body form during growth. Complete metamorphosis (holometabola) involves egg, larva, pupa, and adult stages (flies, fleas), where young stages look different from adults. Incomplete metamorphosis (hemimetabola) involves egg, nymph, and adult stages (lice, bugs, ticks, mites), where nymphs gradually develop and resemble adults.
Larvae are classified by head and leg development: eucephalic (well-developed head), acephalic (reduced head), polypod (thoracic and abdominal legs), oligopod (only thoracic legs), and apodous (no legs). After the larval stage, complete metamorphosis includes a pupal stage (resting, non-feeding) where transformation to an adult (imago) occurs. Pupae can be exarate (free limbs), obtect (limbs attached by fluid), or coarctate (enclosed in puparium).