Male Tarsus in Adephagous Coleoptera. 323 



pedicel which supports the disk and is kept straight and the 

 slit closed by the elasticity of this pedicel. When, however, 

 pressure is applied against the surface of the disk, the pedicel 

 is shortened and the columella bent, thus opening the slit and 

 allowing the fluid to escape into the general cavity of the 

 pedicel whence it is distributed over the surface of the disk. 

 The small palettes, he says, have no columella, the pedicel is 

 pointed, while the disk is cup-shaped and obhquely attached 

 to the pedicel when at rest. When pressure is applied the 

 disk is brought at right angles to the pedicel and opens com- 

 munication with the secreting sac. 



It is not my intention however to discuss the mechanism by 

 which these sexual hairs are rendered serviceable, but rather 

 to bring before the student some of the many patterns of 

 arrangement of the disks and to figure as many as may be 

 available for the purpose. It is quite possible that this struct- 

 ure might be used with profit in the identification of species in 

 many cases, though in others, where the species are easily 

 differentiated by other characters, the feet are almost indis- 

 tinguishable. 



Any classification founded on these hairs must necessarily 

 be artificial, since it is a principle generally accepted among 

 naturalists that those organs which are brought most into 

 contact with the peculiarities of the life of an animal are those 

 which are likely to undergo to the greatest extent the process 

 of adaptation and change; and will consequently offer no 

 reliable index of true fundamental or homological affinity 

 though often valuable for defining groups among species 

 closely allied. 



The short note regarding generic peculiarities is taken 

 from Dr. Sharp's great work on the Dytiscidae.^ The de- 

 scriptions and drawings are original, the latter made after 

 photo-micrographs, which, with the exception of two by Mr. 

 L. B. Robinson, were all made by Mr. Edw. G. Decker, to 

 whom I am greatly obliged for this kind aid. It is hoped to 



1 Scientific Transactions of the Royal Dublin Society. Vol. II, Series II. 



