88 THE BIOLOGY OF INSECTS 



have seen that the cuticle is thickened so as to form a 

 biconvex lens beneath which lie the retinal cells. Here, 

 therefore, is an arrangement by which rays of light coming 

 from surrounding objects may be brought to a focus on the 

 retina so as to produce there an inverted image. From the 

 analogy of structure of a vertebrate's eye and our own 

 experience of seeing, there seems no reason to doubt that 

 an insect's ocellus is adapted for the appreciation of definite 

 form. But in contrast to the vertebrate's eye, the ocellus 

 has no provision for accommodation, and the usual high 

 convexitN' of its lens renders it available for seeing only 

 objects that are close at hand, wliile the extent of its field of 

 vision must be greatly restricted. 



A compound eye consists of a number of elements each 

 consisting of a set of transparent structures — cornea, lens, 

 cone, rhabdom — the last-named surrounded by the receptive 

 retinular cells connected by fibres with the ganglionic 

 centre towards which the closely arrayed elements converge. 

 J. Miiller long ago (1826) pointed out that the general visual 

 sensation induced through such an eye must be regarded as 

 the sum of the multitudinous sensations due to the individual 

 elements which, owing to the surrounding dark pigment are 

 more or less insulated from one another. Such a hypo- 

 thetical '' built up " impression suggested to Miiller the 

 term " mosaic vision " from the analog}' of a picture made 

 up of a number of small apposed pieces ; and this term has 

 been generally accepted as being suitably descriptive of an 

 insect's seeing with its compound eyes. But how far is 

 such vision definite ? Miiller denied that any clear image 

 could be formed in a compound eye and suggested that an 

 insect receives no more than the impression of as many 

 spots of light as there are elements, the slender pencil of 

 rays traversing each element being concentrated at the apex 

 of the cone and the intensit}^ of light appreciated through 

 each element varv'ing with the source whence it comes. 



H. Grenacher (1879) recognised that the lens of each 

 element is adapted for the production of an image, but 

 believed that the position of the image is such as to render 



