We know Arthropoda is the largest phylum, with the highest number of species, and the numbers of individuals are also high. So Arthropods have various types of respiratory systems, some of them described below :
1- Tracheal system
1. Tracheal Respiratory system
It is present mainly in insects, structure, and mechanism of respiration by trachea is described below –
A- Tracheal system
Insects mainly breathe by the tracheal system, consist of elastic air tubes called tracheae, called the tracheal system. The trachea is distributed all over the body like the circulatory system of vertebrates, hence the air is in direct contact with cells and tissue of the body. It means the tracheal system serves for the transport of oxygen and carbon dioxide so there is no respiratory function of blood. So insects can not have the capacity to conduct oxygen by pigments or have a poor capacity for oxygen transport. The tracheal system is consists of the following part.
It is the entry point of air, stigma, or spiracles open side of the body. Spiracles are mainly 10 in number among these 2 are thoracic and 8 are abdominal.
It is an extensive network of branches of tubes, wall of these tubes is made up of three layers, intima, epithelium, and basement membrane.
The branches of the tracheae are called tracheoles.
d- Air sac
In some insects, tracheae are open into air sacs. It can fill and empty by muscular movement
B- Types of the tracheal respiratory system
The tracheal system can be classified as follow –
It is present in most adults and many larval forms. In this type of tracheal system, all spiracles remain open
This type of tracheal system is found in some flies, butterflies, and beetles. In this type of tracheal system, one or a pair of spiracles remain closed.
It is present in endoparasitic insect and aquatic larval forms. In this type of tracheal system, spiracles are lacking.
C. Mechanism of Gaseous exchange
The gaseous exchange takes place directly from tracheoles and air sacs partly by diffusion and partly by ventilation.
2. Blood gills
In some insects, the respiratory organ is blood gills. In this type of gills, tracheae are absent but they contain blood. So-called blood gills.
3. Book lungs
Book lungs are mainly respiratory organs of arachnids such as scorpions, spiders, etc. structure and mechanism of respiration by book lung is described billow.
Cuticle invaginates at the base of rudimentary appendages to form book lungs. Book lungs are mainly present in terrestrial arachnids. Book lungs mainly consist of the proximal or ventral part and dorsal part. The proximal part contains air chambers called atrial chambers, which open outside by the slit-like opening called spiracle or stigma. Stigma is situated ventrolateral side of the sternum. While dorsal part of the book lung is made up of approximately 150 vertical folds known as lamellae, which are arranged parallel as like leaves of the book (page of the book). Hence the name book lungs. Lumens of these lamellae are continuous with the body cavity and filled with blood. There is air space is present between two adjacent lamellae, atrial chamber communicates with the air space of lamellae.
B. Supply of blood in book-lung
Blood enters in book lung from the ventral sinus, which is sent by a diverticulum. And aerated blood from lamellae of book-lung is collected by pulmonary vein.
C. Mechanism of respiration
Inspiration and expiration of air in the book lung are controlled by the action of dorsoventral and atrial muscles. By contraction of these muscles, book-lungs are compressed and air of inter-lamellar spaces is reached into the atrial chamber from there expelled to outside. And when muscles relax the book-lungs come in the normal situation, and air enters into inter-lamellar space. And gaseous exchange takes place between airs of inter-lamellar space and venous blood of lamellae through the lamellar membrane.
Gills are the respiratory organs of some aquatic arthropods such as crustaceans for example Palaemon. The structure and mechanism of respiration by gills is described below –
A. Structure of gills
Gills are somehow crescentic in shape. There are arranged in a manner that each gill is larger than previous gills. Each gill is attached to the wall of the thorax by gill- root at the middle of gills. Two-row gill plates arranged at right angles to the long narrow axis of or gills-base.
B. Mechanism of respiration
A part of the maxilla lies anteriorly inside the chamber of the gill called scaphognathite. Vibrating movement of this scaphognathite expel water from the anterior opening of gills freshwater enters from posterior opening in form of water current. Gills are richly supplied with blood, hence, gaseous exchange takes place easily. Gills plates act as a permeable membrane so gaseous exchange takes place through it by diffusion. Dissolved O2 of water enters into blood and CO2 from blood diffuse into the water.
- A Text-Book of Zoology Invertebrates by R.L. Kotpal tenth edition.
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