Study Notes on Phylum Arthropoda
Phylum Arthropoda
Arthropodization
Suite of evolutionary changes that brought about the arthropods:
Hardening of the cuticle into armor.
Hydrostatic skeleton reduced or lost.
Coelom reduced into hemocoel.
Motile cilia lost.
Diversity
Approximately 1.1 million described species, with estimates suggesting many more (5-10 million).
Distribution of major groups:
Hexapoda: 81.0% of species.
Trilobita: 1.7% of species.
Chelicerata: 9.0% of species.
Myriapoda: 1.6% of species.
Crustacea: 6.7% of species.
General Characteristics
General characteristics of Arthropoda are complex due to diverse bodies, lifestyles, etc.:
Most extant arthropods are small, generally less than 60 cm in length.
Prehistoric arthropods may have been much larger, with the largest extant arthropod measuring 4 meters.
Arthropods are generally very active.
Structural Features
Hardened cuticle provides an excellent point of attachment for muscles.
Most arthropods are herbivorous, although all feeding modalities are represented.
Simultaneously, humans enjoy great benefits from arthropods (e.g., pollinators, food sources) while also encountering costs (e.g., pests).
Unique Characteristics
Jointed legs: unique to invertebrates.
Hardened exoskeleton made of chitin:
Provides protection and waterproofing.
Segmented body:
Some groups show fusion of segments into tagmata.
Well-developed head (cephalization):
Numerous sense organs, including antennae and compound eyes.
Presence of pairs of jointed feeding appendages.
Body Wall Structure
Epicuticle:
Thin, hard, and waxy outer layer.
Procuticle:
Thick layer, may be impregnated with calcium in crustaceans.
Epidermis:
Responsible for the secretion of procuticle and epicuticle.
Respiration
Various structures used for respiration:
Gills, e.g., book gills.
Lungs, e.g., book lungs.
Trachea (important in terrestrial types).
Circulation
Circulatory system characteristics:
Open circulatory system where the coelom becomes a hemocoel filled with blood.
Dorsal heart with few vessels.
Use of hemocyanin and hemoglobin as respiratory pigments.
Excretion
Excretion mechanisms:
Antennal glands (located at the base of antennae in crustaceans).
Malpighian tubules (unique to arachnids and hexapods, draining into the intestine).
Coxal glands (modified nephridia at the base of legs).
Excretion also possible through gills and skin.
Subphyla of Phylum Arthropoda
Trilobitomorpha:
All extinct, primarily marine.
Myriapoda:
"Many feet"; includes centipedes and millipedes; predominantly terrestrial with a distinct head featuring mandibles and one pair of antennae.
Chelicerata:
Includes spiders, ticks, mites, and scorpions; ancient terrestrial group featuring chelicerae and pedipalps for feeding and lacking antennae; body divided into cephalothorax.
Crustacea:
Includes shrimps, crabs, and barnacles; mainly marine with mandibles and two pairs of antennae, plus many appendages.
Hexapoda:
Mostly terrestrial with one pair of antennae, three pairs of legs, and two pairs of wings.
Subphylum Trilobitomorpha
Trilobites
Likely arose before the Cambrian period (over 541 million years ago).
Highly abundant during Cambrian and Ordovician periods; extinct for about 345 million years.
Trilobite Characteristics
The name "trilobite" refers to the trilobed shape observed in cross-section, resulting from longitudinal grooves.
Body plan:
Dorsoventrally flattened and benthic, likely scavengers.
Capable of rolling up similar to isopods.
Size ranged from 2 to 70 cm in length.
Body consists of three tagmata:
Head (cephalon).
Trunk.
Pygidium.
Trilobite Morphology
Head includes:
A pair of antennae.
Compound eyes.
Mouth structure lacking mouthparts.
Leg-like appendages.
Trunk includes:
Biramous appendages, with one branch functioning as gills.
Subphylum Chelicerata
Evolved over 445 million years ago and includes:
Horseshoe crabs, spiders, ticks, mites, scorpions, and others.
General Body Plan
Body consists of two tagmata:
Cephalothorax.
Abdomen.
Possesses six pairs of cephalothoracic appendages:
Chelicerae.
Pedipalps.
Four pairs of walking legs.
Most feed by sucking liquid food from their prey.
Divided into three classes:
Merostomata.
Arachnida.
Pycnogonida.
Class Merostomata
Subclass Eurypterida:
Giant water scorpions; all extinct; largest fossil arthropods were 3 meters long.
Subclass Xiphosurida:
Horseshoe crabs; ancient group of arthropods largely unchanged since the Triassic period.
Three genera survive, live in shallow waters along eastern North America, Japan, East Indies, and Southeast Asia.
Features:
Unsegmented carapace.
Chelicerae and pedipalps.
Four pairs of walking legs.
Broad abdomen with long telson.
Six pairs of broad, flat appendages transformed into book gills and gill opercula.
Feeding habits include eating annelids and mollusks.
Reproductive behavior:
Gather in masses at high tide; females burrow in sand to lay eggs, while males deposit sperm into eggs.
Medical significance of horseshoe crab blood:
Contains amoebocytes used to test for bacterial toxins.
Class Arachnida
Features enormous anatomical variation and primarily free-living forms.
Preference for warm and dry habitats; approximately 80,000 species have been described as the earliest terrestrial arthropods.
Arachnid General Body Plan
Consists of two tagmata:
Cephalothorax with walking legs, chelicerae, and pedipalps.
Abdomen containing reproductive and respiratory organs.
Predaceous nature characterized by fangs, claws, venom glands, and stingers, where:
Fangs are modified chelicerae.
Claws are modified pedipalps.
Feeding Modalities
Possess a powerful pharynx allowing them to feed on soft tissues and fluids.
Release digestive enzymes into prey and consume predigested food.
Order Araneae (Spiders)
Comprises about 40,000 species with body plan:
Cephalothorax and abdomen joined by a pedicel, forming a narrow waist.
Chelicerae bear fangs, pedipalps serve a sensory role, and four pairs of walking legs are present.
Feeding characteristics:
Most are predaceous, exhibiting diverse predation styles.
Respiratory methods include book lungs or tracheal tubes.
Excretion via Malpighian tubules or coxal glands; possess 8 simple eyes but generally poor vision; sensory setae are more functional.
Order Scorpiones (Scorpions)
The order encompasses roughly 1,400 species mainly found in tropical to subtropical regions.
Moreover, they are generally secretive, often hiding in burrows.
Body plan:
Consists of two tagmata: cephalothorax (including chelicerae, pedipalps, legs, eyes) and abdomen (divided into pre-abdomen and post-abdomen).
The telson is modified into a stinger.
Order Acari (Mites and Ticks)
Most medically and economically significant arachnids with about 40,000 described species, but actual estimates could be between 500,000 and 1 million.
Widespread in both terrestrial and aquatic environments; many are parasitic at least during one life stage.
Size range:
Mites usually < 1 mm; ticks typically < 3 cm (but generally much less).
Acari Body Plan
Characterized by the complete fusion of cephalothorax and abdomen, with mouthparts located on a capitulum surrounded by chelicerae, which are designed to pierce hosts.
Medical and Economic Importance of Acari
Ticks are vectors for several diseases:
Lyme disease, Southern Tick-Associated Rash Illness (STARI), Rocky Mountain Spotted Fever.
Mites can be significant agricultural and livestock pests.
Subphylum Myriapoda
Known as "many-footed" organisms, characterized by two tagmata: head and trunk.
Each segment typically bears a pair of appendages.
Class Chilopoda (Centipedes)
Approximately 3,000 species characterized by:
Flattened bodies that prefer moist environments.
Agile carnivore lifestyle.
Prey is killed using venomous claws and then chewed with mandibles.
First pair of legs modified into venom claws, last pair adapted for sensory functions.
Class Diplopoda (Millipedes)
Comprise around 10,000 species:
Have a rounded body and are less active than centipedes.
Prefer dark, moist environments and exhibit herbivorous feeding behavior.
Possess repugnatorial glands that produce a foul-smelling secretion when disturbed.