72 



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



there are certain Crustacea in which the maxillae are as suitable 

 as the maxillipeds, for comparison with the maxillae of insects 

 but specimens of these are inaccessible to me, and one must 

 perforce do the best he can with such material as he is able to 

 procure. 



Unfortunately, the diagrams of hypothetical evolutionary 

 stages employed by many investigators to illustrate the probable 

 mode of origin of various animal structures, all too frequently 

 correspond to nothing which has ever existed "in the heavens 

 above, or the earth beneath, or the waters under the earth," 

 and in order to avoid this pitfall, I have at first purposely used 

 only actual cases occurring in some living or fossil arthropod, to 

 illustrate the probable evolutionary stages in the development 

 of the insectan type of maxilla, since, if modifications occurring 

 in existing cases are used, they at least represent actual devel- 

 opmental tendencies which have manifested themselves in 

 nature, and are not purely a figment of one's imagination. On 

 the other hand, despite the fact that in using diagrams one 

 frequently is obliged to sacrifice accuracy to simplicity, the 

 judicial use of diagrams (safeguarded by reference to actual 

 structures upon which they are based) is an invaluable aid in 

 explaining in a concise and readily understandable form, the 

 ideas one has worked out in his own mind from evidence col- 

 lected from many sources; and in this account, I have ventured 

 to add the appended text figures to aid one in visualizing the 

 probable evolutionary stages through which the mouthpart 

 appendages have passed in the evolution of the insectan type 

 of maxilla. 



Fig. 1, biramous limb of trilobite or crustacean. Fig. 2, maxilliped ofGammarus. 

 Fig. 3, maxilla of neuropteron larva. Fig. 4, Maxilla of MacAi/is. 1, 

 coxopodite or cardo; 2, basipodite or stipes; 3, ischiopodite or palpifer; 

 4, 5, and 6, segments of endopodite or maxillary palpus, ex, exopodite; 

 en, endopodite; ie, ischiendite or galea; be, basendite or lacinia. The 

 stippled segments are the homologues of the cardo and palpifer. 



