606 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxix. 



The second anal furroui. — This furrow also extends from the base 

 to the margin of the wing. It is found only in the hind wings and is 

 situated just behind the second anal vein. It is at the end of this fur- 

 row that there is located the emargination which eventually develops 

 into a slit, the axillary incision, which separates off the hind angle of 

 the wing into a lobe or alula. This alula, which always contains the 

 third anal vein, whether it is separated or not by an incision, is 

 always turned back under the remainder of the wing. 



The inedialfurroir. — This is a straight furrow in many Tenthredin- 

 oidea, starting in the cell R. and extending along just in front of Mj 

 to near the margin of the wing. It usually bends down near the 

 middle of the cell Rr,, so that it is close to the vein. This furrow finds 

 its greatest develoi^ment in the family Tenthredinidas, In most of the 

 genera of this family it extends along close to Mj until near the mid- 

 dle of the cell R5, where it subdivides into two or three branches. The 

 posterior branch crosses Mj+j near its origin and passes obliquel}^ across 

 the cells 1st M^ and Mj. The anterior branch passes midwa}- between 

 Ml and R., to near the margin of the wing; in some cases the anterior 

 branch subdivides, one branch extends just behind Rg, while the other 

 extends just in front of Mj. Onl}^ a casual examination is necessary 

 to see how important the medial furrow nmst be in maintaining the 

 tiexibility of this area of the wing. The so-called bulla? of many 

 writers on the Tenthredinoidea are the clear spots in the veins where 

 these furrows cross them. 



The radiiO fnrrov\ — This is a short longitudinal furrow situated 

 just in front of the radial sector and may be a branch of the medial 

 furrow. 



The codal hinge. — This is a thin area of the front margin of the 

 wing, situated between the apex of costa and Sc„ at the base of the 

 stigma. 



The greatest stress on a wing is alwa3^s on its front or striking mar- 

 gin and on that part of the margin that is most prominent. In the 

 Hymenoptera this is the region in the neighborhood of the stigma. 

 This stress is in a plane parallel with the wing membrane. This is 

 due to two causes, the angle at which the wing strikes the air and to 

 the sail area — that is, approximately the posterior two-thirds of the 

 wing, which maintains the wing-membrane at relatively the same angle. 

 The sail area of the wing has the same effect on the wing as the tail on 

 a kite when it is drawn rapidly through the air near the ground, caus- 

 ing it to maintain practically the same angle at all times. 



If we examine a simple type of truss, as fig. 15, where the sides AB 

 and BC are equal and the distance AD is equal to the distance DC\ we 

 will find that any stress exerted at the point B in the plane of the truss 

 and perpendicular to the line AC will be equally distributed along the 

 sides AB and BC. But if we take such a truss as fig. 16, where the 



