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IOWA ACADEMY OF SCIENCES 
smaller number, but there is reason for believing that their condition is 
not a primitive one, but one derived from that mentioned above. Thus, 
in the genus Tomocerus, each eyespot contains but six ocelli. In Sir 
John Lubbock’s great “Monograph of the Thysanura”, there appear some 
errors concerning the ocelli of certain genera of the Collembola; he 
state that Isotoma and Tomocerus have each seven ocelli on a side. 
His figures, however, are at variance with the statement, and show eight 
for Isotoma, which is the usual number in that genus, and but six for 
Tomocerus, which is likewise correct. Figs. 1 and 7. He states that 
Orchesella has six, and with this his figure of the eye in that species 
agrees. I have examined specimens of four species of Orchesella, and all 
appear to have eight ocelli, though one of these is usually quite small. 
Fig. 6. 
A study of their positions indicates that the individual ocelli are 
probably homologous in all the different species; and that there is not 
only a typical number of ocelli, but a typical pattern for the placing of 
tliem upon the eyespot. This may vary somewhat, but usually seems 
to have the ocelli placed along the course of a more or less elongated 
letter S. Certain of the ocelli in this figure seem to be less tenacious 
of existence than others, and in certain genera we may find these grow- 
ing smaller and less important as we pass from species to species until 
they entirely atrophy and actual numerical diminution results. 
The Collembola show a strong tendency to the cave habit, and thus 
we find more or less profoundly modified species in many of the genera. 
In many cases, the modifications induced by the cave habit have so 
differentiated species along common lines, as to have led to the for- 
mation of new genera for their reception. It is to be expected that 
the visual organs would be most quickly and deeply modified by a 
habit which led to a life in situations where light was obscure or absent. 
I have recently re-examined all the species in my collection, as well 
as all accessible figures of the eyes of species I have not, and find some 
very interesting gradations. The eyespot containing the ocelli is usually 
very dark, often deep black. It often gives bronzy reflections when 
viewed by reflected light, excepting the ocelli themselves which appear 
dense black. In shape the eyespot varies considerably, according to 
species. While it is commonly of a more or less elongated rectangular 
shape, with the longer axis corresponding to that of the animal, it may 
be oval, ovoid, reniform, triangular or crescentic, and is often irregu- 
lar upon the edges. The shape corresponds rather closely to the plac- 
ing of the ocelli; sometimes persisting, however, over areas from which 
the ocelli have atrophied. When the ocelli disappear from the middle 
of an eyespot, as they do in some instances, the pigment may also disap- 
pear from the middle part of the eyespot and still be retained at its 
two ends where the ocelli still persist. This gives rise, of course, to 
the appearance of two eyespots on each side of the head. One of these 
may even disappear, leaving but one small one on each side. Fig. 5 
shows an eyespot which may be considered fairly typical. In this 
species {Isotoma muskegis Guthrie) the ocelli are approximately of the 
same size, while the densely pigmented eyespot is elongate, and pretty 
