|Select image to enlarge in new window|| Tracheal System pages 18-19 | 20-21 | 22 | 23
In the larva, numerous small tracheoles are added to the tracheal system. Several nests of precursor cells of the adult tracheal tree (tracheal imaginal discs or histoblasts, (trhi) are situated along the walls of the larval tracheae. The tracheal histoblasts of the abdomen form cords of cells that link the transverse connectives to the lateral epidermis. These clusters mark the position at which the tracheal placodes had originally invaginated (see above). During late larval and early pupal stages, cells of the tracheal histoblasts proliferate and gradually replace the larval tracheal cells. The tracheal histoblasts also invade the epidermis and form the abdominal spiracles. In the head and thorax, nests of tracheal histoblasts are present along the dorsal cervical trachea (dca, dcp) and along the second dorsal anastomosis (from where tracheal branches associated with the indirect flight musculature [wt] originate).
|The figure of an early pupa (based on Whitten 1980) illustrates the tracheal tree during metamorphosis. In the abdomen, the posterior spiracle and tracheae of A8 have degenerated. The tracheal histoblasts have replaced part of the transverse connectives (tc). From the transverse connectives of A2 and A3, profuse new tracheal branches have grown out toward the gut (it).The former dorsal cervical trachea now rests in the everted head. It circles around the brain hemisphere of either side and produces numerous new branches. The illustration on the facing page (based on Miller 1950) depicts the tracheal system of an adult fly. The pattern of adult abdominal tracheae is basically very similar to the larval pattern (e.g., dorsal trunk [dlt], lateral trunk [lt], transverse connectives [=dorsal segmental tracheae, dsgt], dorsal tracheae [= dorsal commissures, dco]).The main tracheae of the head and thorax have dilated into air sacs. In the head, air sacs (dilations of the former dorsal cervical trachea) are circularly arranged around the brain. In the thorax, two systems of air sacs can be distinguished. The former lateral trunk has formed an interconnected network of superficially located air sacs. Ventrally, the leg tracheae (let) branch off. The former dorsal trunk forms a deep system of air sacs situated between the indirect flight musculature.|