INTRODUCTION IX 



eter, and do the necessary arithmetic for getting proportions. Un- 

 educated guesses tend to lead one astray. 



The length of a structure is ordinarily measured as its greatest 

 visible length, disregarding parts of it that may be hidden in a socket 

 or articulation. The width or depth is measured at its greatest 

 width or depth, disregarding hairs. The length of the front wing is 

 taken from the free edge of the tegula to the extreme apex of the wing. 

 The length of the first tergite is taken from the center of the insertion 

 of its dorsal (extensor) tendon to its apex. The width of the face 

 is taken at its narrowest point between the eyes. The height of 

 the face is measured from the center of the clypeal foveae (= anterior 

 tentorial pits) to the lower margin of the antennal sockets. 



The descriptions of the various aspects of the legs are as if they 

 were extended horizontally at right angles to the body. Thus the top 

 and bottom edges and the front and rear faces of a structure like the 

 hind tibia would be different than if it were in the normal position 

 for walking. In order to arrive at a definite and uniform terminology, 

 it seems necessary to imagine the unnatural position of all legs being 

 stretched horizontally at right angles to the body, even if it seems 

 confusing at first, because the natural position of the legs differs be- 

 tween the front, middle, and hind legs, and puts many of the segments 

 in an oblique position. 



The wing membrane of most Nearctic Metopiinae is hyaline or 

 almost so, and the wing veins and stigma are fuscous. In Metopius 

 there are various wing colors, which give good taxonomic characters, 

 so the wing color in Metopius has been described for each species. 

 For all other species it may be assumed to be approximately hyaline 

 with dark veins and stigma, unless described otherwise. 



