500 EXTERNAL ANATOMY OF INSECTS. 
occurs, placed, like those of many Crustacea, on a_foot- 
stalk, but not a moveable one. An instance of this in cer- 
tain male Ephemera has already been mentioned. In the 
Hemiptera De Geer has figured two species of bugs 
belonging to the Lygeida, that are so circumstanced*; 
as are also all the known Strepsiptera, though in these 
the footstalk is very short: but the most remarkable 
example of columnar eyes is afforded by that curious 
Dipterous genus Diopsis, in which both eyes and an- 
tennze stand upon a pair of branches, vastly longer than 
the head, which diverge at a very obtuse angle from its 
posterior part °. 
In their fgure eyes vary much. Sometimes they are so 
prominent as to be nearly spherical : this is the case with 
some aquatic bugs, as Ranatra, Hydrometra, and several 
male Liphemere*. Very often they are hemispherical, as 
in the tiger-beetles (Czcindelid@), and the ground beetles 
(Zutrechina); but in a large number of insects they are 
flat, and do not rise above the surface of the head.— 
With regard to their outline, they are often perfectly 
round, as in many weevils; oval, as in various bees ; 
ovate, as in other bees (Andrena); triangular, as in the 
water-boatman (Nofonecta). ‘They are also often oblong, 
and occasionally narrow and linear; as in that singu- 
lar beetle Heleus. In many of the Muscide they form 
nearly a semicircle, or rather, perhaps, the quadrant of a 
sphere. The eyes of the Capricorn beetles have a sinus 
* De Geer iil. ¢. xxxiv. f. 17, 18, 24. 00. 
> Mon. Ap, Angl. i. t. xiv. no. 11. f.1.f. Linn. Trans. xi. t, ix. 
f.10. d. 
© Pratre XII. Fic. 9. Fuessly Archiv. t. vi. 
* Schellenberg Cimices t. xiii. ix. f. 1. a. De Geer ii. ¢, xviii. 
f.10. 
