AND THE MOKPHOLOGY OP THE EYE IN INSECTS. 407 



The reflex appears as a disk having a fiery glow in Moths, and as a bright ruby spot in 

 the Cabbage Butterfly. Sometimes six spots, surrounding a central spot, are seen in the 

 eye of this insect : perhaps these are diffraction-images. A similar appearance is seen 

 when the eye of this insect is observed by the naked eye, except that the spots are black. 

 The central spot is always opposite the eye of the observer, whatever the position of the 

 eye of the insect. 



The reflex seen with the micro-ophthalmoscope is green in Tipula and bright yellow 

 in the Diurnal Flies. Coloured diffraction-fringes are usually present around the central 

 bright spot in both these insects ; but the central image is sometimes surrounded by a 

 perfectly black ring. 



The manner in which the luminous reflex scintillates is very suggestive of an altera- 

 tion in the focal plane of the dioptric structures under the control of the insect. 



II. The Anatomy and Functions or the Neuron. 



The neuron consists of a large nerve-papilla, or of a series of papillge, which arise 

 from the side of the procephalic ganglion, and form a retinal expansion on the inner or 

 neural surface of the membrana basilaris (figs. 41 to 07). 



The neuron may be conveniently divided, for purposes of description, into three parts 

 — the retina (rt.). the optic nerve (no.), and the optic ganglion (g. 1 ) (fig. 41). 



The retina consists of a layer of bacilla, supported by a complex neuroglia (fig. 56). 



The bacilla present an inner (i.) and an outer segment (o., fig. 53), like those of the rods 

 and cones of the vertebrate retina. Both the inner and outer segments are very easily 

 destroyed, and these structures are best examined by teasing out the recent retina in a 

 very dilute solution of osmic acid, '05 per cent. 



The outer segments are cylindrical or conical, highly refractive, easily stained by 

 chromic acid and its salts, but unaffected by logwood. In the Dragon-flies the outer 

 segments often split into transverse disks. 



The inner segments are protoplasmic, and are easily stained with logwood and carmine. 

 They are richly supplied with very fine tracheal vessels (fig. 53). The inner and outer 

 segments are of nearly equal length, and measure from 25 to 50 micromillimetres each. 

 The outer segments are usually about 2" in diameter, and the inner are somewhat thicker. 



The bacilla are usually arranged in bundles, which are bound together by the neuroglia, 

 each bundle corresponding to a segment of the dioptron. In the Cockroach and the 

 Blow-fly I have occasionally found some of the bacilla with the outer segment double, 

 like the twin cones of fishes (figs. 65 & 66) ; and in some of the bacilla from the latter 

 insect I have occasionally observed a lenticulus (fig. 64) between the inner and the outer 

 segments. This is not stained by osmic acid, and closely resembles the lenticulus 

 described in some vertebrate rods. I have never observed anything like the coloured 

 globules of the cones of birds and reptiles. 



The inner segment is usually finely granular, and is continued inwards as the axis- 

 cylinder of a nerve-fibre. 



The most remarkable modification of the bacilla is the separation of the inner and 



60* 



