Respiration through tracheae is called tracheal respiration. It is seen in insects centipedes, ticks, some mites and spiders. The respiratory pigments are absent in blood because the tracheal system distributes O2 or air directly to the cells.
Tracheal system- Tracheal system consists of 2 components: (1) Tracheae, (2) Spiracles.
Tracheae- Tracheae are system of highly branched chitin lined air tubes throughout the body. They develop due to invagination of body wall and hence lined internally by ectoderm. Histologically it is made up of outer cuticle, inner epithelial cells and a basement membrane.
In thoracic region the main tracheal trunks form the tracheae and tracheoles. In the abdominal region there are 3 pair’s parallel longitudinal tracheal trunks. These are connected by transverse tracheae. Here the tracheae are divided into smaller tracheae and tracheoles. The tracheoles finallu end up in a terminal cell called tracheal end cell. In tracheal end cells, the tracheoles are lined with a layer of protein called trachein. At the terminal end of tracheolar lumen is filled with tracheolar fluid.
Spiracle- The tracheal system opens to outside through slit-like openings called stigmata or spiracles. In cockroach there are two pairs of spiracles in the thoracic region and 8 pairs located in the first abdominal segements. Each spiracle is surrounded by a ring-like sclerite called peritreme. The spiracles are provided with bristles which act as filters and prevent entry of dust and other particles. The spiracles are guarded by valves which are attached to the occlusor and dilator muscles for closing and opening of the aperture.
Mechanism for Gas Exchange in Tracheal System:
(i) At rest, the tracheoles are filled with a fluid by capillary action due to low osmotic pressure in tissue cells.
(ii) The oxygen of incurrent air dissolves in the tracheolar fluid and CO2 is released to air.
(iii) The fluid is absorbed into the tissue due to increased lactate concentration for\during flight.
(iv) The CO2 released during tissue oxidation is stored temporarily as bicarbonates. They act as chemoreceptor and signal in opening of spiracles. But major part of CO2 is released through cuticle.
Ventilation movement- Ventilation of tracheal system is brought about by contraction of the body wall muscles. In insects expiration occurs due to contraction of the dorsoventral tergosternal muscles of the abdomen.Inspiration occurs when the body assumes normal shape.