General characters & Classification of�Arthropods

B. SUJATHA

GOVT DEGREE COLLEGE, RAJAMPET

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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)

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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

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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.