482 



COMPARATIVE ANATOMY 



CHAP. 



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

 as rudiments. The tracheal system of these larva? is peripneustic. 



4. The tracheal system of peripneustic larvae may be modified in various ways 1 >y 

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

 ing water, as in the larvae 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 larvae except the last pair. The tracheal 

 system is then called metapneustic. The larvae 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 larvae of the water beetle and of many Diptera, 

 which are aquatic or parasitic, are metapueustic. (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 larva 5 

 (Ocstridce., Asilidcc), 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 larvae 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 

 larvae show very clearly to what an extent special conditions of 



existence may influence the organisation of free-living larva?. 



ft) 



FIG. 341. -Right B. The Traeheal 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 res pi r atory organs of aquatic insect larva? in the 



si dp i'^ A. uteri or 



stigma; ;/, posterior section on " wings." Tracheal gills, i.e. delicate mem- 

 stigma ; a, longitud- branous processes of the body into which tracheae 



inal tracheal trunks. extend) are found not on]y in the krva3 of the 



Efjhem&ridce, Trichoptera, and Sialidce there mentioned, but also in the 

 larvae of the Plecoptera (Perlidce), Odonata, and the aquatic larva? of a 

 few species of Diptera, Hymenoptera, Lepidoptera, and Coleoptera. The 

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

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

 (Libdlula, jtEschiut) 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 larva? of the Pcrlidce 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 larva? of Diptera, Hymenoptera, Lepidop- 

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



