398 ME. B. THOMPSON LOWNE ON THE COMPOUND VISION 



two characters which he had observed in these structures — their tendency to split into 

 transverse disks and their pink colour in the lobster. 



The structure and development of the spindles varies considerably in different Arthro- 

 pods. Sometimes they occupy the whole interior of the great rods, extending from the 

 basilar membrane to the apex of the cone, as in the flies, and more obviously in the eye 

 of the Water Boatman (Notonecta) ; in a greater number of insects, however, they are 

 separated from the cones by a considerable space, the interval being occupied by an 

 albuminous fluid or semifluid substance enclosed in a cuticular sheath, which is sur- 

 rounded by pigment-cells (fig. 22. ch.). 



The albuminous contents of this cuticular sheath have a low refractive index, and 

 partially enclose both the cone and the spindle. The diameter of the tube which inter- 

 venes between the cone and spindle is subject to considerable variation in the same species 

 of insect. In moths it is usually a very fine thread in specimens prepared by section ; but 

 in the recent eye of the very species in which it appears in this form in sections, I have 

 frequently found the tube as wide as the base of the cone (figs. 3 & 27). I believe that 

 the contracted condition is produced by the escape of its contents and the great elasticity 

 of the cuticular sheath itself. 



In the perfectly fresh eye of an insect the spindles are very transparent ovoid bodies, 

 attached by their bases, which are truncated, to the membrana basilaris, and surrounded 

 by pigmented fringes from cells, which cover the outer surface of the basilar membrane 

 between the spindles (figs. 2, 3, & 10. sp.). 



The appearances which the spindles assume after death and in sections prepared for 

 microscopic examination are very various. In the noctuid moths they then appear as 

 chitinous rods ; sometimes they even present a stellate transverse section. In a specimen 

 of the eye of a moth (Hemerophila perfumciria), prepared with osmic acid and afterwards 

 mounted in balsam, they have assumed the form of absolutely empty shells, which are 

 blackened intensely by the acid. 



The spiudles of a yellow Underwing (Triphcena pronuba, fig 22 sp.) from a specimen 

 preserved in a solution of chloral hydrate which was not replaced by any other fluid, 

 resemble tubes filled with minute spherical granules, which give them a transversely 

 striated appearance. These spindles are white and opaque when seen with reflected 

 light. 



In all these cases the form and appearance of the spindle are very different from that 

 which it presents in its normal unaltered condition. The outer end always contracts 

 much more than the inner, so that it has a strong tendency to assume the form of a wine- 

 bottle with a long neck. In the Crane-flies (Tipnla) I have found that the length of the 

 narrow part of the spindle varies greatly in different specimens (figs. 10-13), so that there 

 is no difficulty in connecting the normal ovoid spindle with the bottle-shaped organ 

 which is usually seen in microscopic preparations. 



The spindles of the Crane-flies (Tipula) are seen, in transverse sections, to be composed 

 of seven tubes (fig. 11). 



In some recent preparations I have succeeded in isolating these (fig. 13) ; they then 

 curl and twist in a very remarkable manner, in water and glycerine, and they are easily 



