Insect’s Respiratory System

All insects are aerobic organisms. They must obtain oxygen (O₂) from their environment in order to survive. They use the same metabolic reactions as other animals to convert nutrients into the chemical bond energy of ATP.  

The respiratory system is responsible for delivering sufficient oxygen to all cells of the body and for removing carbon dioxide (CO₂) that is produced as a waste product of cellular respiration. The respiratory system of insects is a complex network of tubes, called a tracheal system that delivers oxygen-containing air to every cell of the body. The respiratory system of insects that consists of tracheae and their branches in a complicated form is known as tracheal system. 

The tracheal system of insect consists of following components:

(1) Spiracle

(2) Tracheae

(3) Tracheole cell

(4) Tracheole

1. Spiracles

These are the external openings of the tracheal system. They are lateral in position, usually on the pleura and there is never more than one pair of spiracles on a segment. Each spiracle is slit-like ap­erture in an oval sclerotic area guarded by bristles or hairs to prevent the passage of dirt. Often the spiracle is bordered by an annular sclerite called peritreme. The spiracles are opened and closed by valves regulated by sphincter or spiracular muscles.

Number and distribution of spiracles

The largest number of spiracles found in insects is ten pairs, two thoracic and eight abdominal. Keilin (1944) classified respiratory system of insects on the basis of the number and distribution of the functional spiracle as mention below:

I. Polyneustic Respiratory System: In this case there are at least 8 pairs of functional spiracles.

(i) Holopneustic: Spiracles 10 pairs with 1 mesothoracic, 1 metathoracic and 8 abdominal. Found in bibinoid larvae.

(ii) Perineustic: Spiracles 9 pairs with 1 mesothoracic and 8 abdominal. Found in cecidomyid larvae.

(iii) Hemineustic: Spiracles 8 pairs with 1 mesothoracic and 7 abdominal. Found in mycetophilid larvae.

II. Oligoneustic Respiratory System: In this case there are 1 or 2 pairs of functional spiracles.

(i) Amphipneustic: Spiracles 2 pairs with 1 mesothoracic and 1 post-abdominal. Found in psychodid larvae

(ii) Metapneustic: With 1 pair of post-abdominal spiracle. Found in culicid larvae.

(iii) Propneustic: With 1 pair of mesothoracic spiracle. Found in dipterous larvae.

III. Apneustic Respiratory System: In this case there are no functional spiracles i.e., the tracheal system does not open to the outside. Found in chironomid larvae.

2. Tracheae

 The tracheae are the larger tubes of the tracheal system, running inwards from the spiracles and usually breaking up into finer branches. These are formed by the invaginations of the ectoderm and form a network throughout the body.  The wall of tracheae consists of three layers, viz., outer epidermal layer basement membrane, middle layer epithelium and the inner cuticular layer intima. Air passes through the lumen of tracheae. The intima of tracheae at distinct interval form spiral or ring-like thickenings around lumen are called ctenidia which prevent the tracheae from collapsing. Tracheae transport respiratory gases throughout the body.

In the majority of insects the tracheae are distributed networkly in the body and some of them are arranged in longitudinal and transverse manner. These are called tracheal trunks. There are three pairs of large longitudinal tracheal trunks-one dorsal, one ventral and one lateral in position. All these trunks are connected by transverse trunks, thus, they anastomose to form a network which reaches to every part of the body. The smaller branches of tracheae arises from the tracheal trunks exten to the various tissues and in turn give rise to the tracheoles which run to the cells.

The arrangement of the tracheal system varies between different insects, but in general the heart and dorsal muscles are supplied by branches from the dorsal trunks, the alimentary canal, gonads, legs and wings from the lateral trunks and the central nervous system from the ventral trunks or transverse trunks.

3. Tracheoblast

The ultimate branches of tracheae end in polygonal cells called trcheoblasts. These are derived from the epidermal cells lining the trachea. Like other animal cells these cells comprise of plasmalema, cytoplasm, nucleous and other cellular organelles. These cells form tracheoles.

4. Tracheoles

 These are very fine tubes arise from tracheal cells. The tracheoles have thinned cuticle lining and they end blindly in the tissue cells. Proximally the tracheoles are about 1mm in diameter, tapering to about 0.1mm. The intema of tracheoles is some 16-20 nm thick and may consist only of outer epicuticle. Tracheoles ar very intimately associated with the tissues. In a resting insect when respiratory activity is not high, the tracheoles are filled, not with air but with a tissue fluid of cells in which oxygen dissolves. By means of this system of tracheae the cells of the body or their fluids are in direct communication with the environmental air.

5. Air sacs

The absence of taenidia in certain parts of the tracheal system allows the formation of collapsible air sacs, balloon-like structures that may store a reserve of air. In dry terrestrial environments, this temporary air supply allows an insect to conserve water by closing its spiracles during periods of high evaporative stress. Aquatic insects consume the stored air while under water or use it to regulate buoyancy. During a molt, air sacs fill and enlarge as the insect breaks free of the old exoskeleton and expands a new one.   Between molts, the air sacs provide room for new growth, shrinking in volume as they are compressed by expansion of internal organs

Mechanism of respiration

Due to ab­sence of respiratory pigment in the blood respiratory gases come to direct contact of cells through different parts of tracheal system. Inspiration and expiration take place through the spiracles.  Expiration is an active process but inspiration is passive. In cockroach the first thoracic and first abdominal spiracles remain open all the time, but the second thoracic and last seven abdominal spiracles open during inspiration and close during expiration. 

(a) Inspiration: Air enters the spiracles during inspiration and comes to the tracheae, then it comes to the tracheoles which contain fluids, the O₂ gets dissolved in these fluids and reaches the tissue cells. Opening of spiracles and subsequent diffusion of air occur due to the stimulation of spiracles by carbon dioxide.

 (b) Expiration: In expiration some carbon dioxide may pass out through the spiracles but the major part of it diffuses out through the cuticular covering of the body. Carbon dioxide also dissolves in the plasma and reaches the body surface which is permeable to gases and allows carbon dioxide to pass out. 

When active movement takes place, as in running or flying, the metabolic rate is high and the osmotic pressure of the tissues increases, as a result of these the fluid is withdrawn from the tracheoles into the body cells. This withdrawal makes it possible for a column of air to extend deeply into tracheoles and directly reach the cells, and O₂ is taken by the fluid of the cells. In active movement the abdominal segments expand and relax, these movements are termed respiratory movements and they cause more air to be taken in through the spiracles.

Respiratory movements are co-ordinated by nerves in each segment, but these nerves receive impulses from thoracic ganglia which exercise a controlling influence over all respiratory activities. The co-ordinating centres in thoracic ganglia are stimulated and respond lack of oxygen and also to an excess of carbon dioxide.

 

Other functions of tracheal system

Apart from respiration the tracheal system has a number of other functions as-

(i) The whole system lowers the specific gravity of the insect.

(ii) In aquatic insects it also gives some degree of bouncy and in the larvae of Chaoborus (Diptera) the tracheae form hydrostatic organs enabling the bouncy to be adjusted.

(iii) Air sacs allow for the growth of organs within the body without any marked changes  in the body form.

(iv) Air sacs also permit changes in gut volume as a result of feeding.

(v) In some noctids (Lepidoptera) tracheae form a reflecting tapetum beneath the eye.

(vi) Expansion of the tracheal system may assist in inflation of the insect after a moult.

(vii) Some insects, such as Aeschna (Odonata), have an extensive development of air sacs round the pterothoracic musculature which probably serve an insulating function, helping to maintain the temperature of the flight muscles.

(viii) An important general function of tracheae and tracheoblasts is in acting as connective tissue, binding other organs together.

(ix) The tracheal system may be involved in defense mechanisms. In cockroache Diploptera quinones which probably have a defensive function can be forcibly expelled from the second abdominal spiracle.