THE CLOSED TR AC HEAL SYSTEM 



459 



h. The closed or partly closed tracheal system 



There are two chief morphological tracheal systems : 1. The open 

 or normal and primitive (holopneustic) type, and 2. The closed, or 

 secondary and adaptive, i.e. apneustic, type. The open system is 

 characterized by the presence of the stigmata. Through them the 

 air directly enters into the tracheal tubes, whose delicate walls allow 

 the exchange of gases in the 

 blood. This type occurs in all 

 sexually mature individuals, 

 and also in the greater number 

 of larvae. 



The closed or apneustic 

 tracheal system is distinguished 

 either by the want of stigmata, 

 or, if present, they are not open, 

 and do not function, so that the 

 tracheae cannot communicate 

 with the air. In such cases the 

 direct oxygenation of the blood 

 is effected through the delicate 

 integument, especially over the 

 surface of the body in general, 

 or in certain specialized places 

 where the gill-like expansions 

 of the skin are rich in tracheae ; 

 such outgrowths, generally 

 tubular or leaf-like, are called 

 by Palmen trachea! gills. 



This closed form of the 

 tracheal system only occurs in 

 the larval stage of aquatic or 

 parasitic insects, as in the 

 Plectoptera (Ephemeridae) , Perlidae, Odonata, and Trichoptera, 

 besides single genera of other orders, i.e. among Coleoptera, Gyrinus, 

 Pelobius, Cnemidotus, and the young larva of Elmis ; in the aquatic 

 caterpillar of Paraponyx ; in certain Diptera (Corethra, Chironomus, 

 etc.), and some of the parasitic Hymenoptera (Microgaster). 



Palmen has discovered that in the nymphs of Ephemeridae, Perli- 

 dae, Odonata, and the larvae of most Trichoptera the tracheal branch 

 (stigmatal branch) sent from the longitudinal trachea to where the 

 thoracic stigmata would be situated if present, or where their vestiges 



FIG. 427. The lateral and lower series of sacs 

 ofHombtis terrestriz, tf: a, c, longitudinal tracheae, 

 connected by 6, and dilated at/, and again in the 

 succeeding segments ; i, k, funnel-shaped dilatations 

 passing over the dorsal surface of the abdomen and 

 anastomosing (g) with their fellows opposite ; at I, 

 communicating directly by a large branch. After 

 Newport. 



