A. D. Imms 
121 
by their nut-brown colour, whereas the already existing ones, owing to 
their greater amount of pigment, appear dense black (Fig. 24 a). 
These are the rudiments of the future imaginal compound eye 
(“ Hauptauge ”). As development proceeds, each of the small pig¬ 
mented elements increases in size, with the result that it becomes in 
close contact with its neighbours, and in a larva about 7 mm. long 
they collectively form a conspicuous black crescentric shaped tract on 
either side of the head (Fig. §4 b). The subsequent extension of the 
organ is effected by new elements being added around its periphery. 
A short optic nerve is present on either side in relation with the 
larval eye ; in larvae about 7 mm. long a nerve from the imaginal 
ocular rudiment is distinguishable, and the two nerves of aside combine 
to form the main optic nerve (b. and c.). 
When viewed in sections, the larval eye consists of a central 
densely pigmented area bordered by elongated cells whose narrower 
inner ends are directed towards the centre of the organ (c.). The eye 
is densely loaded with pigment of a rust-brown colour which appears 
black when viewed in thick layers. 
The developing imaginal eye consists of a number of deeply 
pigmented fusiform bodies, placed at right angles with the surface of 
the head. These elements are the ommatidia of the compound eye, 
and they each consist, as seen in transverse section, of a central axis 
or rhabdom surrounded by a circlet of densely pigmented cells (the 
retinulae). Intervening between the ommatidia and the external cuticle 
is a layer of but little modified hypodermal cells (/i). It is destined to 
form on the outside the corneal facets of the imaginal eye, and on its 
inside the crystal cells. 
The cells of the hypodermis immediately bordering on the imaginal 
ocular rudiment are markedly columnar in form, and in places it is 
folded or invaginated into small pit-like areas, which are the very early 
stages in the development of new ommatidia. 
In addition to the Culicidae, certain other families of nemocerous 
Diptera, having eucephalous larvae, are remarkable in that the 
imaginal eye develops in close l’elation with that of the larva and at an 
early stage in the larval life-history. A short comparative study of 
the eyes, in both the larvae and pupae of such Diptera, has recently 
been made by Zavrel (1907). 
