1896.] NATURAL SCIENCES OF PHILADELPHIA. 29 



males. There is also a marked sexual difference in the mandibles 

 of venfralis. The tongue differs in length and in the degree of 

 development of the hairs. As will be seen below, the tongue has on 

 two or three occasions proved useful in distinguishing allied forms 

 (as affinis and senecionis), but it has not been studied throughout 

 the series. In one instance, a useful distinction was found in the 

 relative lengths of the joints of the maxillary palpi. 



The form of the clypeus differs very much both between the spe- 

 cies and the sexes of the same species (e. g., ventralis). For conven- 

 ience I have compared the shapes noted to the outlines of different 

 kinds of hats. 



The degrees of hairiness of the face and cheeks, as also of the 

 thorax (especially of the mesothorax) offer useful characters. The 

 hairs are usually white, but may in part be grayish or brownish, or 

 even, in a yellow species (beata), yellow. They are very rarely 

 (cdbovittata) dense enough on the face to obscure the markings. 



The antennae present different grades of color (usually paler be- 

 neath) from yellow and orange to black. In the albipennis group 

 the color of the flagellum has served to distinguish the males of 

 allied forms. 



The face markings at first seem complicated and hard to describe, 

 but are easily reduced to a simple system. The face may be wholly 

 dark, but if the pale marks are much reduced they are generally 

 seen to linger last upon the clypeus. An exception to this is found 

 however in semiccerulea, with its shining yellow mark on each side 

 of a perfectly dark clypeus. The clypeus may be wholly light, 

 usually retaining a black dot on each side near the margin. The 

 clypeal dark markings appear frequently in the form of two longi- 

 tudinal black bars, as in numerata. 



The lateral light markings of the face are commonly triangular, 

 the inner angle being about opposite the dot on the clypeus, and the 

 upper angle usually on a level with the antennal socket on the 

 orbital margin. Soaietimes the lateral mark extends up along the 

 margin of the orbit much further ; and it may terminate variously, 

 being either pointed or truncate. The shapes of the lateral face 

 marks afford excellent specific characters. 



Above the clypeus, between its upper border and the level of the 

 anteunse, is the siipradypeal mark, which differs very much in its 

 degree of development, and even in its shape in some allied species. 

 It may be produced upward in the median line to an enlarged yel- 



