On Aquatic Cariwvorous Coleoptera or Dytiscide, 953 
out in Sternopriscus oscillator, where the middle tarsi of the male are very elongate 
and their true fourth joint is developed to such an extent that it actually very 
nearly equals the third joint. 
It is a further interesting fact that the only two genera,—and they are widely 
different from one another—in which the true fourth joint of the tarsi is very con- 
spicuous, are both pecuhar to Australia. 
Also it should be noted that some of the species of Sternopriscus, differing as they 
do from all other Dytiscidee by the great development of the true fourth joint of 
the middle tarsi of the males, have no two species in which this part is similarly 
formed. I judge from this that each species has developed the peculiarity 
independent of the others, and that their all possessing it (in various ways) 1s no 
proof of genetic identity. 
There seems reason to believe that the Hydroporides have had ancestors with 
five-jointed front and middle tarsi ; not only because the joint usually wanting (the 
true fourth joint) remains quite visible in certain genera, (Necterosoma, Sterno- 
priscus); but also because the posterior or hind tarsi are always five -joimted 
throughout the tribe. And as in all the carnivorous Coleoptera except these Hydro- 
porides, the front and hind tarsi have the same number of joints (five), and as this 
is at present the case in a certain number of Hydroporides (where the fourth joint 
is greatly reduced though still conspicuous), we may reasonably suppose that these 
Hydroporides which have now only four jomts on the front foot, had formerly five 
joints, and that the true fourth joint has become reduced and lost, in conformity 
with the plant-frequenting conditions of their existence; 1t being the rule that 
beetles which walk on vegetable substances have only four-jointed tarsi. The pos- 
terior tarsi remain always five-jointed because they are used not for walking on plants 
but for swimming, and for this purpose reduction in area would be very 
disadvantageous. 
The scutellum displays in this aggregate great diversity, though as it is concealed 
from view a superficial or misleading conformity among the species is suggested. 
In Hydrovatini it is found in its minimum of development, so that when the 
prothorax is separated from the after or middle body the scutellum still appears to be 
absent, indeed the mesonotum in these insects seems to be excessively small and fra- 
gile, and owing to the perfect way in which the elytra are locked together, I have not 
succeeded in getting a view of the mesonotum without rupturing and destroying it. 
In Dytiscus ovatus (Hyphidrini) the scutellum exists as a moderately large 
membranous plate ; the postericr margin is truncate and presents a sharp edge, but 
this is not produced into a fine triangular plate : the anterior margin of the scutellum 
is also truncate and is narrower than the posterior one. 
In Dytiscus duodecimpustulatus (Hydroporini, Deronectes,) the anterior pieces of 
the mesonotum are larger than in Dytiscus ovatus (Hyphidrini) but the scutellum 
is not larger, and is of a different form, being rhomboidal, and angulate both in 
