Biorhiza Group. 135 



difference can hardly be found ; even the ovipositors 

 have the same form. Although the two flies do not 

 pierce the same parts of the plant {Biorhiza aptcra 

 pricks buds almost entirely, and Teras terminalis only 

 the bark), yet the form of the ovipositor is alike, because 

 the parts of the plant they severally attack are pierced 

 perpendicularly. Biorhiza renuni has a differently 

 formed ovipositor from Biorhiza aptcra, although it is 

 also intended for piercing buds. On the contrary, Trigon- 

 aspis criistalisy the sexual generation, has a quite 

 differently shaped ovipositor, which agrees with that 

 oi Spathcgasier Taschenbcrgi, and like it is destined to 

 pierce leaf-veins. It is evident that variety of form 

 in the ovipositor enables us very easily, in many cases, 

 to separate species which are otherwise very nearly 

 related. Through adaptation to different circumstances, 

 the ovipositor has assumed many modifications of form, 

 while the rest of the fly's organization has remained 

 essentially the same, at least no striking outward 

 deviations are stamped upon it. 



It is interesting to compare the two different genera- 

 tions from the standpoint of their accepted systematic 

 classification. So long as classification regarded only 

 outward marks of distinction, heterogeneous species 

 were constantly being put into the same genus. In 

 many classes of insects outward characters alone 

 might amply suffice to separate the species, because 

 their various habits of life, and their different adapta- 

 tions to them, have served to stamp distinguishing 

 marks upon them ; but in those whose habits of life 

 differ less, we cannot expect to find aids to classifica- 



