The Eyes of Dactylopius. 81 



the inuuth parts to be actually replaced by the ventral pair of acces- 

 sory eyes. 



Development. 



Passing now to a consideration of the development, it is needless 

 to repeat that a thorough knowledge of an adult structure can be had 

 only through a study of its growth. in the present instance, some 

 of the most interesting features in regard to the structures under con- 

 sideration were brought out by such a study. For these reasons it 

 might be profitable to give a short account of the various developmental 

 stages, of which there are roughly five, corresponding in a large measure 

 with the nymphal periods of the insect, beginning with the second. 

 As before, the ventral eyes will be taken as typical. 



In the latter part of the second nymphal period, sometimes at 

 the beginning of the third, the hypodermis, in the region from which 

 the eyes develop, thickens through a proliferation and elongation of 

 the cells. These are irregularly arranged in a single layer and cover 

 most of the ventral surface of the head, except for the mid-ventral 

 line. By the beginning of the third stage these two groups of cells 

 meet along the median line and form a band of about uniform depth 

 across the ventral surface. Before long, however, the cells of the original 

 areas increase still more, both in number and length, so that at these 

 two points they extend some distance entad. There is still but one 

 irregularly arranged layer of cells (Fig. 3). 



Shortly after this, but still at an early period in the third stage, 

 is to be seen the beginning of the visual rods. They are formed by 

 terminal growth at the distal ends of the retinal cells. Patten (87) 

 found that in the eyes of Acutus an undivided strip of chitin forms 

 across the distal end of the retina and then later becomes divided into 

 rods corresponding to the individual cells. The rods in Dactylopius 

 are separate and distinct from the very first. They do not appear to 

 be of a chitinous nature but, on the other hand, to be composed of a 

 granulär substance secreted by the retinal cells. The first indication 

 of their appearance is what seems to be at first sight an uninterrupted, 

 dark line extending across the distal end of a longisection of the optic 

 thickening, and even beyond to the shorter cells that form a transition 

 between the elongated and the normal hypodermis (Fig. 4). Never- 

 theless, careful examination shows that the line is not uninterrupted. 

 It is composed of a succession of numerous, slightly separated, circular 

 areas that stain darkly, each in a hypodermis cell, which latter at 



Zeitachrift f. wissensch. Zoologie. XCIII. Bd. 5 



