IO 



A PHYSICAL STUDY OF THE FIREFLY 



V. HISTOLOGY OF THE LIGHT ORGANS. 



For clearness in understanding and ease in describing the thermal and 

 other measurements given on a subsequent page, the general topography of 

 the ventral side of a firefly (Photinus pyralis) is given in Fig. i. The dark 

 areas, L, represent the regions of the luminous organs. It will be noticed 

 that in the male the luminous region includes the whole of two seg- 

 ments (the second and third from the end), while in the female only 

 about a third of the third segment is luminous. In Photinus pyralis there 

 are two additional points of light, L'L', which are of a more greenish hue 

 than the light emitted from the rest of the body. These points of light are 

 of interest in connection with the temperature measurements to be discussed 

 presently. In the male Photuris pennsylvanica the luminous organs are 

 situated as in Fig. i. In the female the dark area, illustrated in Fig. i, is 

 expanded so that it occupies about three-fourths of each of the two segments 

 (the second and third from the end) . Hence the female is almost as lumi- 

 nous as the male. 



In Fig. 2 is given a transverse section through the entire abdomen of the 

 firefly Photinus mar- 

 ginellus, studied by 

 Townsend,* and in 

 Fig. 3 is given a de- 

 tailed view of the 

 general structure of 

 the luminous organ. 

 Fig. 4 gives illustra- 

 tions of transverse 

 sections of Photuris 

 pennsylvanica and 

 Photinus pyralis re- 

 cently studied by McUermott and Crane. f 



In all these illustrations, L is the photogenic tissue, P is the pigment 

 layer, and R is the so-called "reflecting layer," the properties of which will 

 be mentioned presently. From a physical standpoint these comparisons 

 are of great interest, because the structure of the photogenic organs is prac- 

 tically the same in all three insects, although the Photuris is a distinct genus, 

 widely separated from the Photinus in the scale of development. Evidently 

 the structure of the luminous organs has nothing to do with the great differ- 

 ence in the color of the light emitted by these two insects. 



A further illustration of the great similarity of the essential parts of the 

 photogenic organs in different animals is given in Fig. 5, which shows a 

 transverse section! through a luminous organ situated on the ventral side 

 of a marine fish, Maurolicus pennanti. Here again we see photogenic mate- 

 rial, d. K., backed by the so-called "reflecting layer, i. R. The whole light 

 organ is inclosed with a layer of black pigment, P, on the inner wall of which 



"Townsend, American Naturalist, 38, p. 127, 1904. 



fMcDermott and Crane, Amer. Naturalist, 45, p. 306, 1911. 



JMangold, Winterstein's Handb. der Vergleich. Physiologie, vol. 3, 191 1. 



L = luniinous tissue. R=reflecting layer 



