PHOTOGENIC ORGANS OF LAMPYRIDS 157 



into the nucleus. This is noteworthy, especially from a physio- 

 logical standpoint, for it indicates that the end-cells are the 

 important mechanism for stimulating the light production. 



In the newly-formed pupa a decided advance in development 

 is observed (fig. 28). The two layers can now be clearly sepa- 

 rated. The cells of the photogenic layer, P, are more spherical, 

 have increased in size and in density of contents, and the nuclei, 

 though larger, are less sharply defined. All of the blue and 

 brown granules, once so abundant, have disappeared, and, owing 

 to a fine knotted network of cytoplasm, the cells have a rather 

 granular aspect. It seems probable that the cells of both layers 

 divide, since the cells of the fat-spheres involved in the formation 

 of the organ seem not to be sufficiently numerous to preclude a 

 later proliferation. The cells of the luminous layer are larger 

 and rather more regularly arranged than those of the non- 

 photogenic layer, U, which has resolved itself into several layers 

 of smaller, clearer, more or less flattened cells. 



The differentiation of these two layers is further accentuated 

 by the development of the 'tracheal end-cells' which are almost 

 certainly formed from the tracheal epithelium in the lower or 

 photogenic layer. Tracheal end-cells, so named by Max Schulze 

 ('65) who studied exhaustively the light-organs of the European 

 Lampyris splendidula, are cells whose cytoplasm envelop, at 

 least basally, the tracheoles or fine terminal, capillary tubules 

 of the tracheae. The tracheoles, though chitinous, lack the 

 tsenidial thickenings characteristic of tracheae. Figure 23 illus- 

 trates a tracheal end-cell, modified from a sketch by Lund ('11), 

 of the photogenic layer. Holmgren ('95) uses the term 'transi- 

 tion cell' (Ubergangszelle) for these end-cells, since he claims 

 that they are not the terminal cells, for there exist other very 

 small and elongate cells distad to them. Townsend ('04) uses 

 Holmgren's term. At any rate it seems reasonably certain that 

 the tracheal end-cells are really terminal in the Lampyridae. 

 (Tracheal end-cells are not peculiar to the photogenic organs of 

 insects but occur in other tissues, such as the spinning glands 

 of Lepidoptera, in the Malpighian tubules of Hydrophilus piceus 

 (Schneider, '02), in the fat-body (Schneider, '02, Thulin, '08), 



