144 



AN INTRODUCTION TO ENTOMOLOGY 



the light passing from the tip of a cone may reach several rhabdoms, 

 making the eye a night-eye. These changes in the position of the 

 pigment are probably due to amoeboid movements of the cells. 



Divided Eyes. — In many insects each compound eye is divided 

 into two parts; one of which is a day-eye, and the other a night-eye. 

 The two parts of such an eye can be readily distinguished by a differ- 

 ence in the size of the facets ; the portion of the eye that functions 

 as a day-eye being composed of much smaller facets than that which 

 functions as a night-eye. 



A study of the internal structure of a divided eye shows that the 

 distribution of the pigment in the part composed of smaller facets is 

 that characteristic of day-eyes ; while the part of the eye composed of 

 larger facets is fitted to produce a superimposed image, which is the 

 distinctive characteristic of night-eyes. 



Great differences exist in the extent to which the two parts of a 

 divided eye are separated. In many dragon-flies the facets of a part 

 of each compound eye are small, while those of the remainder of the 

 eye are much larger ; but the two fields are not sharply separated. In 

 some Blepharocera the two fields are separated by a narrow band in 

 which there are no facets, and the difference in the size of the facets of 

 the two areas is very marked. The extreme condition is reached in 



certain May-flies, where the two 

 parts of the eye are so widely separa- 

 ted that the insect appears to have 

 two pairs of compound eyes (Fig 158). 

 The tapetum. — In the eyes of 

 many ariimals there is a structure 

 that reflects back the light that has 

 entered the eye, causing the well- 

 known shining of the eyes in the 

 dark. This is often observed in the 

 eyes of cats and in the eyes of moths 

 that are attracted to our light at 

 night. The part of the eye that 

 causes this reflection is termed a tapetum. The supposed function of a 

 tapetum is to increase the effect of a faint light, the light being caused 

 to pass through the retina a second time, when it is reflected from the 

 tapetum. 



The structure of the tapetimi varies greatly in different animals; 

 in the cat and other carnivores it is a thick layer of wavy fibrous tissue; 

 in spiders it consists of a layer of cells behind the retina containing 



Fig. 158. — Front of head of Cloeon, 

 showing divided eyes; a, night-eye; 

 b, day- eye; c, ocellus (From Sharp). 



