132 



HISTOLOGY 



The organ consists of a round mass of simple polyhedral cells, and is 

 situated in the abdomen facing its ventral surface. Those of the cells 

 which are nearest that surface are the light-cells, and secrete the light sub- 

 stance or luciferase in their abundant cytoplasm. The inner and dorsal 

 layer secretes, instead, a white material (ammonium urate) and serves 



to reflect all light rays downward. 

 The light tissue is most abun- 

 dantly supplied with trachea to 

 bring in air. The tracheae branch 

 freely, and the terminal twigs are 

 distributed so that each group 

 of several cells has one of these 

 twigs in its midst (Fig. 120). 



The relations of such a ter- 

 minal twig to the group it sup- 

 plies are peculiar. It enters into 

 a central cell of the group which 

 is called the tracked end-cell, and 

 in this cell it gives off from its 

 end a number of fine air capilla- 

 ries, the tracheoles, that branch 

 out and pass through the cell 

 group between the cells. From 

 each end twig there are three to 

 five or more of these tracheoles, 

 finely ringed, and each one soon 

 diminishes in size to a still 

 smaller and smooth, capillary 

 tube, which anastomoses with 

 some of the tracheoles coming 

 from other end-cells. In the 

 first part of their course, at least, 

 the tracheoles are followed and 

 surrounded by protoplasmic 

 processes of the tracheal end- 

 cells. In the latter part of their 

 way they apparently pass be- 

 tween the light-cells and never 

 enter their cytoplasm. Bongardt 

 was not able to find any tracheal 

 nuclei in this part of their course. 

 The tracheal end-cells, until proved otherwise, must be looked upon 

 as specialized hypodermal cells that have grown into the tissue with the 



FlG. 121. Vertical section through the median 

 abdominal light-organ of Pyrophorus: ref.c., 

 reflector cells; lum.c., luminous cells; mus.f., 

 muscle fibers in transection. Tracheal details 

 not shown; they may be represented in general 

 by those in the preceding figure. X 850. 



