6 CLASSIFICATION OF THE ALEYRODID^. 



The vertex is rounded in most forms and often possesses a median 

 longitudinal ridge or marking. In some forms, however, as in the 

 genera Udamoselis and Dialeurodicus, it is produced into a large 

 cone-shaped structure. This prominence between the insertions of 

 the antenna? in both of these genera seems to point to their close 

 relation. 



The frons is rounded when viewed from the side and when viewed 

 from the front it is somewhat U-shaped, the bottom of the U forming 

 the suture between it and the clypeus. Its upj^er edges extend back- 

 ward. Below the clypeus is the labium, which is trapezium-shaped. 

 The edges of this, as well as those of the clypeus, are slightly turned 

 back so that as the structure lies on the labium there will be a better 

 situation for the setas. Below the labrum is a triangular sclerite, 

 the epipharynx. All of these sclerites are somewhat united into an 

 elongate triangular structure which, together with the setae, remains 

 attached to the head and is easily separable from the labium. 



The labium is inserted in the base of the head on what is apparently 

 a projection of the thorax. It is of considerable length and is com- 

 posed of three segments. The relative length of these segments seems 

 to vary in the different forms. Near its proximal end the labium is 

 bent strongly caudad and appears to be grooved for its entire length. 

 At the base this groove is wide, but more centrally it becomes nar- 

 rower. The first (proximal) segment is much the longest and is 

 narrow. It is in many forms, armed with a few scattered hairs. 

 The second segment is thicker and shorter and the sutures separating 

 it from the first are not well defined. This has no doubt given rise to 

 Marlatt's statement that the labium is two-segmented.** The third 

 segment is short in Aleyrodes^ while in Aleurodicus it is considerably 

 longer. It is more heavily chitinized than the others and tapers 

 distad. It is covered with numerous hairs and its tip is bilobed. 

 Each of these lateral lobes is armed with three pegged-spiked taste 

 sensoria situated close together (PI. II, fig. 4). It is noteworthy that 

 very similar lobes and the same number of similar pegged sensoria 

 occur on the labium of the Psyllidee. The labium in that family is 

 also bent on its proximal part. 



The maxillaty and mandibular sclerites are present below the 

 frons, the lower extremity of the former becoming projections. The 

 setae, four in number, constitute two pairs. Those of the inner or 

 maxillary pair are closeW pressed against each other and sometimes 

 for this reason appear as only one. The outer or mandibular setae 

 to a certain extent inclose the maxillary ones. 



In his excellent paper on the hackberry Psylla, Stough has pointed 

 out the very great similarity between the mouthparts in this family 

 and those of the Psyllidae. He says:^° 



The mouthparts, exclusive of the labium, are almost identical with, though 

 larger than, those of the Aleurodidaj which I have examined. The same 



