20 INSECT A. 



115), Xenos, and others, the eyes are borne upon 

 fixed stalks ; but these are simply extensions of the 

 head, and are not movable like the eye-stalks of the 

 lobster, so that they cannot be regarded as append- 

 ages. In the median line of the face is a simple eye, 

 or ocellus (PI. I., Fig. 4, oc), and above it, on either 

 side, two lateral ocelli (Fig. 4, oc^). The young locust 

 does not possess compound eyes, but in their place 

 are groups of simple eyes, which during the growth of 

 the embryo increase in number, and finally unite to 

 form the large many-faceted visual organs.^ Some in- 

 sects like Xenos have groups of simple eyes in the 

 adults, showing the transition state. 



Very different views have been advanced in regard to 

 the structure and physiology of the organs of sight. In 

 1826 Miiller advanced the theory of "mosaic vision," 

 according to which each facet of the eye sees only a por- 

 tion of the object, so that but one image is produced. He 

 also maintained that the simple eyes were used for near 

 objects, and the compound eyes for distant views. Grena- 

 cher^ in 1879 published a valuable work but unfortunately 

 there is no English translation. In 1885 Hickson^ figured 

 and described the structure of the compound eye. This 

 author maintains that the retinulas (which correspond to 



1 According to Patten the simple eye becomes differentiated 

 to form the compound eye, so that the latter he considers not 

 many simple eyes joined together, but " a modihed ocellus " 

 (see " Eyes of Molluscs and Arthropods." Naples : abstract in 

 Journal of Morphology, 1887, Vol. I., No. i, p. 67). 



^ See Untersnchungen iiher das Sehoi-gan der Arthropoden. 

 Gdttingen, 1879. 



^ See the " Eye and Optic Tract of Insects," Quarterly 

 Journal of Microscopical Science, April, 1885; also "The 

 Retina of Insects," Nature, Vol. XXXI. 



