53- 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 denned 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 man}- Moths, 

 more especially as the septa between the retinulce 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 tracheae at the 

 inner valve ; this ring receives the insertion of a muscle, which 

 undoubtedly alters the tension of the basilar membrane. In- 



