Dytlsci complk'afi.l adephaga. 163 



ii. Hiiul margius of joiuts of posterior tarsi set with flat- 

 tened and iidpressed cilia externally ; front tarsi of male 

 with dilated joints forming a round disc. 



1. Suture between episternuni of metathorax and wing 

 of luetasteruum rectilinear ; spurs of hind tibia; acu- 

 minate at apex Hydaticixa. 



2. Episterual suture curvilinear; spurs of hind tibijc 



emarginute at apex Thekjionectina, 



HYDROPORINA. 



Thi.=! is by far the most extensive of tlie tribes of the Dytiscidai ; it 

 contains about five hundred species, arranged in twenty-six genera ; it is 

 s[tecia]Iy defined by the structure of the prosternum, which is strongly 

 deflected between the anterior coxfe ; they are easily known by the third 

 joint of the front and middle tarsi being deeply lobed and concealing the 

 fourth joint, which, however, is usually absent or rudimentary ; the true 

 fifth joint (ap]iarently the fourth) is long and slender, and is furnished 

 with claws which are somewhat variable. INIembers of the tribe are 

 found in all parts of the world, except in some of the Pacific Islands. 



Our genera may be divided as follows : — 



I. Posterior coxal cavities widely separated, excised ; apical 



angle of elytra mucrouate Hydeovatus, Muts. 



II. Posterior coxal cavities not excised; apical angle of 

 elytra not mucronate. 



i. First ventral segment soldered to border of posterior 



coxa;, which are not contiguous BiDESSUS, Sharp. 



ii. First ventral segment free. 



1. Posterior coxal cavities separated; posterior tarsal 



claws very unequal IlYPurDEUS, //I. 



2. Posterior coxal cavities nearly or quite coatiguous ; 



posterior tarsal claws equal. 



A. Elvtral ligula distinct ; mesosternal fork not con- 

 nected witli the intercoxal process of metasteruuni CtELAllBUS, Tkoiiii. 



13. Elytral ligula wanting. 



a. Mesosternal fork not connected with the inter- 

 coxal process of metasteruuni DEitoXECTES, Sharp. 



b. Mesosternal fork connected with the intercoxal 



process of metasternum . ,....,.. Hydkopobus, (7/a//v'. 



The elytral ligula is a tongue-like process on the inner face of the 

 side margin of the elytra, for the purpose of making the union between 

 the elytra and the ventral segments more perfect. 



In those genera mentioned above, Cwlanihits and Drrnnecffs, in whii'h 

 the mesosternal fork is not connected with the intercoxal proce.';s of the 

 niesosternum, the character may b(j observed by tlie removal of the 

 thorax, in Avhich case it Avill be seen that the middle coxa; touch one 

 another ; in the genus JIijih-Dporna proper, in which the connection takes 

 place, they do not touch one another; this is also the case obscurely with 

 the genus Ili/pIiijiJnis, but not with JJidcssiis. 



As many of the points of difference given above, although scientifically 

 correct, are rather hard to work with, it is best for students who are 



>i 2 



