182 THK ENTOMOLOGIST. 



I. The Structure of the Ocelli. 



Dr. Grenacher has investigated these organs in the larva 

 of Dytiscus and Acilius, in several spiders, and in some 

 perfect insects ; and he shows, as Leydig did, only more 

 completely, that there is an important relation between the 

 structure of these organs and that of the compound eye. 



The simplest ocelli are those of the larva oi' Dytiscus — the 

 cuticle is swollen slightly to form the lens ; the other struc- 

 tures of the eye, the vitreous and the retina with its pigment, 

 are manifestly differentiations of the hypoderrais, or cellular 

 layer of the integument : in the young larva, the passage 

 from the ordinary hypodermis to the cells of the vitreous is 

 quite gradual ; the pigment, which serves as a choroid, is 

 contained at the outer ends of the cells which form the 

 vitreous; it surrounds the nuclei of these cells; the retina 

 consists of a series of fusiform cells, which are apparently 

 only slightly differentiated from the cells of the vitreous body, 

 but which are furnished with a well-developed layer of rods, 

 so placed that they receive the image formed by the lens 

 upon their surface. 



In the eyes, stemmata, of the larva of Acilius, there is a 

 very considerable advance ; the lens is very convex, and 

 the vitreous is more decidedly differentiated from the hypo- 

 dermis. The retina also exhibits a very remarkable pecu- 

 liarity ; it is deeply cleft by a fissure, extending almost 

 through its entire thickness, both the walls of which are 

 lined by a series of gigantic, but evidently true rods — a 

 condition which reminds us of the yellow spot in the axis of 

 the human eye, at least as far as their probable function is 

 concerned. 



In spiders and Phnlangid<B the principal difference in the 

 ocelli, as compared with those of the already described larvae, 

 is that the retina is more strongly differentiated from the 

 cells of the vitreous and hypodermis. The most remarkable 

 peculiarity in the eyes of spiders is their dimorphism, the 

 same insect having two sets of eyes with very different 

 retinal structure : as an example of this. Dr. Grenacher 

 figures and describes the two forms of eye met with in the 

 common garden spider, Epeira diadema ; in the anterior 

 eyes the retinal cells are much elongated, and bear at their 

 inner extremities a layer of very small rods, closely abutting 



