Tracheole

Abstract: 1. The life-history of Phaenoserphus viator is described.Four larval instars are found, endoparasitic in the larvae of Pterostichus niger. At thee nd of the last larval instar the parasites, which may number as many as 45 in a single host, emerge, and while still attached, pupate without spinning a cocoon.Adults may appear in August or September.The effect of the parasite in inhibiting metamorphosis of the host is discussed.2. The first observed larva is atracheate and incompletely segmented at first and is of the polypod type bearing paired prolegs on the body segments.Subsequent instars are apodate.The tracheal system develops progressively in the several instars, but only becomes functional in the final stage.3. The anatomy of the larva is briefly described with the exception of the musculature.Tracheal development is described. Gas only appears in the tracheae after the development of the tracheole cells puts the tracheae into communication with the body wall and other organs.In the circulatory system an important accessory organ is the neural sinus, formed by the enclosure of the ventral nerve cord beneath a connective tissue curtain.The imaginal discs of the hypodermis are briefly described, these being clearly defined in the head, thorax, and posterior abdominal segments.The nervous system consists of a brain, suboesophageal ganglion and 11 ventral ganglia, the most posterior being tripartite. This system is connected with the sympathetic, by nerves passing from the cerebral commissures to a frontal ganglion which lies above the oesophagus and behind the labrum.

Abstract: The mechanism by which the tracheal system becomes filled with air is reviewed. It is concluded that the fluid contents are actively absorbed by the walls of the system. The air often enters from the atmosphere through the spiracles. In some insects the system fills while the insect is submerged in water or while the spiracles are closed. It is shown by means of simple models how the tanning of the lining of the larger tracheae or the secretion of wax over the walls will bring about the liberation of gas from solution when the fluid is subjected to a very slight negative pressure. The movements of fluid in the tracheole endings of insects are also reviewed. The removal of fluid which takes place during activity, particularly under conditions of deficient oxygen supply, is not caused by secretory activity but by the physical forces produced by the products of metabolism. The evidence supports the view that the fluid in the tracheoles is a cell sap whose passage up the tracheole under the action of capillarity is opposed by the elasticity or swelling pressure of the cytoplasmic sheath of the tracheoles. It is shown by means of a simple gelatin model how osmotic changes in the surrounding fluid, acting by way of a cytoplasmic sheath, will bring about the absorption of such a cell sap. A more exact model, illustrating the greater permeability of the inner wall of the tracheole which the proposed mechanism requires, is provided by the anal papillae of mosquito larvae. These structures show the same adaptation to a saline medium as is postulated in the tracheoles of mosquito larvae from salt water. Alternative mechanisms are discussed. It is suggested that the capillary forces in the tracheoles are probably small. The effects of probable contamination of the tracheole fluid by an oily film (Beament) and the effects of changes in hydrogen-ion concentration on interfacial tensions are illustrated by reference to further models.

Abstract: Epidermal cells deprived of their oxygen supply by tracheal section give off cytoplasmic processes which become attached to air-filled tracheoles in neighbouring areas and draw these into the oxygen-deficient zone. Many of these cytoplasmic strands exceed 100 micrometer in length but may be no more than 50 nm in diameter; they contain mitochondria, ribosomes, microtubules and microfilaments. In basis structure they resemble the tendon cells; and also the tapering conical epidermal cells along the intersegmental borders of the abdomen, the terminal strands of which are inserted into the basement memebrane behind and in front of the segmental boundaries. The cytoplasmic walls of those tracheoles most exposed to tension during the process of tracheole capture become thickened and packed with microtubules. In all these structures the microtubules are believed to be concerned in resistance to tension. Contraction is presumably effected by microfilaments, but no new evidence is given. The possible role of the epidermal strands in the transport of energy-rich metabolites is discussed.

Abstract: The Drosophila trachea, as the functional equivalent of mammalian blood vessels, senses hypoxia and oxygenates the body. Here, we show that the adult intestinal tracheae are dynamic and respond to enteric infection, oxidative agents and tumours with increased terminal branching. Increased tracheation is necessary for efficient damage-induced intestinal stem cell (ISC)-mediated regeneration and is sufficient to drive ISC proliferation in undamaged intestines. Gut damage or tumours induce HIF-1α (Sima in Drosophila), which stimulates tracheole branching via the FGF (Branchless (Bnl))-FGFR (Breathless (Btl)) signalling cascade. Bnl-Btl signalling is required in the intestinal epithelium and the trachea for efficient damage-induced tracheal remodelling and ISC proliferation. Chemical or Pseudomonas-generated reactive oxygen species directly affect the trachea and are necessary for branching and intestinal regeneration. Similarly, tracheole branching and the resulting increase in oxygenation are essential for intestinal tumour growth. We have identified a mechanism of tracheal-intestinal tissue communication, whereby damage and tumours induce neo-tracheogenesis in Drosophila, a process reminiscent of cancer-induced neoangiogenesis in mammals.