6o SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



metamorphosis the wings develop beneath the cuticula, usually in 

 pouches of the hypodermis, their rudiments appearing first in different 

 insects from a late embryonic period to the last larval stage. They are 

 everted from the hypodermal pockets during the prepupal stage of the 

 larva, and become exposed as external organs with the shedding of 

 the last larval skin. 



The relation between wing tracheae and vein channels in wings that 

 develop on the exterior of the body would appear to indicate that the 

 positions of the veins are determined by the original courses of the 

 tracheae, and this consideration has given weight to the idea that the 

 wings originated as gills. In the Holometabola, however, the vein 

 channels are defined in advance of the growth of the tracheae, and the 

 latter, when formed, do not always enter veins corresponding with 

 those they occupy in insects with incomplete metamorphosis. Develop- 

 ment in the Holometabola, therefore, shows that veins may be laid 

 down along definite lines in the wing without the guidance of tracheae, 

 and that the tracheal courses have no fixed relation to particular veins. 

 The conditions met with in the Holometabola may easily be explained 

 as secondary, hut they allow us to question if tracheae necessarily did 

 determine vein formation or the vein positions in the phylogenetic 

 development of the wings. Since the growing wing is a functionless 

 organ in all insects, and follows a more or less aberrant course in its 

 development, its ontogenetic stages are likely to have become adapted 

 to the conditions of growth, and, for this reason, they cannot be taken 

 as representative of the sequence of steps in the evolution of the wing. 

 The veins may have originated independently as strengthening ribs in 

 the primitive wing lobe, limiting to their channels, as the intervening 

 areas became flattened, the courses of the tracheae and the nerves 

 penetrating the wing. 



The adult wing. — The fully formed wing has the same fundamental 

 structure in all insects, regardless of the method of development, or 

 of the specialized form it attains in the imago. The wing of a roach 

 developed externally arrives at the same structural pattern as that of 

 a moth developed internally ; the elytron of a beetle, the halter of a 

 fly retain each the unmistakable features of a wing. 



The typical wing of modern insects extends laterally from the edge 

 of the dorsum where its base is articulated to the tergum above 

 and to the wing process of the pleuron below. Its anterior margin 

 arises behind the prealar bridge of the tergum, if this process is 

 present (fig. 21A, Aiv) ; its posterior margin is continuous with the 

 posterior fold (Rd) of the tergum. The area of the wing is traversed 



