1896.] NATURAL SCIENCES OF PHILADELPHIA. 45 



Summing up, the writer has to express the opinion that variations 

 in Perdita certainly do not occur indefinitely in all directions, but 

 that they do occur independently, so that the several species differ 

 from one another hardly soliuich in absolute characters, as in the 

 various combinations presented of similar or identical characters 

 Furthermore, it is apparent that the earliest distinctions between 

 species are at least often of a very subtle character, so that the work- 

 ings of natural selection during the actual process of segregation 

 are anything but easy to observe. And this need not surprise us 

 when we reflect that among ourselves constitutional characters, not 

 easily identified by any coincident structural features, play so large 

 a part in determining our ability to reach manhood and beget off- 



spring. 



ARTIFICIAL KEY. 



(Note. — The numbers before the specific names coincide with the 

 numbers of the same in the descriptive portion.) 



Entirely yellow, with no conspicuous markings 1 



Yellow or orange, with dark markings 3 



Head and thorax dark 5 



1. 8 mm. long, mesothorax pubescent, pleura with a black 



patch 63 beata $ 



About 4 mm. long, head very large, cheeks armed 15 larrece $ 

 Over 5 mm. long, head ordinary, cheeks unarmed, meso- 

 thorax not pubescent 2 



2. Antennae dark above, a black line before the eyes, 55 hiteola 9 

 Antennse not dark, a black dot before the eyes . 55 luleola S 



3. Extremely small, cheeks armed, mesothorax mostly green, 



16 'inarcialis $ 

 Not so small, vertex with a black band from eye to eye, 

 thorax with black markings 4 



4. Size 6 mm., head very large, abdomen without distinct 



bands 34 cephalotes $ 



Size 4i mm., head not very large, abdomen with distinct 

 bands 35 puiietosignata $ 



5. Abdomen orange, or orange-brown, or ferruginous; not 



banded, unless at base 6 



Abdomen dark brown, or black, or spotted, or banded . . 13 



6. Head large, abdomen short and broad, ferruginous, mar- 



ginal cell obliquely truncate, mandibles bidentate ... 7 

 Not so 8 



