EXTERNAL ANATOMY OF INSECTS. 501 



the last-named beetles, part of the eye is behind and part 

 before the antennae ; but except where there are four 

 eyes, as in Tetrops, they are never placed before or below 

 them. 



Though the eyes of insects are generally sessile, yet to 

 give them a wider range they are sometimes, but it rarely 

 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 

 ( Cimicidae) that are so circumstanced a ; as are also all 

 the known Strepsiptera K., though in these the footstalk 

 is very short b : but the most remarkable example of co- 

 lumnar eyes is afforded by that curious Dipterous genus 

 Diopsis, in which both eyes and antennae stand upon a 

 pair of branches, vastly longer than the head, which di- 

 verge at a very obtuse angle from its posterior part c . 



In their figure eyes vary much. Sometimes they are so 

 prominent as to be nearly spherical : this is the case with 

 some aquatic bugs, as Manatra, Hydrometra, and several 

 male Ephemera d . Very often they are hemispherical, as 

 in the tiger-beetles [Cicindela L.), and the clocks or dors 

 [Carabus L.); 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; 



a De Geer iii. t. xxxiv. /. 17, 18, 24. oo. 



b Mon. Ap. Angl. i. t. xiv. no. 11./. 1./. Linn. Trans, xi. t. ix. 

 /.lO.d. 



c Plate XIII. Fig. 9. Fuessly Archiv. t. vi. 



d Schellenberg Cimices t. xiii. ix. /. 1. a. De Geer ii. t. xviii. 

 /• 10. 



