TISSUES OF LIGHT-PRODUCTIOX 



feet lens. Its shape can be better understood by a glance at the figure 

 than a page of description. 



Stretched over the lens and overlapping even the reflector is a thick 

 corneal layer which is double on its edges and single in the middle. The 

 lower part of this double region is applied to the edge of the reflector. 

 It is heavily striated epithelium, and continues as a sheet of loose cells, 

 which form one of the few means by which the whole organ is connected 

 with the tissues among which it lies. Large blood spaces effect the iso- 

 lation. Above this cornea comes the hypodermis and its layer of cuticle. 

 These structures are continuous with those on the rest of the body. 



The origin of these tissues has not been worked out. Being an Ar- 

 thropod, and remembering that in the insects the light tissue is developed 

 from the mesoderm, it is plausible to consider these structures as also 

 mesodermal. But the general appearance of the various layers, 

 especially in a younger organ, gives a different aspect to the matter 

 and leads one to believe that the whole organ, with the exception 

 of the reflector, is formed by an involved invagination or delamination 

 of the hypodermis in the embryo. 



The second group of Arthrop- 

 oda, the insects, exhibit sev- 

 eral very efficient tissues that 

 produce light. These organs are 

 supposed to be mesodermal in 

 origin, and probably are modified 

 fat bodies. A reflector is usually 

 present, and consists of a layer of 

 closely packed cells that have the 

 same origin as the gland cells. 

 The light secretion is alway sused 

 in situ, in the cytoplasm of . the 

 cells which produced it. It is 

 surprising that a lens is always 

 missing unless some of the cutic- 

 ular structures may be so inter- 

 preted. While several sorts of light- 

 organs occur, more or less simple in 

 structure, the more complex ones 

 may always be easily compared 

 with the simpler. We shall begin 

 with the study of a simple form. 



The light-organ of a Lampyrid will furnish such a type, and that of 

 Lampyris splendidula and noctiluca, as described by Bongardt, will serve 

 our purpose. 



FIG. 120. Part of a section through the light- 

 producing tissue of Lampyris splendidula, 

 showing a single tracheal end-cell (tr.e.c.) 

 into which a terminal twig (ter.t.) of a tra- 

 cheal tube enters. This twig gives off five 

 tracheoles (trl.). The general histology of this 

 tissue is much like that in the next figure. 

 (After BONGARDT.) 



