PHOTOGENIC ORGANS OF LAMPYRIDS 163 



tracheae, T, have already been secreted by the flattened epi- 

 thelial cells. 



The light-organ, which is now quite free from the underlying 

 hypodermis, has assumed a slightly oblong form, and is held in 

 position at several points by membranous strands resembling 

 connective tissue. An outer, somewhat paler layer of cells, U, 

 is now obscurely visible, and appears to surround the entire, 

 denser inner mass, P, the true photogenic tissue. This seems 

 to be more reticulate than granulate, though in neither zone are 

 the cells themselves clearly discernible, as in the earlier stage, 

 though their nuclei are regular and distinct. It is probable that 

 by this time the organ is functional. It is supplied by a trachea 

 and perhaps a nerve. The fat cells (A, fig. 14) are large and 

 vacuolated, with somewhat collapsed nuclei, and contain no 

 blue granules such as are found later. They sometimes lie free 

 in the body cavity like haemocytes, but may also be united into 

 masses. The fat-extruding or -squeezing process through the 

 agency of a partial encasement, described in old larvae and in 

 pupae of Photinus consanguineus, was not observed to take place 

 in the embryo, but such a process seems unnecessary here, since 

 the fat-body has many free cells and is otherwise of a much looser 

 structure than in the later stages. The organ now measures 

 about 24 by 34 micra. 



In the second-year larva the organ (fig. 16) is about 577 micra 

 in its longer and 451 micra in its shorter diameter, and is broader 

 than high. The whole is enclosed in a delicate, non-cellular 

 membrane by which it is secured to the attenuated hypodermis 

 and there flattened. The hypodermis is now very much thinner 

 than the cuticula. The two layers are well differentiated, but 

 it is evident that the organ is by no means as definitely and 

 highly developed as in the adult. While the light-organ is under 

 control of the larva, in that it can be extinguished and made to 

 glow, this operation is incomparably slower -and therefore less 

 perfect than in the adult, where the stimuli are apparently to 

 a large extent transmitted through the optic nerve, a structure 

 which is of course also better developed in the adult. 



