9 6 



that these tracheae exhibit a striking similarity in their origin 

 and mode of branching in the nymphs and pupse of the 

 more generalized members of most of the orders of insects. 

 We have been able, therefore, to present a hypothetical type, 

 which we believe represents approximately what was the 

 arrangement of the tracheae in the nymph of the primitive 

 winged insect (Fig. 5), and, as the wing-veins of the adult 

 are developed about the tracheae of the wing of the nymph, 



FIG. 5. Diagram representing the hypothetical arrangement of the tracheae in 

 a wing of a nymph of the primitive winged insect. 



the same figure will serve to indicate the probable mode of 

 branching of the wing-veins in the adult of the primitive 

 winged insect. 



The radial sector. When the radius preserves its prim- 

 itive mode of branching, it separates at its first fork into 

 two unequal parts ; the first of these is vein R, ; the other 

 gives rise to the remaining four branches of the radius (Fig. 

 5). This second part of the radius, including its branches, 

 is termed the radial sector or vein R s , 



