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T 11 /•; . I M /•; RICA X t TUEALIST 



[Vol. XLV 



fine passages in the true photogenic tissue, where the 

 oxygen of the air is consumed in a biologic oxidation. 

 In the sections of pyralis there are clearly seen bundles 

 of muscle fibers on either side of the center line of 

 the insect, which pass completely through the abdomen, 

 almost vertically, and are attached to the exterior chitin 

 at the top and bottom. At about the same point, 

 other muscle fibers pass inward from the point of maxi- 

 mum width at each side ; these fibers have not been traced 

 to their full extent, but they appear to pass upward and 

 toward the center near the dorsal side of the insect. 

 These fibers are indicated in Fig. 1, a, at M. No similar 

 muscle fibers have been observed in Photuris, although 

 short lengths of muscle fiber passing vertically through 

 the abdominal cavity have occasionally been noted, and 

 these may be fragments of similar muscles to those in the 

 Photinini. The corresponding muscles of Phot inns 

 marginellus are clearly shown in Fig. 1 of Miss Towns- 

 end's paper. Externally, the lower terminations of these 

 bundles of muscle fibers appear as non-photogenic spots 

 on the ventral surface of the luminous segments. 



It may be well here to call attention to certain differ- 

 ences between Photuris and Photinus, as shown by the 

 cross-sections of the insects. While there is a general 

 similarity of outline in the cross-sections of the two 

 species, the section of Photuris is generally a little 

 flatter, and the ventral curvature of a somewhat larger 

 mean radius, than in Photinus. Another difference has 

 been very marked in our sections. While the thickness 

 of the reflecting layer is about the same in both species, 

 the layer of true photogenic tissue is much thinner, both 

 actually and in comparison with the reflecting layer, in 

 Photuris than in Photinus; this difference is clearly seen 

 by reference to Fig. 1, a and 6, This difference may be 

 somewhat significant when considered in connection with 

 the slight differences in the quality of the emitted light, 

 and in the modes of emission of the two species. (See 

 reference No. 8.) In Photinus there are two peculiar 

 organs each consisting apparently of a thick-walled, 



