THE COMPOUND EVE OF THE BLOW-FLY. 531 
rhabdomes; hence, I conclude that it is abnormal. I have 
been unable to discover tracheal capillaries on the pigment 
cells of the great rods or cones in the Blow-fly, but I have no 
doubt they exist; indeed, I suspect that the beautiful net- 
work represented by Patten around the cones of Mantis [289, 
Fig. 118], really consists of fine, empty capillary tracheae—as 
I have detected distinct tracheal capillaries on the great rods 
of some Dragon-flies, Agrion puella [219, p. 587]. 
The Pigment Cells of the Cone are large flat cells, which extend 
from the cornea to the apex of the cone. I am not able to 
state positively the number of cells which surround each cone, 
as transverse sections usually only exhibit one or two nuclei, 
but there are, apparently, only two cells as a rule; I have 
occasionally seen three, and the whole structure of the 
ommatea indicates the probable existence of four. These cells 
are very thin and deeply pigmented, their edges overlap each 
other; probably their number varies in different ommatea. In 
Moths, Dragon-flies, and many other insects these pigment cells 
are represented by fringes from the iris cells, and in the Hymen- 
optera by deeply pigmented palisade-like cells, which appear 
to be connected with the iris cells. I have spoken of these 
elsewhere as ciliary rods [233], and regarded them as sensitive 
to light, and Parker [250] has fallen into a similar error and 
regards them as retinal end organs, his ‘ distal retina.’ 
The Iris.—This term has been used to designate the circle of 
deeply pigmented cells which surround the inner extremity of 
the cone or pseudo-cone. They are the ‘ pigment cells’ of the 
second order of Grenacher and the ‘iris tapetum’ of S. Exner 
[252]. Exner has not, however, distinguished between the iris 
cells and the cells surrounding the cone, which Grenacher terms 
‘pigment cells of the first order’; as these are absent in many 
insects, and are replaced by pigmented fringes from the iris 
cells, it is probable that Exner had such insects in his mind 
when he used the same term for both sets, although he repre- 
sents them as distinct in a figure of the eye of Eristalis 
(252, Fig. 60]. 
The iris cells of the Blow-fly are five in number, and send 
