10 Journal New York Entomological Society, [Voi. xxiv, 



theory, we have no alternative other than to choose the more prob- 

 able and acceptable theory. 



Some of the arguments which may be advanced in favor of the 

 tracheal gill theory of the origin of the wings of insects may be sum- 

 marized as follows : 



1. Wings cannot have suddenly sprung into being (like Minerva 

 from the brain of Jove!) fully formed and immediately functional, 

 but the process of their development must have been a gradual one; 

 and in their early stages they could not have been of use as flying 

 organs, but must have served some other purpose while becoming 

 wing-like. In other words, the locomotor function must have grad- 

 ually become predominant in structures having a prior fiuiction or 

 significance. 



2. Wings, in their immature stages, contain tracheae, and this in- 

 dicates that their prior purpose was respiratory — either in an in- 

 tensely humid atmosphere, or in water. 



3. If the successive segments of an insect's body are mutually 

 homologous, we should expect to find structures which are homo- 

 dynamous {i. c, of the same developmental series) with the wings, 

 on the other segments. Superficially, at least, the tracheal gills on 

 the abdominal segments of certain immature Ephemerids appear to 

 fulfil this requirement. 



4. The tracheal supply of the tracheal gills appears to be some- 

 what similar to that of the immature wings (see Plate I, Fig. 4). 



5. The tracheal gills of some Ephemerid nymphs are remarkably 

 similar, in outline, to the wings of certain insects (see Plate I, 

 Fig. 2). 



6. The gills may be bordered with hairs similar to those of cer- 

 tain wings, and are even stiffened by structures strongly suggestive 

 of nervures (Plate I, Fig. 2). 



7. The gill plates of immature Ephemerids are capable of very 

 rapid movements for setting up currents to keep the water in contact 

 with the respiratory surfaces pure. 



8. Through their movements, the gill plates have become articu- 

 lated to the tergum, after a fashion, thus " paving the way " for the 

 articulation of the wing-like structures to be developed from them. 



9. The muscles involved in the movements of the gill plates could 

 eventually become modified to form muscles of flight. 



