NO. 1438. WINGS OF THE TENTHREDINOIBEA—MAcaiLLTVBAY. 013 



with these changes, though not necessarily due to the same cause, is 

 the migration of the posterior end of the radial cross-vein toward the 

 apex of the wing. 



The costal hinge as shown above is a thin place in the membrane of 

 the wing between the apex of costa and the tip of Sc... This is 

 undoubtedly a weak place in the wing that has l)een handed down 

 from generalized progenitors which did not require such an efficient 

 organ for flight. That it is a weak place in the wing is shown ))y the 

 fact that in those forms that are especially efficient flyers this area 

 has been bridged over. This is the case in the Cimbicina' (flgs. 59-60), 

 the Siricidje (flgs. 8H-91), theCephidje (flgs. 93-96), and in all the higher 

 Hymenoptera. In other forms this weakness has lieen overcome b}" a 

 decided thickening of the apex of costa, which simply rests against 

 Ki + Sc, and the stigma, but never coalesces with them. The hinge 

 is especially prominent in those genera with a ])road area between 

 costa and 8c+li+M, and pro])at)ly serves to make this region more 

 flexible. That the prominence of the hinge in these genera is for 

 flexibility is emphasized by th(> fact that tiie apex of the costa is not 

 decidedly tliickeruHl. This causes a fold in the wing membrane 

 between costa and Sc + R+M very similar to the furrow found in 

 this same region in the Diptera, and consequently tends to stiflen it. 



In the wings of Orysxu-s (flg. 97) occurs the greatest amount of 

 reduction found in the wings of any member of the superfamily Ten- 

 fhredinoidea. It is an interesting fact that the reduction found in 

 this genus is not amena])le to any of the explanations already given. 

 In Ort/sxKK \\\v membrane of the wing has been more strongly chitin- 

 ized than in the wings of other genera, and with the increased chitin- 

 ization of the wing membrane the necessity of veins for stifl'ening the 

 mend^ranc has been done away with, and (•()iis<Miuently they have 

 gradually disappeared, being represented only by l)ands of pigment. 



Akhough it is not within the scope of the present paper, yet it may 

 not be out of place to say something about the dynamical control of 

 the wing type in those orders where appi'oximately all the veins are 

 parallel and extend lengthwise or approximately lengthwise of the 

 wing. This is especially true of the orders Lepidoptera and Diptera. 



Among the meml)ers of the order Le])idoptera the wings are broad 

 and long. The stress exerted on the front margin of the front wings 

 is not applied at one point as in the Hymenoptera, but is spread out 

 along the entire front margin of the wing. Another point that nuist 

 not be overlooked is the fact that there is no marked necessity for a 

 transverse stifl'ening, because this is accomplished by the overlapping 

 scales covering l)oth surfaces, which stiflen it in tlic same manner that 

 the overlapping shingles stiflen a roof. The great majority of the 

 trusses in this order have their apices near the apex of the cell R+M 

 and their basal angles at the margin of the wing. They serve merelv 



