No. I.] EVES OF ARTHROPODS. 161 
Grenacher maintains that they are horizontal like the others, 
but much longer. 
Eye I. of Acilius is the largest and most complicated, and as 
there are several instances in which the posterior dorsal ocellus 
of other Insect larvze is conspicuous on account of its greater 
size, it may be well to mention that a similar condition is to be 
found in Lithobius, for Grenacher says, p. 441, “ Die Einselau- 
gen sind unter sich nicht vollig gleich gross ; auffallig ist freilich 
nur die uberwiegenden Grdsse der jenseite am meisten nach 
hinten gelegenen.” 
We have tried to show above: That there ts a striking resem- 
blance between the eyes of Myriapods and those of larval Insects, 
(1) 22 the pigmentation of the retinal cells ; (2) in the shape of the 
retina; (3) 2x the presence of a retinal furrow, and (4) and (5) in 
the probable presence of dimorphic retinal cells and rods, some of 
which are horizontal and others upright. This similarity is all 
the more important since we seek in vain for any special points of 
resemblance between the larval ocelli of Insects and those of Crius- 
tacea or Arachnids. Even if it should be shown that the retinal 
cells in Spiders and Scorpions are upright, there would still be 
a great difference between the ocelli of Insects and those of 
these groups, for the presence of upright rods in both instances 
is in itself too general a feature to be of much importance in 
determining affinities, and there is no trace whatever in Scor- 
pions, Crustacea, or Spiders of those peculiar features, such as 
horizontal rods, retinal furrow, and dimorphic retinal cells, that 
are common to Insect larvze and Myriapods. 
SPIDERS AND Scorpions. — Although both Graber and 
Sograff maintain that the eyes of Myriapods and the ocelli of 
Insects and Spiders have essentially the same structure, it is 
probable that both writers based their opinions solely on the 
fact that in all these cases the eyes were composed of two 
layers. But granting that such a condition prevails, very little 
is gained thereby, for it leaves out of account altogether the 
fundamental differences in the structure of the retinas. 
In spite of the recent contributions to our knowledge of the 
eyes of Arachnids, by Schwemkewitz, Bertkau, Locy, Mark, 
and Parker, we are still much in the dark concerning the struc- 
ture and development of these eyes. 
