NO. 1438. WINOS OF THE TENTHREDINOIDEA—MacGILLIVRA Y. 609 



in the anj^-le formed by the medial cross-vein and M.^. Of the seven 

 trusses here enumerated, four of them have their apices in the stigiiia, 

 while the remaining three have their apices so situated as to be a direct 

 support to the ti'usses ending in the stigma. This does not take into 

 account the thickened costa and the radio-medial cross-vein, which are 

 also additional supports to this region, while truss 9, which is behind 

 cubitus, is -the main si'pport of the stress transmitted ])y trusses 2, 5, 

 and <). Although each of these trusses is here described as a separate 

 entity, yet the fact should not be overlooked that thei-e is a direct 

 interrelation betAvecn all the ti'usses. Each is dependent on the other. 

 It is like the side of a bridge, composed of a complex of rods and beams 

 that to the casual o}>server do not bear nuich relation to each other, 

 Itut yet can ])e resolved by the engineer into a series of simple trusses, 

 all directly interrelated in the same way as the trusses described here 

 in this wing. 



The anterior three-fourths of the w^ing being so strongl}' ])raced, 

 there is no necessity for so perfect a bracing in the region of the 

 Hrst anal cell, because the stress exerted at this point can not be great, 

 and in addition the stress is applied at a i)oint where it can l)e easily 

 disseminated. There arc three of these trusses, though only two of 

 them are directly connected with the anal area. Truss 10 occupies 

 the first anal cell, with its apex directed toward the hind margin of 

 the wing and opposite the point where the hooks of the hind wing- 

 fasten into the fold of the fore wing. Just in front of the apical half 

 of truss 10, with its apex at the middle of the base of truss 10, is 

 truss S, with one of its basal angles at the apex of M3 and the other 

 at the angle formed by M^ and M^ + Cuj. It is of interest that the 

 stress sustained b}' truss 8 is not transmitted directly to the front 

 margin of the wing, but is disseminated over its apical two-thirds. 

 The stress transmitted 1)y the vein M^ + Cuj one side of truss S, is 

 taken up by truss 11, which has its apex almost opposite this vein. 

 The medio-cubital cross-vein is an excellent example of the interrela- 

 tion of these trusses. It is an important factor in two trusses trans- 

 mitting stress from the stigmatal region, and is e<iually important in 

 transmitting stress from the anal region toward the base of the wing. 



It is not necessary to discuss the trusses of the hind wings in any 

 detail. A glance at the figure of a wing (tig. 81) is sutiicient to show 

 that all the principal trusses are behind the costal area of hooks. They 

 are all arranged so as to spread the stress over as wide an ai'ea as pos- 

 sible and also to stitlen the wing membrane, for one of the principal 

 functions of the hind wing is to furnish sail area. 



In the preceding description no account has l)een taken of trusses 

 12, 18, and 14. They are not of primai-y importance, but serve to dissi- 

 pate the stress transmitted from the stigmatal and anal regions, and 

 to keep the membrane or sail part of the front wing expanded. 



