10 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. 1. 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. 1. In the female the dark area, illustrated in Fig. 1, 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 McDermott 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 
Fic. 2. Fic. 3. 
L=luminous tissue. R=reflecting layer. T=trachezx. 
*Townsend, American Naturalist, 38, p. 127, 1904. 
tMcDermott and Crane, Amer. Naturalist, 45, p: 306, 1911. 
fMangold, Winterstein’s Handb. der Vergleich. Physiologie, vol. 3, 1911. 
