108 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



produce a sharper and more detailed impression than if the same 

 visual area be occupied by a smaller number of larger visual 

 elements. The unknown photo-receptive capacity of the nervous 

 centres within the optic lobes must also be involved. 



It is well known that compound eyes are divisible into two main 

 types, namely, those giving an apposition image, and those giving 

 a superposition image. In the first type the vision is essentially 

 diurnal, and the images are composed of a large number of apposed 

 points of light : such images are true mosaics as implied by the 

 theory of mosaic vision. It has been generally believed that the 

 direct image so formed is an inverted one, but apparently this 

 is not always the case, since Eltringham (1919) has observed a 

 direct image with the eucone eye of a butterfly. Compound eyes 

 giving a superposition image differ from the previous type in the 

 disposition of the components of their ommatidia. The rhabdoms 

 are not in contact with the apices of the crystalline cones, and the 

 two elements are separated by a space filled with a transparent 

 medium. The ommatidia in this kind of eye are consequently 

 greatly elongated. The distribution of the pigment surrounding 

 these elements is capable of adjustment according to the amount 

 of light available, and consequently this type of eye is most highly 

 developed in nocturnal or crepuscular species. At night time the 

 pigment congregates far forward, so as to expose a larger area of 

 the apices of the crystalline cones, with the result that rays of 

 light entering adjacent ommatidia traverse the space already 

 alluded to, and contribute to those entering the same retinula. In 

 this type of vision there is an overlapping of points of light, and 

 the image thus formed is termed a superposition image. It will be 

 evident that, in an eye of this description, a limited amount of light 

 will produce a better image than in an eye giving an apposed image 

 where so much of the light is absorbed by pigment. The eyes of 

 nocturnal insects are adapted, therefore, lor the perception of 

 movement and general form only, and, as already stated, they are 

 able to take advantage of varying degrees of light. In the presence 

 of daylight the pigment moves backwards like a dark sleeve 

 cutting off more and more of the peripheral rays, thus tending to 

 give a sharper image, and one more resembling the apposition type. 



