I :;io. NOR Til A MER ICA N TH YSA NOPTERA —HINDS. 125 



Prothorax rarely longer than broad, but usually transverse, fre- 

 ([uently twice as wide as long, ordinarily rectangular in genei-al outline 

 and scarcely wider at the hind edge than at the fore edge, except in the 

 genus C/t/'rothrij)s, in which it is strongly broadened behind, where it 

 is about t\vice as wide as at front edge. The fore wings are broader, 

 stronger, and much more specialized than the hind wings, shaded 

 darker, if shaded at all. As a rule they have more veins, there ])eing 

 usuall}' two, sometimes apparently only one, fully developed longitu- 

 dinal veins besides frequently a strongly developed vein following the 

 border of the wing and known as the ring vein; cross veins are present 

 in some cases. The veins are usually set with more or less numerous 

 and conspicuous spines which vary in size, the smallest being minute 

 and indistinct, the largest extremely stout and conspicuous, exceeding 

 in length the breadth of the membrane of the wing. The membrane 

 itself is thickly set with numerous microscopic spines. A fringe is 

 always present upon the hind margin, consisting on the hind wing of 

 one, on the fore wing of two rows of long usually wavy hairs. On 

 the fore w ing these rows appear to be placed at different angles to 

 the edge, so that instead of the hairs being parallel when the wing is in 

 action, the}" cross each other at a slight angle, thereby forming a mesh- 

 work W'hicb must add materially to the strength and resistant power 

 of the wing. Spines such as are found on the other veins are wanting 

 upon the hind margin. The fringe upon the front is always shorter 

 than that upon the hind edge and is composed of a single row of 

 stouter, more bristle-like hairs. The development of the fore fringe 

 appears to be in inverse proportion to that of the spines borne upon 

 the costal edge, and when these last are very stout the fringe is ves- 

 tigial, though sometimes both fringe and spines are wanting on the 

 costa. In many cases the shading of the fore wings takes the form of 

 dark cross bands alternating with light or almost white bands or areas. 

 The hind wings are more slender and more delicate than the fore wings 

 and have but one median longitudinal vein, usual 1}^ fully developed, 

 and no ring or cross veins. The median vein is without spines such 

 as are borne upon the veins of the fore wing. The hind fringe is 

 single instead of double and the fore edge always bears a more or less 

 well-developed fringe. Shading of the hind wings is very slight and 

 a distinct banding of them is not known. When at rest the wings are 

 laid straight back upon the abdomen, the fore wing of each side com- 

 pletely covering the hind wing and each pair lying parallel to but not 

 upon the other. The hind fringes are ver}^ flexible or jointed at their 

 attachment to the wings and when at rest point backw^ard between 

 them. The wings are very frequently reduced to small, rounded or 

 oval pads which are usually invisible even when present. Rarel}' they 

 are entirely absent, but when this is the case the structure of the 

 thorax indicates the fact. The fore leo-s are often more thickened 



