EXTERNAL ANATOMY 31 



segments has been much used by systematists. Careful studies of 

 representative species of several genera of Lecaniinae by Thro 

 showed that the setae were practically identical in number and 

 arrangement in the species studied and the differences described 

 were due to overlooking the calices of setae that had been broken 

 off. In certain genera there is a group or transverse row of setae 

 located on the ventral aspect of the head between the articulations 

 of the antennae which have been named by Green the interantennal 

 setae. 



Rostrum. — The rostrum in the heteropterous Hemiptera has 

 been shown to be homologous with the labium of other insects. 

 That the rostrum in the Coccidae is homologous with the rostrum 

 of the Heteroptera and the labium of other insects, there is abso- 

 lutely no doubt. It is an elongate projection attached between 

 the prothoracic legs. The lateral margins of the rostrum are 

 normally folded so that it is U-shaped in cross-section except that 

 the margins meet on the dorso-meson. By this folding a closed 

 furrow is formed, the rostral furrow, which extends lengthwise 

 through the rostrum and serves for holding, guiding, and support- 

 ing the bristle-like mandibles and maxillae. The rostrum consists 

 of three segments in Coelostoma and Phenacoleachia ; of two 

 segments in Icerya, Pseudococcus, and many other coccids ; and of 

 a single segment in the Lecaniinae and Diaspidinae. In the adults 

 of the Margarodinae, Xylococcinae, Callipappinae, and Kuwaniinae 

 the rostrum is always wanting. It is usually stated that the rostrum 

 is lost in the male at the first or second molt and is wanting in all 

 subsequent stages, including the adult. But in the adults of cer- 

 tain species as those of the genus Pseudococcus, it is represented 

 by a minute conical projection. The distal end of the rostrum is 

 frequently provided with large setae, the labial setae. In the 

 Coccidae the rostrum, is always much shorter than in the closely 

 related families, the plant lice and the psyllids, rarely if ever 

 extending beyond the thorax and usually only to the caudal margin 

 of the mesothorax. 



Mandibles, Maxillae. — The mandibles and maxillae are long 

 and bristle-like as in all other Hemiptera. It is very difficult, if 

 not impossible in most cases, to distinguish the mandibles from the 

 maxillae, except where their articulation to the head can be de- 

 termined. They can be distinguished in some heteropterous 

 Hemiptera by the teeth or projections on their distal ends. The 

 margins of the mandibles and maxillae are furrowed in such a 

 way that they are normally so closely fastened together that it is 



