72 INSECUTOR INSCITI^ MENSTRUUS 



comparatively undeveloped; that is, the highest development 

 of the lobes and harpagones is not shown in the same species. 

 For example, in canadensis the lobes are highly developed, 

 but the harpagones are of a comparatively simple type. Con- 

 versely, in trichurus, the lobes are simple and the harpagones 

 highly modified. This is natural, since if one set of organs 

 comes into prominence for a special use, in this case that of 

 prehension, another set must remain comparatively in abey- 

 ance. 



The clasp filament in these American Aides does not come 

 much into use and remains throughout unmodified and sim- 

 ple. The divisions, therefore, which we will recognize among 

 these American forms are mainly those of progressive spe- 

 cialization along one general line and not dichotomous divi- 

 sions. 



The second element in our fauna is what may be called the 

 foreign element. It comprises three species only, not closely 

 related, but all agreeing in the total absence of the harpagones. 

 These organs have not only been unelaborated, but they do 

 not exist in a rudimentary state. The side piece shows the 

 development of a basal lobe, but of an independent order, not 

 homologous with the basal lobe mentioned above for the Amer- 

 ican forms. The development is not marked, however, for in 

 these species specialization has primarily afifected the clasp 

 filament, which has been adopted as the principal organ of 

 prehension. Use also has been made of modifications of the 

 basal membrane, which is entirely unknown to the American 

 forms. These three species, cincrcus Meigen, z'exans Meigen 

 and argentcus Poiret, as has long been known with the last and 

 recently shown for the other two, all have a wide distribution 

 outside of North America. The first two are of temperate 

 distribution, the last of tropical distribution. The first two 

 are presumably endemic with us, having an originally holarctic 

 distribution ; the latter has probably been spread by commerce 

 within historic times. In the monograph we argue for the 

 American origin of the yellow fever mosquito, but it must be 

 admitted that the genitalia lend no support to this view. No 



