CLASSIFICATION OF INSECTS. 57 



ing to any theory which would trace the origin of 

 aerial organs to gills. These last may spring up 

 sporadically upon the abdominal rings, and have usu- 

 ally but one tracheal tube. The wing pads, on the 

 other hand, always arise from similar parts of the 

 mesothorax and metathorax, and have several tracheal 

 tubes. There are also difficulties in the adoption of 

 the assumption commonly made by authors, that the 

 winged forms first arose from aquatic forms having 

 gills on the back of the abdomen. Purely terrestrial 

 insects develop their wings in precisely the same man- 

 ner as those having aquatic larvae, and transitions 

 such as ought to be found, if this theory were true, 

 have not been observed. This leads to the conclusion 

 that the wings of insects must have originated in the 

 same manner as organs of flight in other terrestrial 

 groups, according to the theory advocated by Pack- 

 ard. "Now, speculating on the primary origin of 

 wings, we need not suppose that they originated in any 

 aquatic form, but in some ancestral land insect related 

 to existing cockroaches and Termes. We may imag- 

 ine that the tergites (or notum) of the two hinder seg- 

 ments of the thorax grew out laterally in some leaping 

 and running insect ; that the expansion became of use 

 in aiding to support the body in its longer leaps, 

 somewhat as the lateral expansions of the body aid 

 the flying squirrel or certain lizards in supporting the 

 body during their leaps. Then by continued use and 

 attempts at flight they would grow larger, until they 

 would become permanent organs." ^ 



1 Packard, Third Rep. U. S. Ent. Co?n., 1883, p. 268, in 

 which also are quotations from MUller and other authorities. 



