Arthropoda: Vision and respiration in Arthropods | Zoology Optional Notes for UPSC PDF Download

Aquatic Respiration in Arthropods

Gills or Branchiae

• Occurrence: Gills serve as the primary respiratory organs in aquatic arthropods, with the most developed structures found in crustaceans.
• Location: They are located within the gill chamber, situated on each side of the cephalothorax and covered by the gill cover or branchiostegite.
• Origin of Gills in Crustaceans: Gills typically originate as outgrowths of the body wall, though their origin varies in different crustaceans. In Amphipoda, they develop from thoracic limbs, and in Isopods, the endopodites of the second and fifth pleopods are modified into gills.
• Structure of Gills in Crustacea: A typical gill consists of a crescent-shaped structure with a central axis or rod, and blade-like gill filaments called lamellae arranged on each side. Afferent and efferent branchial channels run through the central axis.

Types of Gills in Crustacea

• A. Based on the shape of the lamellae:

  1. Phyllobranchiate gill: These gills have flat, broad, leaf-like lamellae arranged in two rows, found in crabs and prawns.
  2. Trichobranchiate gill: Gills with tubular-shaped filaments, common in crayfish and rock lobsters.
  3. Dendrobranchiate gill: Gills with branched filaments, found in Penaeus.

• B. Based on the mode of attachment, gills may be of three types:

  1. Podobranch: Attached to the coxopodite of thoracic appendages (e.g., Macrobrachium, Penaeus).
  2. Arthrobranch: Attached to the arthroidal membrane connecting thoracic appendages (e.g., Palaemon, Penaeus).
  3. Pleurobranch: Attached to the lateral thoracic wall (e.g., Palaemon, Penaeus).

Number of Gills in Crustacea: The number of gills varies across different crustaceans. For instance, Lucifer lacks gills, while penaeid shrimp may have 24 gills. The number ranges from 6 to 24 in various decapods.

1. Tracheal Gills

   • Aquatic insect larvae often possess tracheal gills, which are simple and divided external structures attached to abdominal segments. These gills are richly supplied with tracheae and assist in respiration.

2. Blood Gills

   • Some aquatic insect larvae, mainly chironomidae, have branching tubular outgrowths that contain blood vessels, referred to as blood gills. These were previously thought to be respiratory organs, but recent findings indicate that respiration occurs through the entire body surface.

3. Rectal Gills

   • In nymphs of certain insects, gills are present on the inner surface of the rectum and are known as rectal gills.

4. Book Gills

   • Xiphosurids possess specialized plate-like book gills on the abdominal appendages, particularly on segments from the ninth to thirteenth. These gills are formed by the evagination of the posterior borders of the opisthosoma, with each gill containing numerous lamellae resembling the leaves of a book.

Mechanism of Gill Respiration

• In most crustaceans, gills are bathed by a current of water. In decapods, the carapace extends laterally to form a gill chamber, directing the flow of water over the gills. Gaseous exchange between blood and water occurs in the gills. In insects, oxygen diffuses into the tracheal tubes.

Branchial Formula:

• Arthropods often represent the number and arrangement of gills in a branchial formula on each side of the body.

Other Devices of Aquatic Respiration: 

Epipodites: These leaf-like outgrowths on the coxa of the maxillipeds in the first three thoracic segments assist in respiratory functions.

Branchial Basket: Immature Odonates (insects) have a rectum modified into a branchial basket with contractile walls and tracheal branches, facilitating anal respiration.

Aerial Respiration in Arthropods

Aerial respiration is crucial for terrestrial arthropods, and they utilize various organs for this purpose, including:

1. Trachea:

• The primary organ for aerial respiration.
• Present in most land arthropods, including insects, centipedes, millipedes, and many arachnids.
• Two types of tracheae are observed:
  • Ventilation trachea: Oval in section, collapses after exhalation.
  • Diffused trachea: Rigid, does not collapse after exhalation.
• Tracheae originate as invaginations of the body wall.

Structures of Trachea and Associated Parts:

• Tracheae are tubular structures lined with chitin.
• Tracheal walls consist of three layers: the internal layer (intima), a middle layer of epithelium, and an outer layer of basement membrane.
• The intima is lined with spiral cuticular ridges (taenidea) to prevent collapse.
• Tracheal cuticle contains layers similar to the surface cuticle, excluding the cement and wax layers.
• Tracheal openings to the exterior are called spiracles or stigmata, located along the sides of the body.
• Each spiracle opens into a chamber (atrium) and is placed on a plate (penetrene) with two lids for opening and closing.
• Filtering apparatus within the chamber removes foreign particles.
• Some parts of tracheae are dilated to form air-sacs, serving as air reservoirs.
• Finer branches of tracheae, known as tracheoles, lack inner taenidial ridges and reach every cell of the body.
• Tracheoles are permeable to water and remain fluid-filled, facilitating the final oxygen transport to tissues.
• Tracheoles are immersed in a fluid through which gaseous exchange occurs.

Classification of Tracheae

• Tracheae can be classified based on the number of functional spiracles and the presence of spiracles in different stages of development.
• On this basis, they are categorized into:
  • Polypneustic: Having 8 or more pairs of functional spiracles.
    • Subdivided into:
      • Holopneustic: 2 pairs of thoracic and 8 pairs of abdominal spiracles are functional.
      • Peripneustic: 1 thoracic and 8 abdominal spiracles are functional on each side of the body.
      • Hemipneustic: 1 thoracic and 7 abdominal spiracles are functional on each side.
  • Oligoneustic: Having one or two pairs of functional spiracles.
    • Subcategories include:
      • Amphipneustic: One pair of thoracic and one pair of post-abdominal spiracles.
      • Metapneustic: Only one pair of post-abdominal spiracles.
      • Propneustic: Only one pair of thoracic spiracles.
  • Apneustic: No functional spiracles, relying on cutaneous respiration.

Mechanism of Tracheal Respiration:

• Tracheae branch into fine networks of tracheoles that terminate in tissues for gaseous exchange via diffusion.
• Air is drawn in and forced out through the spiracles by the contraction and expansion of the body.
• Spiracles remain mostly closed, with gas exchange primarily occurring through diffusion and ventilation.
• Recent studies suggest brief and sequential opening of spiracles due to a reduction in haemocoelomic pressure.

Other Devices of Aerial Respiration

In addition to tracheae, some arthropods employ alternative respiratory mechanisms:

1. Lungs: In certain Crustacea like Birgus, the upper part of the gill chamber forms a closed chamber with vascular tufts for aerial respiration.
2. Book-lungs: Best observed in Scorpionids and spiders, these are blind sacs originating from opisthosoma evaginations. Their inner lining consists of numerous vascularized folds. Exchange of gases depends on circulation, and book-lungs connect to the exterior via stigmata.
3. Pseudotracheae or Air Tubes: Found in woodlice (Oniscus), these minute tube-like structures in the abdominal appendages assist in respiration.
4. Anal Respiration: Certain crustaceans, like Cyclops, perform rhythmical contractions of the intestine for water intake and expulsion, known as anal respiration.
5. Miscellaneous Devices: Various arthropods use innovative methods, including diving beetles and aquatic bugs that carry air tubes for respiration, and mosquito larvae with a long siphon for drawing air from the water's surface.