1915] Wheeler and Williams — Xeic Zealand Glow-Worm 39 



terminate with rounded tips. It is these dilated tips of the four 

 Malpighian tubules which appear as the four curved, luminous 

 rods in the living larva and, therefore, constitute the photogenetic 

 organ. What to all appearances serves as a reflector is represented 

 by a layer of tissue (R, Figs. 3 and 7) of variable thickness, cover- 

 ing much and in life perhaps all the ventral surface of the dilated 

 ends of the tubules. A pair of large tracheae, shown at T, enters 

 this layer and each breaks up almost immediately into three 

 branches, one of which runs back between each of the two pairs 

 of Malpighian tubules. The reflecting layer seems to be a syn- 

 cytium made up of the hypertrophied and finely vacuolated tra- 

 cheal epithelium, as it reveals no cell boundaries. It contains 

 numerous small nuclei which are all at one level well within the 

 mass (Fig. 7) and close to the chitinous tracheal intima. Anteri- 

 orly the reflecting layer extends for a short distance along the 

 two main tracheal trunks, but posteriorly it thins out into a mem- 

 brane which partly serves to secure the tubules to the rectum. 

 Some slender longitudinal muscles, shown at M in Fig. 3, have 

 the same function. It is probable that the reflecting syncytium 

 may in life contain numerous fine vacuoles of a fatty substance 

 which has been dissolved away in the mounted preparations. 

 We do not believe that this layer is derived from the fat-body, 

 because the latter, as shown in Fig. 5, consists of larger cells with 

 definite boundaries and large, spherical nuclei. 



It thus appears from dissection that the Malpighian tubules are 

 each differentiated into two portions with very different functions, 

 viz., a short distal piece which is photogenetic and a long proximal 

 portion which retains the primitive excretory function of these 

 organs. The portion anterior to the subterminal constriction 

 of the body shows in both stained and unstained material a more 

 opaque and vacuolated structure of its cells, with darker nuclei; 

 the slender intermediate region (Fig. 2S) is much more transparent 

 and its nuclei are paler, while the photogenetic portion is also 

 rather pale but consists of much larger cells. In sections these 

 structural differences are even more marked. No sections were 

 taken near the points of origin of the Malpighian tubules, but it 

 is evident from a specimen stained in toto that the lumen is here 

 quite large. A section through the middle region is shown in the 

 lower of the two drawings in Fig. 4. The wall of the tubule shows 



