142 PROCEEDINGS OF THE ACADEMY OF [Feb., 



of the pupa stage proper the ommatidia are completely formed. The 

 exact method by which this differentiation takes place is difficult to 

 learn, since the growth at this time is so very rapid that it is practically 

 impossible to get all the stages. The head of the insect grows very 

 rapidly and the eyes keep pace with it. The retinular cells become 

 longer and broader, and the retinulae lie closer together. The cone and 

 corneal pigment cells come to lie at the distal end of the retinula by 

 the method previously described. When the pupa stage proper is 

 entered upon, the area of the eye is practically that of the adult eye. 



The various stages of the pupa period are easily distinguishable 

 externally, and this fact is of great value in the selection of material. 

 The eye is first white, like the rest of the body, then pink, then brown, 

 and finally, as the other parts of the body take on their adult colors, 

 black. These changes of color are due to the deposition of pigment 

 in the various cells of the ommatidium, pigment in the corneal pigment 

 cells being red in color, giving the first color externally, and the darker 

 pigments of the other cells obscuring this color at a later period. These 

 changes enable one to choose the desired material by simply uncapping 

 the cells containing pupse without removing the bee from its cell, since 

 the head is always toward the outside. 



From this stage on it becomes necessary to discuss the various parts 

 of the ommatidium separately. Such a method tends to give the im- 

 pression of a lack of continuity in mode and time of development, but 

 the drawings which accompany the description are made of the entire 

 ommatidiimi, and these will show the relative size and degree of devel- 

 opment at various stages. The order followed is from the retinula to 

 the more lateral cells. 



a. The Retinula. — The retinula cells are eight in number normally, 

 but numerous ommatidia are observed in which nine cells are present. 

 In the earliest pupa stage (fig. 3) these cells extend from the proximal 

 end (apex) of the cone cells to the basement membrane, and each cell 

 has a protoplasmic process extending through the openings in the base- 

 ment membrane toward the optic lobes, w^hich later functions as the 

 nervous connection of these cells with the cells of the retinular ganglion. 

 At this time the only indication of the rhabdome is the clear space at 

 the distal end which was described for the larval ommatidium; its 

 differentiation has gone on little, if any, during the rearrangement of 

 cells. The cytoplasm at the distal end of the cells is more granular 

 than elsewhere, and by the time the eye has reached the stage figured 

 pigment is laid down around the forming rhabdome. This is the first 

 pigment laid down in the ommatidium, but at almost the same time 



