PHOSPHORESCENT ORGANS 425 



phosphorescent organ or plate consists, as first stated by Kolliker, of 

 two layers lying one over the other, a dorsal one (Fig. 388, d) which 

 is opaque, chalky white, and non-photogenic, and a lower one (v), 

 the active photogenic layer, which is transparent. Through the 

 upper or opaque layer and on its dorsal surface extend large tracheae 

 and their horizontal branches, from which arise numerous very fine 

 branches which pass down perpendicularly into the transparent or 

 photogenic layer of the organ. Each tracheal stem, together with 

 its short branches, is enveloped by a cylindrical mass of transparent 

 tissue, so that only the short terminal branches or very fine tracheal 

 capillaries project on the upper part of the cylinder. These finest 

 tracheal capillaries are not in Luciola filled with air, but with a color- 

 less fluid, as was also found by Wielowiejski and others in Lampyris. 



These transparent cylinders, with the tracheae within, forming 

 longitudinal axes, resemble lobules. These lobules are so distributed 

 that they appear on a surface section of this plate as numerous round 

 areas in which circular periphery the tracheal capillaries are arranged 

 with the axially disposed tracheal end-cells. These " tracheal end- 

 cells " are only membranous enlargements at the base of the tracheal 

 capillaries (Wielowiejski). The cylindrical lobules are separated 

 from each other by a substance consisting of abundant large granular 

 cells (parenchym cells) among which project the tracheal capillaries. 

 The cylindrical lobules extend to the hypodermis and come in con- 

 tact only by their lateral faces with the parenchym. 



The structure of the upper opaque chalky white layer of the phos- 

 phorescent organ is, compared with that of the photogenic lower por- 

 tion, very simple. In its loose, pappose, mass are no cellular ele- 

 ments, but when treated with different reagents it is seen to be filled 

 with countless urate granules (guanine) swimming in the fluid it con- 

 tains, the cell plasma appearing to be dissolved, the cells having lost 

 their cohesion. 



In comparing the phosphorescent plate or organ of Luciola with 

 that of Lampyris, the general structure, including the clear cell ele- 

 ments of the cylindrical lobules, which envelop the perpendicular 

 tracheal twigs and their branches, and also the grannler paren- 

 chymatous cells are alike in both, though the arrangement and distri- 

 bution of the elements in Luciola is more regular, in Lampyris the 

 tracheal stems being irregularly scattered through the parenchym. 



Wielowiejski found in the larval and female Lampyris a higher 

 degree of differentiation than in the male, and Luciola has a more 

 differentiated photogenic organ than Lampyris, as seen in the more 

 regular structure of the lobules. 



