Arachnids – Characteristics, Classification, & Physiology of Arachnid

Overview

The scientific study of spiders, scorpions, and pseudoscorpions is called arachnology. Arachnids are the vast class of phylum Arthropoda.
They can live and thrive in almost all places from peak of mountains to the depth of caves.

They can also be found in damp marshes and deserts. Some arachnids also live on the surface of freshwater lakes. Arachnids are carnivorous.
They prey upon other arthropods mainly insects but some arachnids also eat small vertebrates and rodents.

Like all arthropods, these have segmented bodies with tough exoskeletons and jointed appendages.
Most abundant of the arachnids are ticks and mites found in soil, fresh and marine water, and as parasites of animals including humans.

Arachnids

Arachnids is the large Class of arthropods that includes over 60,000 described species (and more than likely a large number of so far undescribed ones). Spiders comprise most of these (over 50,000 known species); with termites and ticks next largest (around 48,200 species).

The Arachnida likewise consists of a varied variety of smaller sized groups, including scorpions (1200 species), whip scorpions (100 species), palpigrades (60 species), pseudoscorpions (2000 species), solpugids (900 species), and harvestmen (5000 species). Nearly all species are terrestrial. Very few of particularly Mesostigmata, Sarcoptiformes, and Trombidiformes– are the only chelicerates habitually found around freshwater habitats. Among these, members of just a couple of groups of Sarcoptiformes and Trombidiformes are truly aquatic.

Characteristics of Arachnid

To be categorized in the class Arachnida, an arthropod should have the following attributes:

  • Arachnid bodies are generally divided into two unique regions, the cephalothorax (anterior) and the abdominal area (posterior).
  • Adult arachnids have 4 pairs of legs, which attach to the cephalothorax. In immature stages, the arachnid may not have 4 sets of legs (e.g., termites).
  • Arachnids do not have both wings and antennae.
  • Arachnids have actually simple eyes called ocelli. Most arachnids can detect light or its lack, however, do not see detailed images.

Characteristics-of-Ar

Arachnids belong to the subphylum Chelicerata. Chelicerates, consisting of all arachnids, share the following qualities:

  • They lack antennae.
  • Chelicerates typically have six sets of appendages.
  • The very first set of appendages are the “chelicerae,” likewise known as the fangs. The chelicerae are present in front of the mouthparts and look like modified pincers. The 2nd set is the “pedipalps,” which operate as sensory organs in spiders and as pincers in scorpions. The remaining 4 pairs are the walking legs.

Although we tend to think of arachnids as being carefully related to insects, their nearest relatives are horseshoe crabs and sea spiders. Like arachnids, these marine arthropods possess chelicerae and come from the subphylum Chelicerata.

Subphylum Chelicerata

Freshwater arachnids represent a small portion of Chelicerata, among the most diverse of the living arthropod subphyla with about 100,000 known species. Their unique mouthparts (paired chelicerae and pedipalps) and 4 sets of walking legs separate arachnids from all other arthropods.

Although a lot of arachnids and other chelicerates populate terrestrial ecosystems, they are a crucial, though largely overlooked group of freshwater invertebrates with about 5000 described species around the world– all but a handful of which are mites. Water mites inhabit most places on all continents except Antarctica.

Classification of Arachnids

The class Arachnida is divided into orders and subclasses, arranged by typical qualities. These include:

  • Order Amblypygi– tailless whip scorpions
  • Order Araneae– spiders
  • Order Uropygi– whip scorpions
  • Order Opiliones– harvestmen
  • Order Pseudoscorpiones– pseudoscorpions
  • Order Schizmoda– short-tailed whip scorpions
  • Order Scorpiones– scorpions
  • Order Solifugae– wind scorpions
  • Order Acari– ticks and termites

Order Scorpiones

Scorpions are fairly big arachnids. Like other arachnids, their body consists of a cephalothorax, which bears the mouthparts and legs, and an abdomen. They are defined by having four pairs of legs, big claw-like pedipalps, and a five-segmented abdomen with a sting on the distal end.

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Order-Scorpiones

Order Araneae

The spiders are characterized by having four sets of legs and the cephalothorax and abdomen (usually unsegmented) joined by a narrow pedicel. The pedipalps of some spiders might appear leg-like. Many spiders have eight simple eyes (ocelli) on the cephalothorax.

There are roughly 3,700 species of Araneae in The United States and Canada and more than 38,000 worldwide. Although nearly all spiders have venom glands, only a few species are thought-about hazardous, such as the black widow and the brown recluse. The cephalothorax consists of the eyes, mouthparts, legs, and stomach. The abdominal area houses the primary reproductive structures, respiratory system, intestinal tract, silk glands, and spinnerets, from which the silk is spun.

Order-Araneae

Order Acari

The ticks and termites are a large group of small arachnids. The body is generally oval with little distinction in between the cephalothorax and abdomen, which are broadly signed up with. The larvae have only three pairs of legs and get an extra set after the very first molt.

Ticks and termites occur just about anywhere animal life is discovered. They may be terrestrial or aquatic, free-living or parasitic. Some feed on organic debris, while others live on plants or animals. On plants, they might become major pests while parasitic forms can be pests of humans and other animals. With over 30,000 explained species of ticks and termites, this is the biggest order within the class Arachnida.

Order-Acari

Physiology of Arachnids

Arachnids have a pair of tagmata called a prosoma and opisthosoma. The prosoma is partially or totally covered with a carapace-like guard. The opisthosoma might be segmented or unsegmented. The appendages on the opisthosoma are missing or modified, being utilized as spinnerets (spiders) or pectines (probably sensory in function, found in scorpions). Respiration is through the tracheae or book lungs; it is cutaneous in many small arachnids.

Multiple Choice Questions (MCQs) with Answers: Arachnids

  1. What is the scientific study of spiders, scorpions, and pseudoscorpions called?
    • A) Entomology
    • B) Arachnology
    • C) Mycology
    • D) Herpetology

    Answer: B) Arachnology

  2. How many described species are there in the class Arachnida?
    • A) Over 100,000
    • B) Over 50,000
    • C) Over 60,000
    • D) Over 30,000

    Answer: C) Over 60,000

  3. In which subphylum do arachnids belong?
    • A) Crustacea
    • B) Chelicerata
    • C) Hexapoda
    • D) Myriapoda

    Answer: B) Chelicerata

  4. What distinguishes arachnids from other arthropods in terms of appendages?
    • A) Antennae
    • B) Chelicerae and pedipalps
    • C) Wings
    • D) Antennae and wings

    Answer: B) Chelicerae and pedipalps

  5. How many pairs of legs do adult arachnids typically have?
    • A) 2
    • B) 6
    • C) 8
    • D) 4

    Answer: C) 8

  6. Which order includes tailless whip scorpions?
    • A) Order Uropygi
    • B) Order Scorpiones
    • C) Order Araneae
    • D) Order Solifugae

    Answer: A) Order Uropygi

  7. What is the function of pedipalps in scorpions?
    • A) Walking legs
    • B) Sensory organs
    • C) Pincers
    • D) Spinnerets

    Answer: C) Pincers

  8. Which order includes spiders?
    • A) Order Uropygi
    • B) Order Scorpiones
    • C) Order Araneae
    • D) Order Solifugae

    Answer: C) Order Araneae

  9. How many species of Araneae are estimated to be worldwide?
    • A) Over 50,000
    • B) Over 3,700
    • C) Over 1200
    • D) Over 900

    Answer: A) Over 50,000

  10. What is the distinctive feature of the opisthosoma in ticks and termites?
    • A) Segmentation
    • B) Spinnerets
    • C) Pectines
    • D) Broadly joined cephalothorax and abdomen

    Answer: D) Broadly joined cephalothorax and abdomen

  11. Where are the respiratory structures located in arachnids?
    • A) Book lungs
    • B) Tracheae
    • C) Spinnerets
    • D) Pectines

    Answer: A) Book lungs

  12. Which arachnid order includes pseudoscorpions?
    • A) Order Solifugae
    • B) Order Pseudoscorpiones
    • C) Order Araneae
    • D) Order Amblypygi

    Answer: B) Order Pseudoscorpiones

  13. What is the primary habitat of freshwater arachnids?
    • A) Deserts
    • B) Caves
    • C) Freshwater lakes
    • D) Mountains

    Answer: C) Freshwater lakes

  14. Which arachnid order includes scorpions?
    • A) Order Scorpiones
    • B) Order Araneae
    • C) Order Solifugae
    • D) Order Amblypygi

    Answer: A) Order Scorpiones

  15. What is the primary diet of arachnids?
    • A) Plants
    • B) Other arachnids
    • C) Insects and small vertebrates
    • D) Algae

    Answer: C) Insects and small vertebrates

  16. Which subphylum do horseshoe crabs and sea spiders belong to?
    • A) Chelicerata
    • B) Hexapoda
    • C) Myriapoda
    • D) Crustacea

    Answer: A) Chelicerata

  17. How many pairs of walking legs do scorpions typically have?
    • A) 2
    • B) 4
    • C) 6
    • D) 8

    Answer: B) 4

  18. What is the primary role of the chelicerae in arachnids?
    • A) Walking
    • B) Sensory perception
    • C) Preying and defense
    • D) Respiration

    Answer: C) Preying and defense

  19. What is the primary function of spinnerets in spiders?
    • A) Sensory perception
    • B) Walking
    • C) Producing silk
    • D) Preying

    Answer: C) Producing silk

  20. Where are the ocelli located in arachnids?
    • A) Abdomen
    • B) Pedipalps
    • C) Cephalothorax
    • D) Spinnerets

    Answer: C) Cephalothorax

  21. How many pairs of walking legs do pseudoscorpions typically have?
    • A) 2
    • B) 4
    • C) 6
    • D) 8

    Answer: B) 4

  22. Which order includes wind scorpions?
    • A) Order Solifugae
    • B) Order Scorpiones
    • C) Order Amblypygi
    • D) Order Araneae

    Answer: A) Order Solifugae

  23. What is the distinguishing feature of the cephalothorax in spiders?
    • A) Eyes, mouthparts, legs, and spinnerets
    • B) Pedipalps
    • C) Fangs
    • D) Abdomen

    Answer: A) Eyes, mouthparts, legs, and spinnerets

  24. Where are the pedipalps located in arachnids?
    • A) Abdomen
    • B) Cephalothorax
    • C) Legs
    • D) Spinnerets

    Answer: B) Cephalothorax

  25. Which order includes harvestmen?
    • A) Order Uropygi
    • B) Order Araneae
    • C) Order Opiliones
    • D) Order Scorpiones

    Answer: C) Order Opiliones

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Frequently Asked Questions (FAQs) – Arachnids

  1. What is arachnology, and what organisms does it study?
    • Arachnology is the scientific study of spiders, scorpions, and pseudoscorpions.
  2. How diverse is the class Arachnida in terms of species?
    • The class Arachnida is vast, with over 60,000 described species, including spiders, scorpions, termites, ticks, and more.
  3. Where can arachnids be found in terms of habitats?
    • Arachnids can thrive in various environments, from mountain peaks to caves, damp marshes to dry deserts. They are also found on the surface of freshwater lakes.
  4. What is the primary diet of arachnids?
    • Arachnids are carnivorous, preying mainly on other arthropods such as insects. Some arachnids also feed on small vertebrates and rodents.
  5. How are arachnids classified within the animal kingdom?
    • Arachnids belong to the class Arachnida, which is a part of the phylum Arthropoda.
  6. What are the key characteristics of arachnid bodies?
    • Arachnid bodies are generally divided into two regions: the cephalothorax (anterior) and the abdomen (posterior). They have four pairs of legs, simple eyes (ocelli), and lack both wings and antennae.
  7. What distinguishes arachnids within the subphylum Chelicerata?
    • Arachnids lack antennae, have six sets of appendages, and the first pair are chelicerae (fangs) located in front of the mouthparts. The second pair is the pedipalps.
  8. Are arachnids closely related to insects?
    • Contrary to common perception, arachnids are not closely related to insects. Their nearest relatives are horseshoe crabs and sea spiders, and they all belong to the subphylum Chelicerata.
  9. How do arachnids respire?
    • Arachnids respire through tracheae or book lungs. Cutaneous respiration is common in many small arachnids.
  10. What are the major orders within the class Arachnida?
    • Major orders include Scorpiones (scorpions), Araneae (spiders), Acari (ticks and termites), and more.
  11. Which order includes spiders, and how many species are estimated worldwide?
    • Order Araneae includes spiders, and there are roughly 3,700 species in North America and over 38,000 worldwide.
  12. Do all arachnids have venom glands?
    • While almost all spiders have venom glands, not all arachnids possess them. Some species, like ticks, do not have venomous bites.
  13. What are the distinguishing features of scorpions within the class Arachnida?
    • Scorpions are characterized by four pairs of legs, large claw-like pedipalps, and a five-segmented abdomen with a sting on the distal end.
  14. Where are the pedipalps located in arachnids?
    • Pedipalps are located in the cephalothorax and serve as sensory organs in spiders and as pincers in scorpions.
  15. How are ticks and termites classified within the order Acari?
    • Ticks and termites are part of the large order Acari, characterized by oval bodies with little distinction between the cephalothorax and abdomen.
  16. What is the primary function of spinnerets in spiders?
    • Spinnerets in spiders are used for producing silk, which is essential for various functions, including web-building and prey capture.
  17. Are there freshwater arachnids, and where are they found?
    • Yes, there are freshwater arachnids, though they represent a small portion of Chelicerata. They inhabit freshwater habitats on most continents, except Antarctica.
  18. How many pairs of walking legs do scorpions typically have?
    • Scorpions typically have four pairs of walking legs.
  19. Do arachnids have antennae?
    • No, arachnids lack antennae.
  20. Are all arachnids terrestrial?
    • While most arachnids are terrestrial, there are a few exceptions. Some members of Sarcoptiformes and Trombidiformes are found around freshwater habitats, and water mites are aquatic.

 

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Concluding Arachnids – Characteristics, Classification, & Physiology

The tutorial on arachnids provides a comprehensive exploration of the characteristics, classification, and physiology of these fascinating arthropods.

  1. Overview
    • Arachnology, the scientific study of spiders, scorpions, and pseudoscorpions, delves into the vast class of phylum Arthropoda known as arachnids.
    • Arachnids exhibit remarkable adaptability, thriving in diverse habitats ranging from mountain peaks to caves, damp marshes, and dry deserts.
    • As carnivores, arachnids play a crucial role in ecosystems by preying upon other arthropods, including insects, as well as small vertebrates and rodents.
  2. Arachnids
    • The class Arachnida encompasses over 60,000 described species, including spiders, termites, ticks, scorpions, and various smaller groups.
    • Ticks and mites, abundant in soil, freshwater, and marine environments, are prominent members of this class.
  3. Characteristics of Arachnid
    • Key characteristics include body division into cephalothorax and abdomen, four pairs of legs, absence of wings and antennae, and simple eyes (ocelli).
    • Belonging to the subphylum Chelicerata, arachnids lack antennae, possess six sets of appendages, and feature specialized structures like chelicerae (fangs) and pedipalps.
  4. Subphylum Chelicerata
    • Freshwater arachnids contribute to the global diversity of over 5,000 described species within the diverse Chelicerata subphylum.
  5. Classification of Arachnids
    • Orders include Amblypygi, Araneae (spiders), Opiliones (harvestmen), Scorpiones (scorpions), and Acari (ticks and termites).
    • Distinctive features of scorpions, spiders, and ticks are discussed in detail.
  6. Physiology of Arachnids
    • Arachnids possess tagmata (prosoma and opisthosoma) with appendages serving specialized functions.
    • Respiration occurs through tracheae, book lungs, or cutaneous means.

In summary, the tutorial provides a rich understanding of arachnids, encompassing their diverse characteristics, classification, and physiological adaptations. From their ecological roles to the intricacies of body structures, the tutorial offers a comprehensive overview of these intriguing arthropods.