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 feature 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 number 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). Fi g- I53- Two visual cells from 

 ~, an ocellus of a pupa of Apis 



1 he 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, 



mellifica. A, longitudinal sec- 

 tion ; B, transverse section; ?z, 

 n, nerves; nu, nucleus; r, 

 rhabdom; p, pigment (After 

 Redikorzew), C, diagram il- 

 lustrating the structure of a 

 rhabdomere; r, rhabdomere; 

 c t cell-body (From Berlese after 

 Hesse). 



