TISSUES OF LIGHT-PRODUCTION 133 



tracheae to form and maintain these necessary tubes. Theoretically a 

 thin layer of their cytoplasm should follow all branches of any tracheal 

 ending. As so far actually seen, it follows these branches only part 

 way, and the finest branches must be formed, if this is true, by the 

 activities of the light-cells which lie in a trophic contact with them. 



It should be noticed that, in this tissue, the end-cells and light-cells 

 are irregularly grouped in alveoli of sufficient size throughout the light 

 tissue. This is also true of the elatrid beetle, Pyrophorus, of Jamaica, 

 from whose median, abdominal organ, the Figure 121 was drawn. 



We shall now study briefly the luminous tissue of our common 

 American firefly, Photinus marginalis, which has been recently worked 

 out by Miss Townsend. 



In this insect are found the same two layers, a light-producing layer 

 next the integument, and proximal to this a reflecting layer to send out 

 all rays. Here, too, the trachety come down through the reflecting layer Q&. 

 and enter the photogenetic layer, but not so haphazardly as in Lampyris. 

 They descend at regularly spaced intervals into little cylinders which 

 reach vertically the whole distance from reflector to integument through 

 the light-cell layer (Fig. 122). In their downward course they give off 

 laterally about a hundred terminal twigs, which each pass directly into 

 a cell lying next to the main tracheae. These cells form the walls of the 

 cylinder, and from the way that the lateral terminal twigs break up inside 

 of them, we can recognize them at once as the tracheal end-cells that were 

 found in Lampyris. The only difference is that in Lampyris the end- 

 cells were scattered among the light-cells at random, so long as each one 

 was the center of a round group of the light-cells that it could supply with 

 oxygen, while in Photinus there is an organization of the end-cells into 

 a layer that surrounds each main tracheal stem as a cylindrical tube, 

 and outside of and between these cylinders lies the mass of light- 

 cells. 



There is here an evident structural economy. The length of tracheal 

 tube is shorter, and the oxygen-laden air can be brought in larger quan- 

 tities and more suddenly and efficiently into contact with the light-cells. 

 The result is also evident when the insects are observed in life. The 

 Photinus gives a quick, short, and dazzling flash, while a glowworm (the 

 common ground glowworm, a lampyrid larva), with its diffuse form of 

 tissue, glows slowly and softly for a few seconds. 



Several centipedes show a light which is produced by a discharged 

 external secretion. It is thrown and rubbed on their enemies as a slime 

 containing tiny granules. 



Among the tunicates are some members of the group that are pro- 

 vided with photogenetic organs. Pyrosoma is one in which this is very 

 evident. The tissues consist of two cell masses in the integument, one 



