236 PTILOTA: IMAGO EXTERNAL ANATOMY. 



entire breadth to the head ; but in some instances they are 

 placed at the extremity of footstalks formed of a continuation 

 of the corneous covering of the head, and consequently inca- 



pable of separate motion. This is especially the case in the 

 dipterous genera, Achias, Diopsis (fig. 60), Plugiocephala, 

 and others, in some of which the footstalks are nearly as long 

 as the entire body ; the same occurs in some species of strep- 

 sipterous insects, as well as in some exotic bugs (Heteropte- 

 ra). Again, the eyes of insects are generally either almost 

 flat, or but very little elevated above the surface of the head 

 (fig. 53) ; but in many cases they are almost globose and very 

 prominent (fig. 54), a peculiarity respecting which, and con- 

 nected with their habits, has been noticed by the late cele- 

 brated Swedish entomologist Dalman, who observes that the 

 very prominent eyes of insects generally indicate their habita- 

 tion to be either in sandy situations or upon the margins of 

 water; this is exemplified in the coleopterous genera, Cicin- 

 dela, Elaphrus, Omophron, and Stenus, and in the henii])tcr- 

 ous genera, Salda and Alydus. Moreover, insects with pro- 

 minent eyes seem to be especially rapacious in their hubits. 

 as in the genera above mentioned, as well as Libellula, 

 Hemerobius, &c. 



In many dipterous insects a remarkable diversity exists in 

 the size of the eyes in the two sexes, those of the mak-s IHMIIU' 

 very large, and united upon the crown of the head, wlim-as 

 those of the females are much smaller, leaving n space bctu i-en 



