Arthropods: Characteristics and Diversity

Phylum Arthropoda

• Arthropods are multicellular, bilaterally symmetrical, triploblastic, and have a true coelom (protostomes)
• They are segmented, with two out of every three known species of animals being arthropods
• Members of the phylum Arthropoda are found in nearly all habitats of the biosphere

General Characteristics of Arthropods

• Arthropods have achieved great diversity and abundance due to their segmentation, hard exoskeleton (made of chitin), and jointed appendages
• Segments have combined into functional groups called tagmata, which have specialized purposes
• As arthropods evolved, segments fused, and appendages became more specialized
• Arthropods have an open circulatory system with an exoskeleton that is protective and flexible
• The exoskeleton prevents desiccation, provides places for muscle attachment, and does not allow for growth, requiring molting (ecdysis)

More Efficient Locomotion

• Each segment bears a pair of jointed appendages, which have sensory hairs and may be modified for sensory functions, food handling, or walking & swimming

Class Arachnida - Order Araneae (Spiders)

• In spiders and insects, Malpighian tubules serve as excretory structures, conserving water and allowing organisms to live in dry environments
• Many spiders have coxal glands, modified nephridia, at the base of legs
• Reproduction involves a courtship ritual, with males storing sperm in pedipalps and inserting them into female genital openings
• Eggs develop in a cocoon in the web or are carried by the female
• Young hatch in about two weeks and may molt before leaving the egg cocoon
• Not all spiders are dangerous, with some being allies of humans in the battle with insects
• Some species of black widow spiders, brown recluse spiders, and Australian and South American spiders are the most dangerous and aggressive

Class Arachnida - Order Scorpiones (Scorpions)

• Scorpions feed on insects and spiders, seizing them with their pedipalps
• The last segment contains a bulbous base and a curved barb that injects venom
• Scorpions are viviparous or ovoviviparous, with females brooding young within their reproductive tract

Class Arachnida - Order Opiliones (Harvestmen)

• Harvestmen differ from spiders in that their abdomen and cephalothorax are broadly joined rather than constricted
• Some harvestmen have live young (viviparous)
• Young resemble little adults, with no metamorphosis

Class Diplopoda (Millipedes)

• Millipedes have two pairs of legs on each segment
• The head has one pair each of antennae, mandibles, and maxillae
• The body is more cylindrical
• Millipedes live in dark, moist places, feeding on decayed plant matter or occasionally living plants
• They are slow-moving, coiling up when disturbed
• Toxic or repellent fluids are secreted when disturbed from repugnatorial glands
• After copulation, females lay eggs in a nest and guard them
• Larvae have only one pair of legs per segment

Class Pauropoda

• Pauropods live in moist soil, leaf litter, decaying vegetation, or under bark and debris
• They are soft-bodied, small (2 mm or less), and have approximately 500 species
• The head lacks true eyes, has branched antennae, and a pair of sense organs
• There are 12 trunk segments, with 9 pairs of legs but none on the first or last two segments
• One tergal plate covers two segments, lacking tracheae, spiracles, and a circulatory system

Class Symphyla

• Symphylans live in humus, leaf mold, and debris
• They are soft-bodied, with 14 segments, 12 of which bear legs and one bears a pair of spinnerets
• Males place a spermatophore at the end of a stalk, which the female stores in special pouches
• Eggs are fertilized and attached to moss or lichen
• Young hatch with only 6 or 7 pairs of legs
• Small (2–10 mm) with centipede-like bodies

Subphylum Crustacea

• Crustaceans constantly threatened by over-dilution with water
• Freshwater crustaceans have urine that is isosmotic with blood
• Marine crustaceans have urine that is isosmotic with seawater
• Crustaceans have a pair of supra-esophageal ganglia connected to eyes and two pairs of antennae
• The nervous system includes a ventral nerve cord with a pair of ganglia for each somite
• Eyes and statocysts are the largest sensory organs
• Chemical sensing of taste and smell occurs in hairs on antennae and mouth

Class Maxillopoda

• Maxillopods generally have 5 cephalic, 6 thoracic, and 4 abdominal segments plus a telson
• The nauplius of maxillopods has a maxillopodan eye – unique to this group

Class Maxillopoda – Subclass Copepoda

• Copepods are planktonic crustaceans, lacking a carapace
• They retain the simple maxillopodan eye in adults
• Antennules are used in swimming
• They are very diverse, with some species being parasitic and highly modified
• Free-living copepods may be the dominant consumer
• Some species are intermediate hosts of human parasitic tapeworms and nematodes

Class Maxillopoda – Subclass Tantulocarida

• Subclass Tantulocarida is a recently described group
• There are approximately 12 species
• They are tiny copepod-like ectoparasites of deep-sea benthic crustaceans