Vol. 1.] WoodwortJi. — Wing Veins of Insects. 11 



That region which is most susceptible to the stimulation that 

 produces growth generally — that where growth last occurred — 

 is the region where the gills most readily form. We can con- 

 ceive that the bases of the appendages and the end of the 

 abdomen where gills most commonly occur are such regions of 

 easy stimulation. That the region of the thoracic segment at 

 which the wings are developed belongs to this same category 

 we can readily believe, on account of the frequency with which 

 another growth, the so-called margin, occurs in the corre- 

 sponding parts of adjacent segments, and in these segments in 

 very young stages. 



The simplest form of tracheal gill is a tube-like process con- 

 taining a simple tracheal twig. The modifications consist of 

 the development of a brush-like bunch of such simple elements, 

 a tree-like branching of a simple element, or a leaf-like expan- 

 sion of the same; all intergradations exist between these types. 

 There is a practical limit to the length of a gill, chiefly depend- 

 ent upon the danger of injury in the case of a long organ; 

 these modifications are the result of an effort to increase the 

 surface without increasing the length. 



Of these three forms of modifications the last is evidently 

 the most efficient, since the whole gill surface is exposed to 

 open water and none of it faces an adjacent surface with which 

 it must compete for the oxygen contained in the intervening 

 water, as is the case in the other types. There are compensat- 

 ing disadvantages in the increased danger of injury, as the 

 individual portions can not give wa}^ as readily when coming 

 in contact with objects liable to injure them. The necessity, 

 therefore, arises of devoting a portion of the surface to strength- 

 ening ribs or veins, and a greater or less thickening of the 

 whole upper surface, making it somewhat more resistant and, 

 therefore, less useful as a gill. The production of the wing 

 from a gill of this sort would result in the early loss of its 

 utility as a gill, so we can not look upon it as the precursor 

 of the wing, even though there may be a striking similarity in 

 general appearance. 



Another specialization, one that affords a better transition 

 toward a wing, is seen in those insects in which a portion of 

 the gill is devoted to the function of protecting the remaining 

 parts, permitting them to retain their most efficient gill struc- 

 ture even under rather adverse conditions. Just such an organ 



