Chap. XIII.] THE COCKROACH. 273 



There has been much discussion as to the nature of insect 

 vision. If we cut out the faceted cornea of a beetle's eye and 

 clean its inner surface, mount for the microscope, and having 

 focussed the facets raise the objective, we shall see, if the flat 

 side of the mirror be used, a great number of images of the 

 window or lamp, one for each facet. Some, relying on this fact, 

 maintain that the compound eye is a multiple structure consist- 

 ing, in the drone for example, of more than 13,000 eyes the 

 two compound eyes giving the brain or ganglia some 26,000 

 images to co-ordinate. It would seem, however, that in the 

 water-beetle (Hydrophilus) the focus of a corneal lens is about 

 three millimetres away and altogether behind the eye. It is on 

 the whole more probable that each dioptric element transmits 

 the image of a point, and that a number of the elements combine 

 to form a stippled image, or a picture in mosaic. 



In addition to the many-faceted eyes, some insects have simple 

 eyes or ocelli ; the bee, for example, has three such, arranged 

 in a triangle on the vertex of the head (queen) or on the fore- 

 head (drone). These would seem to be used for near vision, 

 the many-faceted eyes being used for far vision. Each has a 

 lens arising by differentiation and thickening of the cuticle; 

 the crystalline cones are not developed as such ; the hypodermic 

 cells give rise to a retinal layer. 



Reproductive Organs. (1.) In the Female. The paired ovaries 

 consist of groups of eight beaded ovarian tubules, the apices of 

 which are filamentous and united. In them the ova are 

 developed (Fig. 83, A.). The upper part (germogen) of each 

 tubule contains protoplasm, in which nuclei are imbedded; 

 further down separate nucleated masses are found ; and lower 

 still these nucleated masses are arranged in single file, and 

 are recognisable as ova. Around each ovum small nucleated 

 cells arrange themselves as a single-layered follicle. A vitelline 

 membrane is secreted by the inner surface of this egg-follicle, 

 and a chitinous chorion by its outer surface. The lowest egg 

 is much larger than the others The two oviducts (o. d.) unite to 

 18 



