STOUGH: THE HACKBERRY PSYLLA. 138 



lumina in each mandible may be seen. These may also be seen 

 running to the tip when the mandibles are viewed extended, as 

 they are generally filled with air, which renders them visible. 

 No indication of lumina could be found in the maxillae. They, 

 however, by their concave surfaces fitting together, form a 

 canal through which the plant juices are sucked. The tip of a 

 mandible is very slender, sharp, and sickle-shaped, the convex 

 side bearing blunt serrations, the teeth pointing proximad 

 (26-7). The tip of a maxilla is of similar shape but with a 

 sharp projection on the convex side some distance from the 

 apex (fig. 6). The apices of these setae are so transparent 

 that the details of their structure were difficult to discern 

 clearly. 



The homologies of the hemipterous mouth have been much 

 discussed in the past, and the opinions held have been various. 

 I have followed the nomenclature used by Meek for Cicada, 

 which seems to be in harmony with late investigations along 

 this line. In their place I have compared the parts of the mouth 

 as seen in Pachypsylla with the corresponding parts in Cicada. 

 I also found in my work that comparison with other homop- 

 terous forms was helpful, showing the close relationship of 

 the Psyllidae to the other families. 



The labium of the Aphididae is quite similar in general struc- 

 ture to that of Pachypsylla except in relative length and num- 

 ber of segments and in the fact that it does not possess the 

 bend characteristic of the latter insect. 



The mouth parts, exclusive of the labium, are almost iden- 

 tical with, though larger than, those of the Aleurodidae which 

 I have examined. The same sclerites are present, arranged 

 similarly, though somewhat differently shaped. The labrum is 

 much shorter and smaller proportionately than in this family, 

 and the setae seem to be relatively shorter. The end of the 

 labium has the same lateral processes, but these are relatively 

 smaller. The Aleurodid labium has a bend similar to that of 

 Pachypsylla. 



The coccid mouth parts also correspond in many ways. Here 

 the labium is very short and the setae are retracted into a 

 special pouch when not in use. The parts exclusive of the 

 labium are similar in general structure to the same parts in 

 Pachypsylla. Berlese's nomenclature and figures in his work 

 on Italian Coccidae were very useful to me. According to this, 



