482 



COMPARATIVE ANATOMY 



CHAP. 



t-e -- 



of wings in the larvse. The stigmatic strands belonging to these parts are present 

 as rudiments. The tracheal system of these larvse is peripneustic. 



4. The tracheal system of peripneustic larvse may be modified in various ways by 

 adaptation to different modes of life : (a) it may become apneustic in larvse inhabit- 

 ing water, as in the larvse of the Phryganidce and Sialidce, which breathe through 

 tracheal gills, (b) By adaptation to life in water or parasitic life all the stigmata 

 may remain closed in the larvse except the last pair. . The tracheal 

 system is then called metapneustic. The larvse then obtain air 

 at the surface of the water or of the host, by means of this 

 posteriorly placed pair of stigmata, which is often elongated 

 like a siphon, or provided with other suitable structural adapta- 

 tions. The larvse of the water beetle and of many Diptera, 

 which are aquatic or parasitic, are metapneustic. (c) There is 

 occasionally besides the posterior an anterior prothoracic open 

 pair of stigmata (Fig. 341). This amphipneustic tracheal system 

 is found in many parasitic or half - parasitic Diptera larvse 

 (Oestridce, Asilidce), which stretch only their anterior and 

 posterior ends beyond the medium which surrounds the rest of 

 the body. The larval stigmata of the meta- and amphipneustic 

 fly larvse disappear during metamorphosis. 



In all cases where the larva is not holopneustic, the stigmatic 

 branches of the tracheal system are present as rudiments. "We 

 must distinguish between such first rudiments remaining latent 

 during the larval period, and those rudiments of stigmatic 

 branches which are found in the imagines of the various Insecta. 

 The latter are the remains of stigmata which have disappeared. 

 Several pairs of such stigmata are often found in the abdomen. 



The peculiar arrangements of the tracheal system in insect 

 larvse show very clearly to what an extent special conditions of 

 existence may influence the organisation of free-living larvse. 



FIG. 341. -Right B. The Tracheal Gills (Figs. 342 and 343). 



side of the tracheal 



system of a fly mag- Something has already been said about these 

 got, seen from the respiratory organs of aquatic insect larvse in the 

 stigma ;!'*, interior section on " wings." Tracheal gills, i.e. delicate mem- 

 stigma ti, longitud- branous processes of the body into which trachese 

 inai tracheal trunks. exten( j j are found not only in the larvse of the 

 E ' [jhemeridce, Trichoptera, and Sialidce there mentioned, but also in the 

 larvse of the Plecoptera (Perlidce), Odonata, and the aquatic larvse of a 

 few species of Diptera, Hymenopt&ra, Lepidoptera, and Coleoptera. The 

 tracheal gills of the Odonata are either external (Agrion) in the form of 

 3 branchial leaves on the last abdominal ring, or they are internal 

 (Libellula, jEschna) in the form of folds in the rectum. In the latter 

 case water is alternately drawn in and expelled through the anus. 

 The tracheal gills of the larvse of the Perlidce are very variously 

 formed; they are pouch -shaped or tufted, etc., and occur at very 

 different parts of the body. The same is the case with the tracheal 

 gills which occur singly in the larvse of Diptera, Hymenoptera, Lepidop- 

 tera, and Coleoptera. Larvse which are provided with tracheal gills are 



