STOlJGH: THE HACKBERRY PSYLLA. l47 



The number of groups averages thirty to thirty-five. They 

 tend to be irregular and not polygonal. Groups of "four's" 

 are perhaps the most common, being generally found singly, 

 though sometimes two or three groups are associated together 

 to form a figure, generally a chain. 



III. STRUCTURE. 



Fore Wing.^Ot the sixty wings that I examined, the fore 

 wings averaged 2.94 mm. long by 1.19 mm. at greatest width 

 in the male, and 3.27 mm. by 1.37 mm. in the female. Varia- 

 tions in outline of the wings and in the shape of the cells were 

 very slight. There was, however, variation noticed in the 

 length and breadth of the wing between rights and lefts in 

 both sexes. 



In the membrane at the base of the fore wings are found two 

 small sclerites, called by Audouin "articulatory epidemes," 

 already mentioned (27-1, artep). The cephalic of these is 

 seen in cross section to be more or less spherical in outline. 



On each side of the wing veins is a row of rather stiff, curved 

 hairs in circular pits, seen in 33-1 and 5. On the anterior 

 portion of the costa there are three rows of hairs, the an- 

 terior of which is separated from the others by a central, 

 dorsally projecting ridge of the costa. From a point near the 

 proximal edge of the light band, already referred to under 

 "pattern," these hairs are found in a single row until the place 

 is reached where the anal vein and the posterior costa seem to 

 cross. From here on the costa bears no hairs, but in their 

 place a row of very minute, sharp teeth on its outer edge. 

 These teeth are also found on the edge of the anterior costa 

 below the smooth-edged cuticular margin of the wing ; they are 

 somewhat larger here, but become obsolete a little before the 

 apex of the wing is reached. 



Upper and lower surfaces of the wing are essentially the 

 same, as regards number and position of hairs. The structure 

 of the membrane has already been treated. It is submem- 

 branaceous in texture. 



Hind Wing. — The hind wing averages 2.5 mm. long by 0.97 

 mm. wide. The apical portion is wider than the basal and is 

 well rounded. The anterior margin is fairly straight, though 

 somewhat undulating in outline. No hairs are found on the 

 veins of the hind wing except two or three on the fused portion 

 of the subcosta, radius, media and cubitus, and on the basal 



