THE INTERNAL ANATOMY OF INSECTS 



137 



Fig. 152. — Head of a 

 larva of Corydalus, 

 dorsal aspect. 



The ocellus of Gastropacha rubi, which is described and figured by 



Pankrath ('90), resembles in structure, to a remarkable degree, an 

 ommatidium, and the same is true of the ocellus 

 of the larva of Arctia caja figured by Hesse ('01). 

 The structure of a visual cell. — The dis- 

 tinctively characteristic featiu-e of eyes is the 

 presence of what is termed visual cells. In 

 insects, and in other arthropods, a visual cell 

 is a nerve-end-cell, which contains a nucleus 

 and a greater or less amount of pigment, 

 and bears a characteristic border, termed the 

 rhabdomere; this is so called because it forms 

 a part of a rhab- 

 dom. 



The visual 



cells are grouped in such a way that the 



rhabdomeres of two or more of them 



are united to form what is known as a 



rhabdom or optic rod. A group of two 



visual cells with the rhabdom formed by 



their united rhabdomeres is shown in 



Figure 153, A and B. 



The form of the rhabdomere varies 



greatly in the visual cells of different 



insect eyes; and the niimber of rhab- 

 domeres that enter into the composi- 

 tion of a rhabdom also varies. 



Figure 153, C represents in a dia- 

 grammatic manner the structure of 



rhabdomere as described by Hesse ('01). 



The rhabdomere (r) consists of many 



minute rodlets each with a minute knob 



at its base and connected with a nerve 



fibril. 



The structure of a primary ocellus. 



— The primary ocelli vary greatly in 



the details of the form of their parts, 



but the essential features of their structure are illustrated by the 



accompanying diagram (Fig. 154). 



In some ocelli, as for example the lateral ocelli of scorpions, 



the visual cells are interpolated among ordinary hypodermal cells, 



Fig- 153- — Two visual cells from 

 an ocellus of a pupa of Apis 

 mellifica. A, longitudinal sec- 

 tion ; B, transverse section; n, 

 n, nerves; nil, nucleus; r, 

 rhabdom; p, pigment (After 

 Redikorzew), C, diagram il- 

 lustrating the structure of a 

 rhabdomere; r, rhabdomere; 

 c, cell-body (From Berlese after 

 Hesse). 



