TEACHE/E OF AETHEOPODA. 289 



anastomoses provides for the equal distribution of the respiratory 

 medium. When the number of stigmata is reduced the longitudinal 

 trunks become of great physiological importance, for they give off 

 tracheal branches to those portions of the body in which there are 

 no stigmata. The above-mentioned tracheal vesicles may be 

 developed on the principal trunks, as well as on their branches and 

 twigs ; their development is correlated with the development of the 

 power of flight. A very large number of them may be found in the 

 Coleoptera (Lamellicornes) ; in the Lepidoptera, Hymen optera, and 

 Diptera they are not so numerous, but are larger ; in the Diptera 

 they are sometimes represented by a large pair of vesicles, which 

 almost fills up the abdomen. 



As the tracheal system is developed in correlation with aerial 

 respiration, and therefore with a non-aquatic habitat, the modifica- 

 tions which are caused by the aquatic habitat of the larval or adult 

 stages of many Insects, must be regarded as secondary arrange- 

 ments. They are adapted to their altered mode of life. Thus in 

 the larvae of many Diptera there is but a single pair of stigmata, 

 which are placed in the hinder end of the body (Corethra). A still 

 further adaptation in the aquatic Hemiptera (Nepa, Ranatra) is the 

 respiratory tube, which projects from the abdomen. 



§ 224. 



When Insects are most completely adapted to an aquatic habitat, 

 all the stigmata, and the tracheal trunks from them, are atrophied. 

 This leads to the formation of the closed tracheal system, which 

 distinguishes the larva3 of the Pseudoneuroptera. The longitudinal 

 trunks, which are also present in the open tracheal system, form the 

 chief part of the apparatus. They give off branches to the viscera 

 (enteron), as well as to the wall of the body. In both parts they 

 give rise to the development of organs, in which the exchange of 

 gases is effected. The relations between this closed tracheal system 

 and the open one are explained by the presence of chords, which 

 connect the longitudinal trunks with the body-wall, and are inserted 

 at the very points where stigmata are found later on. The chords 

 therefore appear to be obliterated tracheal trunks. And this view 

 is confirmed by the fact that when the larvae undergoes ecdysis, the 

 intima of a portion of the tracheal system is also cast off by means 

 of some of these chords and is found in the exuviae (Ephemcrida, 

 Perlida). Part of these chords are cast off again in the last moult, 

 and form an open tracheal system by giving rise to a stigma at their 

 point of junction with the skin. 



The tracheae, which branch in the integument, allow of a dermal 

 respiration during the closed condition of the apparatus (many 

 Perlida). This determines the development of superficial growths, 

 which lead to the formations of processes, in which a large number 

 of tracheae ramify (Tracheal gills, cf. § 190). These organs 



u 



