Phylum Arthropoda: Characteristics and Metamorphosis

• Phylum Arthropoda is one of the most significant animal divisions, with arthropods making up 85% of all known animals.
• Arthropods are bilaterally symmetrical invertebrates with segmented bodies, jointed appendages, and exoskeletons.
• Arthropods' size varies greatly, from the Atlas moth (12-inch wingspan) to the follicle mite (less than 1/250 inch long).
• Metamorphosis is the change in form/structure during an arthropod's development; some insects lack it, with young resembling smaller adults.
• Gradual metamorphosis involves three stages where the young are similar to adults, as seen in cockroaches and lice.
• Complete metamorphosis, as seen in mosquitoes and butterflies, has four stages with significant differences between immature and adult forms.
• Arthropods are adaptable and can be found in diverse environments, establishing colonies.
• They defend themselves with chitinized exoskeletons, detachable/regenerating appendages, hairs/scales/spines, and defensive body fluids.

Classification of Arthropods

• Phylum Arthropoda contains at least 740,000 species.
• Medically important arthropods are mostly in Insecta and Arachnida classes, but also Chilopoda, Diplopoda, Crustacea, and Pentastomida.
• Insecta (flies, mosquitoes, bees etc.) is a the largest class, representing ∼70% of the phylum.
• Insecta typifies the arthropod's external/internal structures and is medically important.
• The insect body has a head, thorax, and abdomen which are clearly differentiated in many insects.

Insect Head

• The head bears eyes, antennae, and mouthparts.
• Antennae are located in the front of the head and are often have characteristic shapes
• Insects have simple eyes (ocelli) or compound eyes with many facets, and their size varies.
• Insects have an upper lip (labrum), a lower lip (labium), maxillae (upper jaw), and mandibles (lower jaw), shaped according to feeding habits.
• Cockroaches have chewing mouthparts, with mandibles for grinding food and palpi for sensing it.
• Houseflies have sponging mouthparts adapted for sucking up liquid or readily soluble foods.
• Mosquitoes have piercing-sucking mouthparts with long, slender mandibles, labrum, and maxillae forming a proboscis.
• Honeybees have chewing-lapping mouthparts to manipulate wax or grasp prey.

Insect Thorax

• This region is made up of the prothorax, mesothorax, and metathorax.
• Each segment in the Thorax bears a pair of walking legs.
• Wings, when present, attach to the mesothorax and metathorax.
• Wings are extensions of the body wall consisting of upper and lower layers supported by veins.
• Leg segments include the coxa, trochanter, femur, tibia, tarsus, and pretarsus.

Insect Abdomen

• The Abdomen which is made up of 11 segments, bears the spiracles and external reproductive organs.
• Spiracles are external openings of the respiratory system.
• Some have cerci on the 11th segment, more noticeable in females.

Insect Internal Anatomy

• Insect blood (hemolymph) is colorless and contains phagocytic hemocytes.
• Circulation is maintained by hemolymph flowing through ostia.
• Heart is dorsal, carrying nutrients to tissues and waste to Malphigian tubules.
• The body cavity is the hemocoel.
• Oxygen is directly exchanged through spiracles on the thorax and abdomen that lead to the trachea.
• The trachea is a small tube where oxygen diffuses, and this system also regulates water levels.
• The nervous system is a brain connected to a nerve cord with ganglia, and nerves from these reach sensory organs.
• These sensitive sensory organs include compound eyes, ocelli, antennae, halteres, palpi, and hairs
• The digestive system starts with the mouth and comprises of the pharynx, oesophagus, and proventriculus.
• The posterior of the esophagus (crop) serves as temporary food storage and the muscular proventriculus prevents regurgitation
• Digestion occurs in the midgut, which secretes enzymes for insect meal digestion.
• The beginning of the hindgut is marked by the Malphigian tubules.
• Malphaigian tubules act as excretory filters.

Insect Reproduction and Senses

• Insects are dioecious and reproduce via mating.
• Oviparous insects lay eggs; viviparous insects deposit larvae.
• Females have ovaries and oviducts.
• The eggs are fertilized by sperm cells stored in the spermatheca.
• The males have testes and seminal vesicle for sperm storage.
• Insects have senses of touch, taste, smell, hearing, and sight, as well as balance.
• Sensory hairs connected to nerves facilitate the sense of touch.
• Insects use hairs on the mouthparts, antennae and tarsi to detect touch, and the palpi bear olfactory organs.
• Insects respond to specific noises, picked up by sensory hairs or auditory organs.
• Insects cannot move or focus their eyes, leading to blurred vision.
• Insects react to stimuli like light, heat, gravity, hunger, and smell with fixed behavior.
• They act independently and behavioral reactions are immediate.

Medically Important Classes and Orders under Phylum Arthropoda

• Arthropods like ticks, fleas and mosquitos can cause direct and indirect injuries to humans.
• Class Insecta, including Diptera (mosquitoes, flies), Siphonaptera (fleas).
• Class Insecta includes Hymenoptera (bees, wasps, ants), Lepidoptera (moths, butterflies).
• Class Insecta includes Hemiptera (bed bugs, kissing bugs), Anoplura (sucking lice), Coleoptera (beetles).
• Class Crustacea arthropods are aquatic in nature with cephalothorax and abdomen body divisions.
• Class Crustacea includes Copepoda (cyclops), Decapoda (crabs, lobsters, shrimps).

Class Arachnida, Chilopoda, Diplopoda & Pentastomida

• Class Arachnida arthropods which are both aquatic and terrestrial in nature and whose bodies are divided into cephalothorax and abdomden.
• Class Arachnida includes Scorpionida (scorpions), Araneida (spiders) and Acarina (mites, ticks)
• Class Chilopoda (centipedes) are terrestrial arthropods, whose bodies are dorsoventrally flattened and whose first body segments are poisoin claws.
• Class Diplopoda (millipedes) are terrestrial anthropods whose bodies are clyindrical with two pairs of legs per body segment.
• Class Pentastomida adults usually live in the lungs or air passages of their hosts, while larvae live free or encysted in the viscera of some other hosts.