614 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxix. 



as ribs for stiffening the wing membrane and keeping- the sail area of 

 the wing expanded. If the wings of such generalized families as the 

 Hepialidfe, Pyromorphida% Megalop^^gida', and Eucleida? be examined, 

 it is found that thi« elongate type of truss is present not only on the 

 hind but also on the front margin of the wing. But in the wings of 

 the specialized families, Sphingidas, Saturniida?, Papilionida^ and 

 Nymphalida3, which are noted as being rapid ilyers, there is a very 

 different condition. In these families all the branches in front of vein 

 Rr, have been crowded close to the front margin of the wing, forming 

 a compact series of five stiff braces for supporting the area subject to 

 the greatest stress. 



In the Diptera, as in the Lepidoptera, the stress is applied along the 

 entire front margin of the wing, but in the wings of this order the 

 covering of overlapping scales is wanting. As there is only one wing 

 on each side of the body, and this is sublanceolate in outline, the factor 

 of a sailing surface is reduced to the minimum. Since the stress is 

 applied along the entire front margin, and there is no posterior wing 

 to exert anj^ influence, there has not arisen any necessity' for a trans- 

 verse stiffening across the middle of the wing. In the generalized 

 families the veins radiate out from the center of the wing to the mar- 

 gin somewhat like the spokes around the hub of a wheel. The reason 

 for this is seen in the necessity for the stiffening of all parts of the 

 wing. Most of the species are light bodied, and consequently the 

 wing membrane is delicate and the wings light in w^eight. Those 

 species that are predacious or hover about flowers are generall}' very 

 active flyers. In these families there has been developed a marked 

 tendency toward the coalescence of the tips of the veins, so as to pre- 

 vent the fraying of the wing margin. There has also been developed 

 along the front margin from the base to the apex of the wing a heavy 

 vein in which the longitudinal veins terminate. This is especiall}' 

 marked in the families Bombyliidai (tig. 21), Apioceridte, and Midaidse 

 (tig. 28), where the tips of all the branches of radius curve forward and 

 terminate close to the wing margin, thus accomplishing the double pur- 

 pose of protecting the wing margin and at the same time stiffening 

 that part of the wing subject to the greatest stress. The wing of 

 Midas, which has been referred to before because of the great number 

 of the tips of its veins that have migrated forward, illustrates this 

 point well. It shows how the stress applied on the front margin of 

 the wing is transmitted to the base along the radial stem, while that 

 on the hind margin is transmitted along the medial stem. We tind 

 here the reason for the coalescence of the branches of the radial sector 

 to Rj rathei- than to M,, as happens in the Hymenoptera; namel}', 

 because the stress in this wing is applied only on its front margin, and 

 there is a greater need for a stiffening in this direction. That this is 

 the correct interpretation is shown by the change in the contour of the 



