TYPE TRACHEATA. 



491 



body, two being situated on the sides of the thorax and eight 

 on the abdomen, but in Campodea the number is reduced to 

 three pairs, which occur upon the thorax. In the winged 

 forms the number also varies somewhat, but there are again 

 typically ten pairs, arranged as in the Thysanura. They lead 

 into short trunks, which, in Campodea, ramify through the 

 body without anastomosing, but more usually they are united 

 on each side of the body by a longitudinal tube, from which 

 pass off numerous branches penetrating to all parts of the 

 body, and transverse connecting tubes passing between the 

 systems of the two sides (see Fig. 215). In certain forms 

 which are active flyers the longitudinal tubes are frequently 

 dilated to form air-sacs, as in the 

 Bees, or numerous air-sacs may occur 

 which may be more or less emptied 

 or expanded according to the will of 

 the insect, the specific gravity of the 

 body being thus altered. In the 

 aquatic larvae which occur in some 

 forms, such as the May-flies (Epheme- 

 ridae, Fig. 226), adaptations occur for 

 the breathing of air dissolved in the 

 water, the sides of the body in the 

 abdominal region being prolonged into 

 a number of pairs of platelike process- 

 es, into which branches of the tracheae 

 project, an interchange of the gases 

 contained in the tracheae for those 

 dissolved in the water taking place 



through the walls of the plates, simi- 



111 . , . FIG. 226. LARVA OF AN 



larly to what occurs in the branchiae DI 



' JiiPHEMERIU (horn LEUNIS). 



of the Crustacea, though in these forms a = tracheal bmuchise. 

 the exchange is directly with the gases 



of the blood. These structures are consequently termed 

 tracheal branchiae, and while they are functional, the stigmata 

 are closed, only opening when the adult stage is reached and a 

 terrestrial life adopted. As a rule the tracheal branchia3 are 

 thrown off at the moult bv which the adult form is reached, 



y 



but in a few forms they persist throughout life. 



