382 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



3. Basis of the rhythmic flashing oj fireflies. — Since when the photo- 

 genic cells are exposed directly to the air or when the intact animal 

 is placed in one atmosphere of approximately pure oxygen, lumines- 

 cence is continuous, it is clear that the regulation of the flashing 

 rhythm is a result of regulation of the oxygen supply. 



Because destruction of the brain or severing of the nerve cord 

 anterior to the photogenic organs causes an immediate cessation of 

 light production (see also Peters, 1841; Macaire, 1822; Lund, 1911; 

 Williams, 1916-17) and because electrical stimulation of the ventral 

 nerve cord affects flashing (Heinemann, 1886; Perkins, 1931; Snell, 

 1931, 1932), the mechanism of oxygen regulation must be under 

 nervous control — probably a result of spontaneous discharges from 

 the brain. The presence of the brain is not necessary, however, if the 

 ventral nerve cord is electrically stimulated (Snell, 1931, 1932). 



Lund (1911) showed that the control of flashing is not exercised 

 by a regulation of the blood flow (as was believed by Dubois, 1886) 

 or of the muscular respiratory mechanism (as was believed by Heine- 

 mann, 1886, and Watase, 1895). He noted that no movements of 

 the skeleton, observable with a binocular, are synchronous with 

 flashing. 



It has been the general opinion in America, among those interested 

 in the subject, that the nervous system exerts its control by acting on 

 the tracheal end-cells, especially since Schultze (1865), Bongardt 

 (1903), and Geipel (1915) sometimes observed fibers in contact with 

 these cells. The realization that even the ordinary hypodermal 

 cells of arthropods are generally richly supplied with a network of 

 multipolar nerve cells (Schleip, 1914; cf. Hanstrom, 1928; Tonner, 

 1933) would tend to weaken any conclusions from the above evidence. 

 Because the tracheal end-cell was sometimes stained darker than the 

 tracheoles, Dahlgren (1917) suggested that there might be a con- 

 tractile layer of cytoplasm around it. He then stated that "the large 

 body of cytoplasm surrounding it sometimes shows a radial structure 

 that may point to a contractile power." His figure which illustrates 

 such a structure is, however, not convincing. As far as I know, no 

 one else has observed any such intracellular radial structure. Fur- 

 thermore, even if we did rest upon the hypothesis of a contractile 

 mechanism, it is incomprehensible how such a mechanism could 

 produce a rapid obliteration of oxygen diffusion into the photogenic 

 cells since some gaseous oxygen would still be present in the proximal 

 portions of the tracheoles and contraction of the end-cells would 

 merely compress the gas farther into the tracheoles and deeper into 

 the photogenic tissues. There seems to be nothing back of the 

 tracheal end-cell theory and, actually, the concept of a contractile 

 tracheal end-cell is in direct antithesis to facts that we know. Thus, 

 if contraction of the end-cells obliterates the oxygen supply, why 



