Phylum Arthropoda: Characteristics

Phylum Arthropoda

• Consists of over 80% of all known species
• Can be found in practically any environment
• Over one million described species (926,990 of hexapoda alone), and estimates as high as 100 million
• The largest assemblage of species is within the Animal Kingdom

General Characteristics: Segmentation

• The body is metameric
• The arthropods were once thought to be descendants of the Annelida
• Loss of segmentation has occurred in three ways:
  • Segments have disappeared
  • Segments have fused together
  • Segmental structures have become structurally and functionally differentiated and modified from their counterparts on other segments (e.g., appendages)
• Specialization of groups of segments are known as tagmatization
• Two major arthropod groups have three tagmata (regions): head, thorax, and abdomen

General Characteristics: Nervous System and Appendages

• Possess a dorsal anterior brain with a ventral nerve cord containing ganglionic swellings in each segment
• Each arthropod segment has one pair of appendages
• Rigid, lever-like appendages and cross-striated muscles allow faster movement than annelids
• Jointed appendages are present
• They are encased in a chitinous exoskeleton (compared to annelids with an epidermis through which gas exchange occurs)
• Live in marine, freshwater, and terrestrial environments

Appendages

• Characterized by specialized, jointed appendages
• Appendages have evolved for movement or into jaws, gills, egg depositors, sucking tubes, claws, antennae, paddles, and pincers
• Relationships may be indicated by appendages that have evolved into specific structures, like mandibles

Segmentation (Metamerism)

• The arthropods are segmented
• Their bodies are shorter with fewer segments compared to annelids
• The number of segments has become fixed
• There is more specialization of segments
• Tagmata is produced by the fusion of segmental skeletal plates
• Generally have three tagmata: head, thorax (sometimes fused together to form the cephalothorax), and abdomen
• Evident in adult in terms of appendages, musculature, and the nervous system

Exoskeleton - Chitin

• It is articulated and jointed
• It is also referred to as a cuticle
• Chitin has significant commercial interest, as it is strong, nonallergenic, and biodegradable
• It can be used in fabrics, surgical sutures, biodegradable capsules for slow-release drugs, as a substitute for plastic wrap, as a food additive to reduce caloric and cholesterol uptake, and as a binding agent to remove toxic organic and inorganic compounds from drinking water and during sewage treatment
• The cuticle of each segment is divided into four primary plates: dorsal, two lateral, and ventral
• It is secreted by, and attached to the underlying

Exoskeleton (Cuticle) Functions

• Protects against predators and ectoparasites
• Often waterproof, keeping exterior water out and interior water in (due to wax in epicuticle)
• Modified to form food grinders in the foregut, to form wings, and to form tactile hairs
• Modified to form the lens of the arthropod eye
• At the joints, the exoskeleton is flexible and thin (lacks exocuticle), permitting free movement
• Pairs of muscles antagonize each other through a system of rigid levers
• When muscles attached on the inside of the exoskeleton contract, the exoskeleton moves at its joints
• Some arthropod joints, such as wing joints and those involved in jumping, contain a substance called "animal rubber" or resilin, which stores energy upon compression and efficiently releases it
• Movement is precisely controlled
• It is not added gradually at a growing edge (like a mollusc) but is secreted over all regions of the body simultaneously and then hardened

Disadvantages of an Exoskeleton

• It cannot expand and must be molted (ecdysis) as the arthropod grows
• Old exoskeleton is discarded, and a new exoskeleton is formed
• At molting time, proenzymes are secreted by the epidermis, activating chitinase and protease, which dissolves the endocuticle
• A new skeleton, not yet hardened, forms under the old one
• The old exoskeleton is split and shed, and the soft new skeleton is stretched
• The newly-molted animal is susceptible to predators, and terrestrial forms can be subject to water loss
• Many arthropods hide until the cuticle hardens
• It has a high metabolic cost
• Some insects and freshwater crustaceans reabsorb the dissolved inner layer and eat the outer layers of the old exoskeleton

Molting - Ecdysis

• A process of removing the existing exoskeleton
• The old cuticle is split by uptake of water and air and by increased blood pressure, causing the body to swell
• A new cuticle is secreted before the old one is shed
• The cuticle does not harden until morphological alterations and increase in body size have taken place
• Although increases in size are discontinuous, growth of tissue (biomass) is continuous
• The number of cells in growing tissue (e.g., epidermis) typically increases continuously, with the additional tissue becoming folded or pleated until the old cuticle is shed, and the increase in body size can take place
• Stages between molts are known as instars, where the number of instars may either be fixed or be indeterminate

Waterproof Exoskeleton

• Evolved during the evolution of terrestrial arthropods
• Occurred by the late Ordovician (440 million years ago)
• Arthropods are the only invertebrates with many species adapted to withstand the drying action of air

Internal Features

• Lack cilia, even in the larval stages
• Possess a tubular gut with a distinct mouth and anus
  • Foregut and hindgut are lined with chitin
  • The foregut is involved in ingestion and storage of food
  • The midgut is the site of enzyme production, digestion, and absorption
  • The hindgut functions in the absorption of water and the formation of feces
  • Food is moved through the digestive tract by muscular activity rather than ciliary activity because the lumens of the foregut and hindgut are lined with cuticle
  • Digestion is generally extracellular, and nutrients are distributed to the tissues through the hemal system
• Have a coelom that is very reduced (like molluscs)
  • Consists of a main cavity called a hemocoel, which is part of the blood circulatory system and the cavities of the reproductive and excretory organs
  • Spiders and other arachnids extend their legs by increasing blood pressure within the hemocoel, but otherwise fluids and body cavities have little to do with arthropod motion

Respiration

• Tracheae are found in some insects and other terrestrial forms
  • Tubes are used in respiration
  • Consists of a system of cuticle-lined ducts that pipe air directly into the various parts of the body
• Open to the outside by spiracles that may close to conserve water
• The tracheal system delivers oxygen directly to the cells
• Gases pass through the tracheoles by diffusion
  • Limits the size of insects because all cells must be reached by diffusion

Book Lungs

• Found in chelicerates
• Spiders may have book lungs in addition to, or instead of tracheae
• Resemble pages on a partially opened book

Excretion

• It has two types of excretory organs
  • In terrestrial forms, excretion occurs via Malpighian tubules
    • They are blind tubular invaginations of the gut which lie within the hemocoel
    • The tubules absorb wastes from the body cavities (water and nitrogenous wastes from the hemocoel)
    • They secrete them into the midgut or hindgut
    • Wastes (in the form of uric acid or guanine) are excreted with feces
  • Nephridia (end sac)
    • Blind tubules open by ducts to the outside of the body, adjacent to an appendage
    • The end sac wall is the site of filtration of blood and selective reabsorption and secretion

Nervous System

• High degree of cephalization
• Well-developed sense organs are present
• Most arthropod nervous systems consist of a double chain of segmental ganglia running along the ventral surface
  • Double chains part to encircle the esophagus
• Many arthropod activities are controlled at the segmental level (similar to annelids)
• There is local control of functions by ganglia
• If the brain is removed, some arthropods can still move, eat, and carry on normal function
• The brain appears to act as an inhibitor rather than a stimulator (similar to annelids)
• With the brain removed, a grasshopper will continue to walk, jump, or fly, and responds to stimuli by jumping or flying

Reproduction and Development

• Sexual reproduction is the rule
• Most arthropods are dioecious
• Some free-living species exhibit various degrees of asexual reproduction
  • Parthenogenesis (production of offspring from unfertilized eggs) is encountered in insects, branchiopods, and some freshwater copepods
  • Majority of species, and all terrestrial species, have copulation and internal fertilization; marine arthropods often have external fertilization
  • Many arthropods, including insects, scorpions, and arachnids, transfer sperm to females indirectly using spermatophores that are passed to the female in a variety of ways
  • Various appendages are involved in courtship and sperm transfer

Important Arthropoda Taxonomic Groups

• Subphylum Crustacea
  • Class Malacostraca: Crabs, Crayfish, Lobsters, Amphipods
  • Class Brachiopoda: Daphnia, etc.
  • Class Copepoda: Cyclops sp.
  • Class Thecostraca: Barnacles
• Subphylum Hexapoda
  • Class Insecta
• Subphylum Myriapoda
  • Class Diplopoda: Millipedes
  • Class Chilopoda: Centipedes
• Subphylum Chelicerata
  • Class Pycnogonia: Sea Spiders
  • Subclass Merostomata: Horseshoe Crabs
  • Subclass Arachnida: Spiders, Scorpions, Mites, Ticks