THE COMPOUND EYE 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 [239, 

 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 



