88 ON THE ANATOMY OF THE FLY. 



plexus, between each pair of rods, which runs directly towards 

 the cones, tapering gradually to a fine blind point between 

 them, but giving off no branches. These vessels are pro- 

 portionally of large size, the diameter of the largest near its 

 origin being quite 1 -2000th of an inch. The use of these 

 vessels is unknown ; but the quantity of air supplied to them is 

 manifestly controlled by the epineuron. They may be concerned 

 in altering the shape of the cones and rods, so adapting the eye 

 to distances or to changes of light ; they certainly have a 

 special function distinct from the supply of air to the structures 

 of the eye. 



The optic nerves connect the common retina with a layer of 

 nerve cells, nearly l-100th of an inch in thickness, which 

 surrounds the outer portion of the optic or external ganglion 

 I shall call it the optic zone. This is largely supplied with 

 long loops of trachea3, which commence in a fine plexus between 

 the zone and optic ganglion. The plexus is formed of vessels 

 which dip in between the zone and ganglion, and give off 

 very fine almost straight tracheae, which form long narrow 

 loops by again uniting on the surface of the optic zone, after 

 having traversed its entire thickness. 



The development of the compound eyes commences very 

 early in the formation of the imago. When the pupa is 

 removed from the pupa case on the fourth day, they appear as 

 narrow transparent oval spaces externally. The optic ganglion, 

 even at that early period, appears separated into two parts by a 

 groove. The more external of these eventually forms the optic 

 zone, and the rods and cones are represented by several layers 

 of cells. On the fifth day, these parts are sketched out, as it 

 were, in the cellular layers. Pigment appears on the sixth day 

 of a faint rust colour, and all the parts of the eye are recog- 

 nizable. The compound cornea consists at this period of n. 



