Annals of the Transvaal Museum. 
61 
tissue of Pkotinus tlie traclieal epithelium is very thin, with large 
flattened nuclei, and there is no indication of abnormal development. 
Another striking difference from the conditions in Lampyris 
splendidula is that in the latter Bongardt found the “ end-cells ” 
and tracheoles more abundant in the dorsal layer than in the ventral. 
In Pkotinus these structures are clearly confined to the ventral layer. 
As to the derivation of the photogenic tissue, the form and 
content of the cells has led to a general idea that it is modified fat 
body. Raphael Dubois, from a study of Lampyris noctiluca and 
Pyropkoms noctiluc-us , claims that it arises by proliferation of the cells 
of the hypodermis. His statements are based on a study of the develop- 
ing organs from the ovum to the adult insect. He further finds that in 
the development of the organs by means of the physiological changes 
incident to photogeny, the dorsal layer becomes filled with opaque 
granules and ceases to be photogenic. The marked difference in 
structure between the two layers in Pkotinus make it difficult to 
believe that in the adult insect, at least, there is any growth of one 
at the expense of the other. There is no apparent difference in the 
relative thickness of the two laj^ers at the beginning and at the end 
of the flying season. 
Summing up the structure briefly, we find that there are two 
distinct layers. The ventral layer is made up of two elements, trie 
cylinders with their tracheae and the parenchyma. Throughout the 
parenchyma is a network of fine air capillaries. 
When a fresh light organ is studied under a microscope, tne 
dorsal layer is found to be non-photogenic, opaque, and chalky white. 
The ventral layer is seen to be the truly photogenic layer. When 
the ventral surface is studied under a microscope in the dark-room, 
the light is found to be uniformly distributed through the parenchyma, 
the area of the tracheolar network. The cylinders stand out against 
this luminous background as non-photogenic rings. It is thus 
evident that the process of photogeny takes place in that portion of 
the ventral layer which lies between the cylinders, and it is exactly 
this portion of the tissue which has a provision of air not known 
elsewhere in insect histology. 
The light of the fire-fly is entirely independent of the life of the 
photogenic cells. When the light organs are crushed the light becomes 
more brilliant as the tissue comes more directly in contact with the 
air and continues for several hours if the tissue is kept moist. 
Crushed light organs become brilliantly photogenic under the influence 
of oxygen, and the light continues for several hours. If slides with 
crushed organs are placed in a jar of C0 2 the light is quickly 
extinguished, but when the slides are transferred to a jar of oxygen 
the light reappears instantaneously. 
With the exception of Raphael Dubois, all modern workers on 
organic light agree that it is a process of oxidation. In the fire-fly 
it is supposed to be the pxidation of some substance secreted by the 
parenchyma cells of the ventral layer. The exact nature of this 
substance has not yet been determined. Watose states that it is of 
a fatty nature. Radziszewski, through his study of the artificial 
luminosity of lophin, discovered a series of carbon compounds similar 
to those found in living organisms and capable of becoming luminous 
under conditions compatible with life. These conditions he found to 
be slow chemical action in the presence of alkalies. He found that 
