KETINULA—PIGMENT. 151 



as eight, originally separate, but closely combined cells. 

 They converge on the optic lobe, and form an outer 

 nucleated sheath, enclosing a strongly refractive, 

 generally quadrangular, rod (the rhabdom, Bm), the 

 relation of which to the filaments of the optic nerve 

 is not yet well understood. 



4. The pigment (Pg). 



Between each separate eyelet (ommateum, or omma- 

 tidium, as it is termed by Hickson), is — at least, in 

 some insects — a long, tubular, thin- walled trachea. 

 These are difficult to see in prepared specimens, but 

 have been mentioned by several observers. They were 

 first, I think, figured by Leydig,* and more recently 

 by Hickson. 



Finally, the eye is bounded by a basilar membrane, 

 which is perforated by two sets of apertures, a series of 

 larger ones for the passage of the tracheal vessels, and 

 of smaller ones for the nerve-fibrils. 



The crystalline cone is not, however, always present, 

 and Grenacher divides the compound eyes of insects 

 into 'three types : acone eyes, in which the crystalline 

 cone is not present, but is represented throughout life 

 by distinct cells ; pseudocone eyes, in which there is a 

 special conical and transparent medium ; and, lastly, 

 eucone eyes, with true crystalline cones." f 



* " Zum feineren Bau der Insekten," Muller's Arch, fur Anat. u. 

 Phya., 1855. 



f Acone eyes occur in Neraatocera (gnats), Hemiptera (bugs), For- 

 ficula (earwigs), and those Coleoptera (beetles), which have less than 

 five tarsal joints. Pseudocone eyes occur in the true flies (Muscidse). 

 Eucone eyes prevail among other insects : Lepidoptera, Hymenoptera, 

 Neuroptera, Orthoptera, Cicadidse, the Coleoptera with five tarsal 

 segments, and among Diptera the single genus Corethra, which, more- 

 over, is remarkable as possessing compound eyes, even in the larva 

 and pupa. 



