Arthropoda: Overview | Zoology Optional Notes for UPSC PDF Download

Definition of Arthropoda

Arthropods, the fascinating members of the animal kingdom, are defined as bilaterally symmetrical, triploblastic creatures with metamerically segmented bodies. Their coelom, although reduced and modified, plays a crucial role in their overall biology. What sets arthropods apart is their chitinous exoskeleton, which covers their bodies and undergoes periodic molting. Additionally, their appendages exhibit distinct jointed structures.

Characteristics of Phylum Arthropoda

Arthropods exhibit a multitude of defining characteristics that make them a diverse and intriguing phylum:

1. Bilateral Symmetry: Arthropods display bilateral symmetry, meaning their bodies can be divided into two equal halves.

2. Triploblastic Organization: These organisms possess a triploblastic body structure, consisting of three germ layers: the ectoderm, mesoderm, and endoderm.

3. Metameric Segmentation: The body of arthropods is metamerically segmented, divided into repetitive, distinct segments.

4. Organ-System Level: Their body organization operates at the organ-system level, highlighting the complexity of their internal structures.

5. Chitinous Exoskeleton: Arthropods are known for their robust chitinous cuticle that forms an exoskeleton, providing protection and support.

6. Lateral and Jointed Appendages: Body segments typically bear lateral and jointed appendages with diverse functions, including jaws, gills, and legs.

7. Head, Thorax, and Abdomen: The body can be divided into three main regions: head, thorax, and abdomen. It's common for the head and thorax to fuse, forming a cephalothorax.

8. Striped Muscles: Arthropods possess separate striped muscles, allowing for rapid contractions and precise movements.

9. Hemocoel Body Cavity: Their body cavity is a hemocoel, as the true coelom is limited to the spaces occupied by the genital and excretory organs.

10. Complete Digestive System: Arthropods boast a complete digestive system, featuring both a mouth and an anus. Mouthparts are adapted for various feeding modes.

11. Open Circulatory System: Their circulatory system is open, characterized by a dorsal heart and arteries. However, it lacks capillaries.

12. Respiration Strategies: Arthropods engage in respiration through their general body surface. In aquatic forms, gills come into play, while terrestrial forms rely on trachea or book-lungs.

13. Excretion Organs: Unlike true nephridia, arthropods utilize a variety of excretion organs, including green glands, Malpighian tubules, or coxal glands.

14. Nervous System: Their nervous system typically resembles that of annelids, featuring a dorsal brain connected to a nerve ring, which, in turn, links to a double ventral nerve cord.

15. Cilia Absence: Unlike many other organisms, arthropods lack cilia on any part of their bodies.

16. Sensory Organs: Arthropods possess a rich array of sensory organs, encompassing simple and compound eyes, chemo- and tactile receptors, as well as balancing and auditory structures.

17. Sexual Characteristics: Most arthropods exhibit separate sexes, being dioecious. They have paired reproductive organs and ducts, engaging in internal fertilization. Reproduction modes can be oviparous or ovoviviparous.

18. Development: Typically, arthropods undergo indirect development through various larval stages. Parthenogenesis, reproduction without fertilization, is observed in some cases.

19. Parental Care: Many arthropods display marked parental care, emphasizing their complex social and reproductive behaviors.

20. Habitat Diversity: Arthropods reign as one of the most diversified groups, thriving in terrestrial, aquatic, and aerial environments.

Phylum Arthropoda Classification: An In-Depth Exploration

The phylum Arthropoda encompasses a vast array of organisms, each exhibiting unique characteristics and adaptations. This comprehensive classification system, influenced by notable researchers like Vandal (1949), Snodgrass (1960), and Strorer (1979), serves as a guide to understanding the diverse groups within this phylum. It's important to note that Onychophora, once considered arthropods, is now recognized as an independent group of segmented animals in modern trends.

Subphylum 1: Trilobitomorpha
Definition: Represented exclusively by fossil trilobites, this subphylum comprises marine organisms that primarily dwell on the ocean floor.

Characteristics:

• The body is trilobed, featuring three distinct lobes separated by two longitudinal furrows.
• Notable head structure with one pair of antennae.
• Except for the final segments, all body segments bear biramous appendages.

Examples: Triarthrus, Dalmanites

Subphylum 2: Chelicerata
Definition: Chelicerata displays a division into an anterior cephalothorax (prosoma) and a posterior abdomen (opisthosoma). This subphylum is home to various terrestrial and predaceous arthropods.

Characteristics:

• Six pairs of appendages in the prosoma: first pair of preoral chelicerae with feeding claws, followed by the second pair of postoral pedipalpi and four pairs of walking legs.
• Lack of antennae and true jaws.
• Often terrestrial, with some predaceous habits.

Class 1: Merostomata
Definition: This class consists exclusively of aquatic, marine arthropods characterized by simple median and compound lateral eyes, abdominal appendages adapted for respiration, and a distinctive telson or spine at the end of the abdomen.

Characteristics:

• Respiration through lamelliform gills or book-gills attached to abdominal appendages.
• Presence of coxal glands for excretion.

Subclass 1: Xiphosura
Characteristics:

• Convex prosoma covered by a broad horseshoe-shaped carapace.
• Six pairs of appendages in the prosoma.
• Unsegmented abdomen with a long terminal telson.
• Paired genital openings covered by a genital operculum.

Examples: Limulus (horseshoe or king crab)

Subclass 2: Eurypterida
Characteristics:

• Extinct marine arthropods, often recognized as giant water scorpions.
• Small cephalothorax with a dorsal carapace.
• Abdomen is 12-segmented and follows the narrow cephalothorax.
• Six pairs of appendages in the cephalothorax.

Examples: Eurypterus, Pterygotus

Class 2: Arachnida
Definition: Arachnida is home to both terrestrial and aquatic arthropods, characterized by simple eyes, a distinctive prosoma with six pairs of appendages, and respiratory structures such as tracheae, book-lungs, or book-gills.

Characteristics:

• Lack of abdominal appendages.
• Respiration through tracheae, book-lungs, or book-gills.
• Dioecious (separate sexes), often oviparous, with courtship preceding mating.

Order 1: Scorpionida (Scorpiones)
Characteristics:

• Terrestrial arachnids often found under stones in tropical and subtropical regions.
• Prosoma and opisthosoma joined by a narrow pedicle.
• Presence of a sharp telson and poison sting.
• Ventral comb-like pectines on the 2nd abdominal segment.

Examples: Buthus, Palamnaeus, Androctonus

Order 2: Pseudoscorpionida (Chelonethida)
Characteristics:

• Tiny false scorpions typically found under tree bark.
• Prosoma consists of six fused segments.
• Abdomen has 11 segments, lacking a sting and telson.
• Chelicerae with comb-like secretions.

Examples: Chelifer, Microcreagris

Order 3: Palpigradi
Characteristics:

• Small-sized micro whip scorpions.
• No eyes.
• Prosomal carapace divided into large anterior and smaller posterior portions.
• Abdomen is 10-segmented and jointed to prosoma by a pedicle.

Examples: Koenenia

Order 4: Solifugae (Solifugida)
Characteristics:

• Often referred to as false spiders, sun spiders, or wind spiders.
• Body consists of prosoma and opisthosoma.
• Prosoma divided into a large anterior and a small posterior part.
• Chelicerae are large and chelate.

Examples: Galedodes

Order 5: Amblypygi (Phrynichida)
Characteristics:

• These are flattened scorpion-spiders or taillers whip scorpions.
• Undivided carapace and large, rhaptorial pedipalps.
• Moderate-sized chelicerae and abdomen with no flagellum.

Examples: Charinus

Order 6: Uropygi (Pedipalpi)
Characteristics:

• Commonly known as whip scorpions.
• Presence of a pair of eyes.
• Prosomal carapace is whole, with moderate-sized chelicerae.
• Large and heavy, with a long telson at the end of the abdomen.

Examples: Thelyphonus, Mastigoproctus

Order 7: Araneae
Characteristics:

• True spiders, with prosoma and opisthosoma.
• No visible segments and jointed by a narrow pedicle.
• Prosoma bears six pairs of appendages.
• Respiration by book-lungs, tracheae, or both.

Examples: Argiope (writing spider), Aranea (house spider), Lycosa (wolf spider), Agelena (funnel-web spider)

Order 8: Ricinulei (Podogna)
Characteristics:

• Rare, small, tick-like arachnids.
• Body includes prosoma and opisthosoma.
• Prosoma features an anterior hood-like movable plate (Cucullus).

Examples: Cryptocellus, Ricinoides

Order 9: Phalangida (Opiliones)
Characteristics:

• Resembling spiders, these arachnids have small, oval bodies and extremely long, slender legs.
• Prosoma is unsegmented, and the opisthosoma bears ten segments.

Examples: Phalangium, Leiobunum

Order 10: Acarina
Characteristics:

• Commonly known as ticks and mites, these arachnids can be free-living or parasitic.
• Small, oval, unsegmented bodies, with the prosoma fused with the opisthosoma.
• Chelicerae and pedipalpi adapted for biting, piercing, and sucking.

Examples: Chorioptes (Mites), Sarcoptes (Itch-mite), Ixodes (Tick)

Class 3: Pycnogonida
Definition: This class, commonly referred to as sea spiders, consists of very small organisms with a cephalothorax, reduced abdomen, and long walking legs.

Characteristics:

• Usually eight pairs of long walking legs.
• Mouth located on a long proboscis.
• No identifiable respiratory or excretory organs.
• Dioecious, with females possessing a pair of ovigers for carrying eggs.

Examples: Pycnogonum, Nymphon

Subphylum 3: Mandibulata - A Classification Overview

Subphylum 3: Mandibulata is a diverse group of arthropods with terrestrial and aquatic representatives. Their bodies can be divided into different regions, and they exhibit distinct characteristics. This subphylum includes several classes, each with its own unique features.

• Terrestrial and Aquatic Mandibulata: Mandibulata includes both terrestrial and aquatic species. They can be found in freshwater and marine environments.
• Body Division: The body of Mandibulata is divisible into different regions. In some, it is divided into cephalothorax and abdomen, while in others, it's divided into head, thorax, and abdomen.
• Head Appendages: Mandibulata's head appendages consist of 1 or 2 pairs of antennae, 1 pair of mandibles, and 1 or 2 pairs of maxillae. They also typically have compound eyes.
• Respiration: Respiration in Mandibulata occurs through various means, including gills or tracheae.
• Excretory System: The excretory system in Mandibulata is managed by structures like Malpighian tubules or green glands.
• Sexual Dimorphism: Most Mandibulata species have separate sexes, exhibiting sexual dimorphism.
• Developmental Stages: The development of Mandibulata species often involves larval stages, varying across different classes.

Now, let's delve into the different classes within Subphylum 3: Mandibulata:

Class 1: Crustacea (Crustaceans) - A Closer Look

• Crustacea are predominantly aquatic, with some species found in freshwater and moist terrestrial environments.
• The head is often fused with the thorax to form a cephalothorax, covered dorsally by the carapace.
• They have chitinous, hard, and limy exoskeletons.
• The head bears 2 pairs of antennae, 1 pair of mandibles, and 2 pairs of maxillae.
• Typically, they have biramous appendages.
• Respiration can occur through gills or general body surfaces.
• Their reduced coelom takes the form of the hemocoel.
• The blood vascular system includes a dorsal contractile heart.
• Excretion can be managed by maxillary glands or antennary (green) glands.
• Sexual dimorphism is common, and their development includes a nauplius stage.

Class 1: Crustacea - Subclass 1: Cephalocarida

• Cephalocarida have a unique body division, with a horse-shoe-shaped head and a 19-segmented trunk.
• Only the anterior 9 trunk segments bear appendages that appear triramous.
• They lack eyes and carapace.
• The head has 2 pairs of antennae, a pair of jaws, and 2 pairs of maxillae.
• Genital openings are on the 19th segment.
• These are marine and bottom-dwelling forms, with Hutchinsoniella as an example.

Class 1: Crustacea - Subclass 2: Branchiopoda

• Branchiopoda are freshwater, free-living, small-sized, and primitive forms.
• They can have either shield-like, bivalved shell-like, or no carapace.
• Their thoracic appendages are flattened, lobed, and leaf-like, serving for respiration, locomotion, and filter-feeding.
• There are no appendages in the stomach, but they bear a pair of caudal styles.
• Antennules and 2nd maxillae are reduced or absent.
• They commonly undergo parthenogenesis, and their larval stage is known as nauplius.
• Branchiopoda includes several orders, such as Anostraca, Notostraca, and Diplostraca.

Class 1: Crustacea - Subclass 3: Ostracoda

• Ostracoda are small crustaceans found in both freshwater and seawater.
• They have a poorly segmented body enclosed in a bivalved carapace.
• Two pairs of stout and cylindrical thoracic appendages are used for swimming.
• Their mandibles have palps, and they often reproduce through parthenogenesis.
• Ostracoda are commonly called "minute mussels" or "seed shrimps" and include several orders: Myodocopa, Podocopa, Platycopa, and Cladocopa.

Class 1: Crustacea - Subclass 4: Mystacocarida

• Mystacocarida are small-sized crustaceans with an elongated body divisible into the head, 5-segmented thorax, and 6-segmented abdomen.
• They have well-developed head appendages, including the 1st thoracic pair (Maxillipedes).
• A single median eye is present, and there are no compound eyes.
• Mystacocarida live in marine and brackish water.
• They are filter-feeders and burrowers, commonly found in sandy substrates.

Class 2: Hexapoda (Insects) - A Closer Look

• Hexapoda comprises terrestrial insects.
• The body is divided into head, thorax, and abdomen.
• They have 1 pair of antennae, 1 pair of mandibles, and 2 pairs of maxillae.
• The head often bears compound eyes.
• Respiration occurs through tracheae.
• Excretion is managed by Malpighian tubules.
• Hexapods are usually sexually dimorphic, and their development includes egg, nymph, and adult stages.
• The subclass Insecta includes a vast diversity of insects, such as Diptera (flies), Coleoptera (beetles), Lepidoptera (butterflies and moths), Hymenoptera (ants, bees, and wasps), Orthoptera (grasshoppers and crickets), and more.

Class 3: Myriapoda - A Closer Look

• Myriapoda includes millipedes and centipedes.
• Their body is divided into head, trunk, and abdomen.
• The head has 1 pair of antennae and 1 pair of mandibles.
• Respiration occurs through tracheae.
• They exhibit sexual dimorphism, and development involves egg and juvenile stages.
• Millipedes are herbivorous, while centipedes are carnivorous.

Class 4: Chelicerata - A Closer Look

• Chelicerata encompasses arachnids, horseshoe crabs, and sea spiders.
• The body is divided into cephalothorax and abdomen.
• The head has 1 pair of chelicerae, 1 pair of pedipalps, and 4 pairs of walking legs.
• Respiration occurs through book lungs, tracheae, or gills, depending on the group.
• Excretion is managed by Malpighian tubules.
• Sexual dimorphism is common.
• Chelicerata includes various orders like Araneae (spiders), Scorpiones (scorpions), Opiliones (harvestmen), and more.

Class 5: Pycnogonida (Sea Spiders) - A Closer Look

• Pycnogonida are marine arthropods.
• They have a long, slender body with a reduced abdomen.
• The head bears a proboscis, a pair of chelifores (chelicerae), and 4 pairs of leg-like appendages.
• Respiration occurs through the body surface or through tubular extensions of the coxal plates.
• Excretion is managed by coxal glands.
• Pycnogonida exhibit sexual dimorphism and external fertilization.

This overview provides a glimpse of the diverse arthropods within Subphylum 3: Mandibulata, highlighting the distinct features and classes within this taxonomic group. Each class has its unique characteristics, adaptations, and ecological roles in terrestrial and aquatic ecosystems.