788 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



of the mandibles to the antennse. They are separated by a 

 space much greater than that which separates the mandibles 

 from the ist maxillas. This appearance, which in Hydrophilus 

 proved so puzzling to Heider, is due to the presence of the 

 so-called "intercalary," " Vorkiefer," " premandibular," or " tri- 

 tocerebral " segment. As the evidence all tends to show that 

 this segment is the homologue of the second antennal segment 

 of the Crustacea, it should be designated either so or as the 

 tritoccrcbml segment rather than by the indefinite and non-com- 

 mital term " premandibular." 



Wheeler ('93) was the first to demonstrate in Anurida a pair 

 of vestigial appendages, which bear to its ganglion the same 

 relation as do the other appendages to the ganglia of their 

 respective segments. Wheeler's work has been abundantly con- 

 firmed by workers on the embryology of the Apterygota, Uzel 

 ('98) and Folsom ('99) having determined that in certain forms 

 these appendages persist even to the adult stage. 



Though these results have been generally accepted as applying 

 to the Apterygota, embryologists have been unwilling to accept 

 the scant evidence of the presence of such vestigial appendages 

 in the Pterygota. As long ago as 1870 Butschli observed in the 

 embryo of Apis a paired rudiment just behind the mouth, which, 

 as he said, sich fast wie ein Paar innerer Antennen ausnimmt." 

 Hiitsclili himself did not regard this structure as homologous 

 with the otlKM- appcMidages. (irassi ('84) also noted these ap- 

 pendages, hut contrary to Hutschli, who thought that they fused 

 1') loim a transitory under lij), (irassi states that they quickly 



