The tracheae in Rhodnius and many other insects are parallel-sided tubes which branch in such a way that the cross-section of the diffusion path remains constant. Growth in length results mainly from stretching the trachea in the preceding inotar. Growth in diameter of the tracheae varies with feeding and the organ supplied. The proportions of the tracheal system are maintained by co-ordination of the growth of the main tracheae and new terminal branches. This is brought about by the property of tracheae which enables them to react to stimuli for growth only upon the spiracle side of the point of application. The stimuli for growth probably originate from the tissues and from the nodes. They determine first the diameter of the tracheae farthest away; that is, the tracheae begin to form at the spiracles. The nodes controlling the diameter of the lateral tracheae are influenced by the blood.

Growth in the tracheal system of Tenebrio larvae varies inversely with the oxygen tension both above and below atmospheric.

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