PROC. ENT. SOC. WASH., VOL. 24, NO. 2, MAR., 1922 73 



In textfigure 1 is represented the typical buccal appendage 

 of a trilobite (compare Fig. 6 of Plate 8) having a gill-like 

 exopodite "ex" and a locomotor endopodite "en," in which 

 segments two and three bear short flat endites "be" and "ie" 

 (the endite of segment four has been omitted for simplicity). 

 In textfigure 2, based upon the maxilliped of Gammarus (com- 

 pare Fig. 10) the exopodite "ex" of texfigure 1 has been lost, 

 and the endopodite "en" assumes a palp-like appearance, while 

 the endites "be" and "ie" of segments two and three become 

 more elongate. In textfigure 3, based upon the maxilla of a 

 larval Neuropteron (compare Fig. 11) the endopodite "en" has 

 become the maxillary palp, and the endites "be" and "ie" of 

 segments two and three have become modified to form the 

 slender lacinia and galea; while in textfigure 4, based upon 

 Machilis (compare Fig. 12) segment number two becomes more 

 elongate and segment number three is more closely applied to 

 it, forming the palpifer still distinguishable as a distinct segment, 

 but tending to unite with the second segment or stipes. In 

 the higher insects, the union of the palpifer segment with the 

 stipes segment is usually complete, although traces of the palpi- 

 fer are retained in certain forms. 



Comparison of Structural Details. 



In the foregoing discussion, it was shown that the maxilla of 

 an insect can be readily derived from crustacean prototypes 

 (although it would be impossible to derive an insect's maxilla 

 from any of the chilopod buccal appendages I have been able to 

 examine), and in the following brief discussion, I would point 

 out the fact that even the minuter details of structure bear out 

 the contention that we must seek the prototypes of the parts 

 of insects among the Crustacea, rather than among the chiol- 

 pods and after all, it is the evidence of the minute details of 

 structure which is of the greatest value in any phylogenetic 

 study, since convergent development may bring about a general 

 resemblance in the various parts, but when this resemblance 

 extends to the more minute details of structure as well, the possi- 

 bility of the resemblance being due to convergent development 

 is very slight, and we are forced to the conclusion that a common 

 heredity is the cause of the resemblance, when it extends to the 

 structural details of many organs from widely separated regions 

 of the body as is the case with numerous structures in insects 

 and Crustacea. 



In the maxilla of the primitive crustacean Nebalia shown in 

 Fig. 4, Plate 8, the endite, or mesal lobe, of each of the two basal 

 segments is bordered by an endofimbrium or mesal fringe of 

 seta-like, or spine-like structures, shown more highly modified 



