ITS OUTER SKELETON. 65 



assumed, and which, according to Gegenbaur, was a normal 

 event among primitive Insects, the trachea! gill is supposed to 

 disappear, and in its place, at the next moult, an opening, the 

 stigma, is formed by the rupture of an air-tube. Gegenbaur 

 supposes that the primitive Insects were aquatic, and their 

 tracheal system closed. The tracheal gill he takes to be the 

 common structure which has yielded organs so unlike as the 

 wing and the stigma. 



The zoological rank of the Insects (Ephemeridae, Perlidae, 

 and Libellulidse), in which tracheal gills are most usual, is not 

 unfavourable to such an explanation. Lubbock has given 

 reasons for regarding Campodea and the Collembola (of the 

 order Thysanura) as surviving and not very much altered 

 representatives of the most primitive Insects, and he has shown 

 that no great amount of modification would be required to 

 convert the terrestrial Cnmpodea into the aquatic Chloeon- 

 nymph.* We must not forget, however, that tracheal gills 

 are by no means restricted to these families of low grade. 

 Trichoptera, a few Diptera, two Lepidoptera (Nymphula and 

 Acentropus), and two Coleoptera (Gyrinus and Elmis),^ have 

 tracheal gills, and a closed tracheal system in the larval condi- 

 tion. We cannot suppose that these larvae of higher orders 

 represent an unbroken succession of aquatic forms, but if we 

 refuse to adopt this alternative, we must admit that the closed 

 tracheal system with tracheal gills may be an adaptive modifi- 

 cation of the open system with stigmata. 



It is well known J that in certain Ephemeridoo (e.g., Tricorythus 

 and Ccenis) a pair of anterior tracheal gills may become trans- 

 formed into large plates, which partly protect the gills behind 

 (fig. 31). A similar modification of the second and third 

 thoracic gills in Prosopistoma and B&tisca brings all the 

 functional respiratory organs under cover, and these enlarged 

 plates resemble stiff and simple wings very closely. 



* Palmen cites one striking proof of the low position of Ephemeridce among 

 Insects. Their reproductive outlets are paired and separate, as in Worms and 

 Crustacea. 



f These examples are cited by Palmen. 



% Eaton, Trans. Ent. Soc., 1868, p. 281; Vayssiere, Ann. Sci. Nat., Zool., 1882, 

 p. 91. 



F 



