AND THE MOEPHOLOGY OF THE EYE IX l\si;< 399 



ruptured by pressure ou the cover-glass, when a highly refractive oil-like fluid escapes 

 from them. The axial structure of the great rods in other insects [Notonecta) exhibits 

 the same appearances, some of which are figured by Dr. Grenacher*, and I have no 

 doubt that the very fine axial threads seen in the great rods of dies arc the shrivelled 

 remains of the spindle. 



In the true flies (Musckke) the great rods are very highly modified ; each consists of 

 six cells (sheathing-cells) ; these enclose the axial spindle. This can only be satisfactorily 

 examined, in the recent eye, by very careful teasing. The great rods may be thus sepa- \ 

 rated either in normal saline solution, or in very dilute osmic-acid solution, -05 per cent. ; 

 in the former fluid the sheathing-cells undergo very rapid disintegration by vacuolation, 

 but this occurs less rapidly in the osmic solution. 



The sheathing-cells surround a structure which is fluted like a column, the " Rhab- 

 dom" of Dr. Grenacher. This is the spindle; it consists of an elastic sheath, apparently 

 composed of from four to seven component tubes filled with a highly refractive fluid. 

 The fluid rapidly escapes from the injured tubes in minute glistening drops, whilst tin 

 tubes themselves become converted into fine threads. In the recent condition the 

 slightest pressure suffices to empty the spindles. The various appearances of the com- 

 ponent tubes are represented in the figures (figs. 16-18). 



The tubes of which the spindle is formed apparently intercommunicate at the inner 

 portion of the spindle. They twist and curl in glycerine and water as well as in normal 

 saline solution. They are very elastic, and are easily drawn out to several times their 

 proper length, which they regain when the tension is relaxed. 



The fluid contents of the spindle are soluble in ether and clove-oil. The fluid is 

 perfectly colourless in the Blow-fly; but the same fluid in the cabbage butterfly has a 

 bright ruby tint when seen through the length of the spindle. 



In ordinary transverse sections (fig. 20) the spindle is seen to contain a cavity sur- 

 rounded by a plicated wall. The boundary of the irregular lumen exhibits six or seven, 

 more rarely four, bright spots : these are the sections of the threads. All these details are 

 correctly figured by Dr. Grenacher f. The outer extremity of the spindle in the Hies [s 

 nearly hemispherical, and is imbedded in a structure which I formerly described as a 

 C tetrasome. This structure, in preparations hardened in chromic acid and its salts, has the 

 appearance of four small spheroid bodies ; but in some remarkably distinct sections which 

 were fixed with osmic acid these are seen to be the four segment sofa minute lens. The form 

 and relation of this structure to the spindle will be best understood by a reference to fig. 19. 



In the Lepidoptera the distance of the external end of the spindle from the cone 

 appears to depend on the habit of the insect. It is furthest removed in the Noctuids, 

 and closest in the Diurna: that is, in the Noctuids it is a larger lens of longer focal 

 length, and therefore it receives a much larger cone of light-rays; in the Diurna the 

 body which I formerly named the "tetraphore" $ appears to be the external portion of 

 the posterior refractive system. 



In the Hymenoptera and Neuroptera, it is very difficult to sec the parts in their normal 



* L. c. figs. 51-53. + L. c. fig. 62. t Thil. Trans. /. 1 . 



ry.i 



