52 JoJui Henry Coins tock 



hope to base conclusions upon them as to the limits of larger 

 groups. But it does not follow, that because an organ is a su- 

 perficial one, it is of little value in suggesting broad general- 

 izations. We find that often the most superficial of structures 

 are among those that were developed very early in the history 

 of a large group, and have persisted almost unchanged in 

 form, although more central structures have been greatly and 

 variously modified. Thus the form of mere dermal appen- 

 dages may present characters of very high value, as the hair 

 of mammals and the feathers of birds ; even the Cetacea have 

 hair, and Archopteryx had feathers. As "the proof of a 

 pudding is in the eating," so the value of a character for tax- 

 onomic purposes can be determined only by its use. 



As to the origin of wings we have no knowledge We have 

 not even a generally accepted theory to account for the appear- 

 ance of these structures. Many writers believe that they are 

 modified tracheal gills. We find in many aquatic nymphs 

 plate-like gills, some of which would need to be modified but 

 little to function as organs of locomotion. This is especially 

 true of the covering pieces forming part of the respiratory ap- 

 pendages of an Ephemerid nymph. These pieces not only 

 protect the gills beneath them ; but probabl}- 

 also serve by their flapping to cause a cur- 

 rent of water to pass over the gills. Fig. i 

 represents the covering piece of a tracheal 

 gill of an Ephemerid nymph collected at 

 Fig. I. -Covering- Ithaca. The step from such a structure to 



'^in^of^May-fly^^ ^'^^ ^^^^ would aid in locomotion is not a 

 great one. 



But other writers think that the wings arose as keel-like 

 expansions of the sides of the thorax. Such expansions 

 would function as a parachute in a falling insect, as does the 

 folds of skin in a flying squirrel, or would function both as a 

 kite and a parachute in a leaping insect. 



In support of the latter theory the netted-veined triangular 

 prolongations at the sides of the prothorax of certain fossil 

 insects {Chorcedodis and Lithomantis, Fig. 2) are brought for- 

 ward ; and it is also said that a species of Tingis from Texas 



