Introduction & Classification of Arthropods Dr. Mohammed Iqbal Siddiqui

Features of All Arthropods All have exoskeletons for support and protection All have jointed appendages Body regions include: a head, thorax, and an abdomen. Some have a cephalothorax (head and thorax are fused together) All arthropods molt (shed exoskeleton)

General Characteristics Cont’d Arthropods use either gills, tracheal tubes, or book lungs for gas exchange. Acute senses include simple and compound eyes, brains, and many ganglia. Arthropods use pheromones to communicate. They have open circulatory systems, complete digestive system, and efficient means of wastes excretion. Arthropods reproduce sexually, but some exhibit parthenogenesis.

Phylum Arthropoda is classified into four subphylla 1- Trilobita Extinct (Fossilised ) 2- Crustacia (All aquatic forms ) 3- Chelicerata Class Arachnida ( includes three important orders) (A) Acarina (Ticks and mites) (B) Scorpionida (All forms of scorpions) (C) Areinida (all spiders) 4- Uniramia – Three classes-Hexapoda; Chilopoda; and Diplopoda

Class Arachnida Spiders, scorpions and mites belong to this class. Spiders are the largest group of arachnids. Most arachnids have only 2 body regions- a cephalothorax and the abdomen. Arachnids have 2 chelicerae for poisoning prey and 2 pedipalps for sensing and handling food. 4 remaining appendages aid in locomotion.

Arachnids Cont’d Spiders have silk glands for spinning webs and for wrapping their eggs in a cocoon. Ticks and mites have only one body segment. Ticks feed on mammalian blood and can expand up to 1 cm. Mites are very small and cannot be seen by the naked eye. However their bites, called chiggers, can be felt. Scorpions have many body segments and 2 pincers and have a long tail with a venomous stinger at the end.

Class Crustacea Most crustaceans are aquatic and use gills for gas exchange. 2 pairs of antennae for sensing. Mandibles for crushing food that move side to side. Eyes are located on movable stalks. Many have five pairs of walking legs used for seizing prey and cleaning other appendages. First pair is often modified into strong claws.

Crustaceans Cont’d Crabs, lobsters, shrimp, crayfish, barnacles, water fleas, and pill bugs are crustaceans. Some have three body sections, and others have only two. Land crustaceans like pill bugs must live in damp environments for efficient gas exchange.

Class Chilopoda and Diplopoda Centipedes belong to class Chilopoda and millipedes belong to class Diplopoda. Centipedes are carnivorous and eat soil arthropods, snails, slugs, and worms. Centipede bites are painful to humans. Both have Malphigian tubes for waste excretion. Both have tracheal tubes for gas exchange. Millipedes eat mostly plants and dead material. Millipedes also have stink glands for scaring predators.

Class Merostomata Horseshoe crabs belong to this class. Have remained unchanged since Cambrian period. Have exoskeletons similar to trilobites Feed on seaweed, mollusks, and worms on bottom of oceans. Females lay eggs on land.

Class Insecta Insects are the largest group and most successful arthropods. Insects may only mate once or twice in their lifetimes. Internal fertilization Large number of eggs are produced to ensure a large number of offspring. Females lay eggs in wood or the ground. May go through complete or incomplete metamorphosis depending on species.

Insects Cont’d Complete metamorphosis: egg, larva, pupa, and adult. Incomplete metamorphosis: egg, nymph, and adult. Nymphs compete with adults for same resources and are not sexually mature.