604 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxix. 



and the transverse part of the second anal is curved toward the base 

 of the wing, while Cephus (fig. 06) dift'ers only in lacking the spur. 



The third anal vein is almost universally present, and extends as a 

 slightly curved vein near the anal lobe of the wing from the base of 

 the wing to the margin. It is represented in O/'ys^us by a fold and in 

 Acordtdecera by a pale band of pigment. In many of the generalized 

 genera there is present a prominent transverse spur on its hind margin 

 near the base of the wing, which is joined to the spiral vein, a cord- 

 like thickening which extends along the base of the wing to the scu- 

 tellum. 



v.— DYNAMICAL CONTROL OF WING TYPE. 



It has already been pointed out in a number of cases that certain 

 modifications were due to mechanical causes. The wing of an adult 

 insect is a machine purely for locomotion, and the rapidity and skill 

 of the locomotion is directly dependent on the perfection of the 

 machine. It is a fact that those insects are the swiftest flyers whose 

 wings approach most nearl}" a triangle in outline, that is, having wings 

 broad at base and pointed at tip. This is illustrated by the wings of 

 the hawk-moths, the bee-flies, and the bees. The efficiency of a Aving 

 is dependent not only upon its outline but upon the arrangement and 

 construction of its various parts. This construction consists in the 

 arrangement of the veins in such a manner as to best fit it to with- 

 stand the stress exerted upon it in striking the air and at the same 

 time without increasing the weight of the organ. 



The different kinds of insects fl\' in two ways — by a soaring flight, 

 for which a broad expanse of wing is required, and by a swift dashing 

 flight, for which a narrow, stifl' wing is necessary. It is also a fact, 

 at least so far as insects are concerned, that those species whose wings 

 are broad and approximate closely the arrangement of the veins found 

 in the hypothetical type are never swift flyers, while those in which 

 there has been a marked reduction in the number of veins, together 

 with a trussing of that part of the wing subject to the greatest stress, 

 are always swift flyers; that is, there is alwaj^s a direct correlation 

 between the structure of a wing and its efficiency as an organ for 

 flight. 



Where insects possess four wings, the wings of a side are generally 

 fastened together in some manner to insure a more synchronous 

 motion. This is accomplished in the Lepidoptera by a jugum on the 

 hind margin of the front wing, or by a frenulum on the front margin 

 of the hind wing, or by an expansion of the front margin of the hind 

 wing so that the two wings overlap. These fastenings are all located 

 at the base of the wing, and consequently can not exert much influence 

 over the course of the veins found ncai- the middle of the wing. 

 With the Hymenoptera in general and the Tenthredinoidea in particu- 

 lar the conditions are different. The wings of the Tenthredinoidea 



