TI 



AXTEXXA TAV'IXGS 



457 



performed before they became exclusively organs of flight is, however, entirely a 

 matter of conjecture. The following view is at present the most acceptable. (1) 

 The ancestors of the Hexapodn were, like the now living Apterygota, wingless land 

 animals breathing through tracheae. (2) The Aptery goto, -like ancestors of the 

 I't'-njcjvtan racial group became adapted to living in water. Dorsal integumental 

 folds served for breathing in the water. The rise of such respiratory folds offers no 

 difficulty, since every increase of surface, small or large, is of service. (3) The 

 p'spiratory appendages (into which tracheae were continued) became movable and 

 may perhaps have assisted in locomotion (swimming). This assumption also offers 

 no difficulty, since the gills of many aquatic animals are movable, and their power 

 of moving is an advantage on account of the exchange of water thus caused. (4) 

 In a new gradual change to land life the respiratory function became less important 

 and the locomotory function came to the front. Here, however, lies the greatest 

 difficulty. It may, however, be assumed, that the animals while still living in water 

 were capable of gliding over the surface of 

 the water by the swinging of their branchial 

 leaves, just as flying fish do by means of 

 their thoracic fins. 



The limitation of the wings to the 2 

 pairs of the meso- and nietathorax must be 

 explained mechanically, as more suited 

 for the propulsion of the body in flight. 

 We still see among living insects an un- 

 doubted tendency to the stronger develop- 

 ment of one of the pairs of wings. 



The so called tracheal gills of the larva; 

 of the Phryganidce, Sialidce, and Eplicmcridae 

 may serve as an example for this conjectural 

 formation of integumental folds serving fin- 

 breathing in water. The Phryganid larvae 

 live in the water in tubes of their own con- 

 struction, and possess on their soft-skinned 

 abdomens thread-like appendages into which 

 tracheal branches enter. Such appendages 

 are called tracheal gills. Similar append- 

 ages are found on the abdomens of the Sialid 

 larvfe. In the Eplicmcricl larvae, which live 

 free in water, there are found, on the seg- 

 ments of the hind-body, 6 or 7 pairs of lateral, movable, tracheal gills (Figs. 318, 

 342, 343), which are sometimes tufted, sometimes leaf-shaped, sometimes thread- 

 like. An anterior pair may even be developed as a sort of branchial cover for the 

 posterior pairs. All these tracheal gills are evidently integumental folds and re- 

 spiratory organs which have arisen as adaptations to aquatic life. When they are 

 leaf-shaped, the tracheae which enter them branch more or less richly. They begin 

 to form in a manner altogether similar to ordinary wings, and persist in the later 

 larval stages together with the wing rudiments (Fig. 318). 



Unsuccessful attempts have been made to trace back the wings of Insects to other 

 organs in other more or less remote animals, e.g. to the dorsal gills of the <'Jh<tinpoda 

 or to the dorsal folds of the Crustacea. If there is any such connection the 

 rudiments of the wings as primitive organs ought to appear in Insect embryos ; this, 

 however, is not the case. 



Some information has already been given in the Systematic Review about the 

 special form and arrangement of the wings in the various orders of the Insecta. 



FIG. 31S. Thorax and anterior ab- 

 dominal Segments of the Larva of Cloeon 

 dimidiatum (Ej'hemerUl), with tracheal gills 

 (tl;i, tJ; 2 , tk s ) and the rudiments of the fore- 

 wings (VF) and hind-wing (HF). tl, Tracheal 

 longitudinal trunks (after v. Graber). 



