REPORT OF THE STATE ENTOMOLOGIST IQ07 



223 



But this analogy might easily be carried too far. For the wing 

 is not a sail, to belly with the wind and hold against its pressure, 

 but it is a flexible scull, to be swung rapidly against still air, up and 

 down, rigid in front, yielding behind, giving a resultant in forward 

 motion of both strokes rapidly repeated. Hence beyond the bolt 

 cord that may be discerned in the line of cross veins and forks, 

 and behind the boom that the stem of the cubital vein represents, 

 there is a wide border that has no counterpart in a sail. Further- 

 more, the analogy will be misleading if held too rigidly even for 

 the inner triangular area. For the " bolt cord " is often not con- 

 tinuous to vein R^ but stops at the radial sector,^ and it is cut by 

 one or more furrows, that greatly increase its flexibility.- 



The analogy will have served its purpose if it fixes our attention 

 upon the triangular outlines of the supporting framework of the 

 wing, for it will abundantly appear in the following pages that 

 along these lines of support have played the forces that have 

 evolved the Tipulid wing. 



This general arrangement of parts is not peculiar to Tipulidae, 

 nor even to Diptera, but is characteristic of the vast majority of 

 winged insects. It seems strange that so little attention should 



have been paid to 

 it hitherto by 

 students of vena- 

 tion. I desire to 

 emphasize it here 

 by means of the 

 accompanying dia- 

 /'^- '^ gram [fig. 13] in 



which the arrows indicate the outer line of transverse joinings. 

 For this I think it would be convenient to use the simple and sug- 

 gestive term cord. The shaded mainmast strip at the front is the 

 costo-subcosto-radial combination. The posterior boundary of 

 the larger triangle is the cubitus, and of the lesser included tri- 

 angle, the radial sector. 



' Thus making, however, a trapezoidal, instead of a triangular sail area. 



* These furrows allow the wing to close somewhat on the up stroke, and 

 are doubtless a compensation for the pull of gravity on the heavy body 

 of a crane fly. They are best developed in the Tipulinae with the heaviest 

 bodies (Holorusia, etc.). One can see how they work by holding a fresh 

 or a softened wing firmly by its base and blowing air alternately against its 

 upper and lower surfaces through a bent tube while looking horizontally at 

 its tip. The anal furrow (behind Cu) allows almost complete flexion on the 

 up stroke. On the down stroke the whole wing tends to flatten out. 



