532 THE SENSES AND SENSORY ORGANS. 
processes between the great rods, which are probably drawn 
from between the rods in dim light and extended again in sun- 
light. The changes in the position of the iris-pigment is most 
marked in nocturnal insects, such as noctuid moths. S. Exner 
[244] has investigated the disposition of the pigment in insects 
which have been kept in the dark and in those exposed to light, 
and finds that in the latter the pigment fringes extend more 
deeply between the great rods than in the former. 
The Membrana basilaris (the Membrana fenestrata of Exner). 
—This membrane is merely spoken of by Grenacher as a cuti- 
cular layer. It is so distinct and sharply defined that it cannot 
be overlooked, yet it has not received the investigation which 
it deserves, although it is very carefully figured by Grenacher 
as a continuous membrane in several of his figures [222, 
Figs. 44-46, 114-117]. In immature flies and in mature 
nymphs it is seen to lie between two layers of cells, the inner 
pigment cells of the rhabdomes and a layer of epithelioid 
branching cells, which in most insects become the pigment 
cells of the true retina. That it is perforated by the tracheal 
vessels of the dioptron in insects, and by the blood vessels in the 
Crustacea, is indubitable, but I have sought in vain for any 
connection between the retina and the great rods by nerves 
perforating this membrane, and, as I have already stated, no 
such nerves traversing it are figured by Grenacher. Certainly 
there is no appearance of such a perforation in the Diptera, 
although in some insects, the Diurnal Lepidoptera especially, 
this membrane forms a kind of honeycomb, each cell enclosing 
a fasciculus of retinal end-organs, a retinula, so that it is diffi- 
cult to see the same marked division between the dioptron and 
the neuron which is so plainly seen in Flies and many Moths, 
more especially as the septa between the retinule are deeply 
pigmented. 
Mechanism of Accommodation.—In the Blow-fly the edge of 
the basilar membrane is thickened, and forms a ring closely 
resembling that which surrounds the thoracic trachez at the 
inner valve ; this ring receives the insertion of a muscle, which 
undoubtedly alters the tension of the basilar membrane. In- 
