THE OLFACTORY RECEPTORS OF THE BLOWFLY 109 



the most numerous sensilla on the antenna. One or more of these types 

 has an olfactory function. Since among them the fine structure differs in 

 minor details only, it is possible to derive from their description an idea 

 of the structure of the olfactory receptors. 



The olfactory sensillum may be described as a hollow cuticular peg 

 innervated by one to four bipolar neurons. The dendrites of the neurons 

 extend into a cuticular scolopoid sheath which is partially compartment- 

 alized to accommodate a particular number of dendrites. The sheath 

 appears to fuse with the base of the peg and terminate there as a discrete 

 structure. There is a continuous lumen from sheath to tip of peg. In this 

 respect the relationship of the sheath to the peg differs from that described 

 by Slifer et al. (1957, 1959) for the olfactory sensilla of grasshoppers. In 

 the long thick-walled sensillum of grasshoppers the sheath extends to the 

 tip of the peg which thus is left open ; in the short thin- walled sensillum it 

 opens at the base of the peg. In both instances, as the insect mohs and a 

 new peg and sheath are secreted, the old one is discarded through the 

 opening. Since the fly does not molt, it is not surprising that the scolopoid 

 sheath fails to open to the outside. 



There is some diversity in the appearance of the neural material filling 

 the lumina of the pegs, but thus far it has not been possible to relegate a 

 particular internal appearance to a particular type of peg. The interiors of 

 the dendrites are markedly reticulate as the cross-sections in Plate IV 

 illustrate. In longitudinal section (Plate III, Fig. 21) this reticulum is 

 more clearly seen and within it there appear to be contorted tubules seen 

 here both in cross and longitudinal aspect. The empty space between this 

 material and the wall of the peg probably results from shrinkage occuring 

 during fixation. Higher magnifications (Plate II) emphasize the irregularity 

 of the reticulum and the marked tubular appearance (Figs. 4 and 6). In 

 some sections the tubular aspect is absent (Plate II, Fig. 5) and the ground 

 material is more finely uniform. In other sections (Plate I, Fig. 2, Plate III, 

 Fig. 5) a tubular construction is pronounced. 



A marked tubular appearance suggests that the dendrite has subdivided 

 into many parallel branches. Where evidence of tubules is absent the 

 appearance suggests that a single expanded dendrite fills the lumen. 

 Further work is required to clarify this point. In the thick-walled sensillum 

 of the grasshopper the four or five neurons send their dendrites unbranched 

 to the top of the peg. In the thin-walled ssnsillum of the grasshopper the 

 forty (average number) neurons possess dendrites which become multiple 

 branches within the lumen of the peg. In the fly the dendrites, whether 

 branched or unbranched, occupy nearly all the space in the peg and are 

 closely appressed to its wall throughout. 



The walls have a peculiar structure. They are uniformly pitted or per- 

 forated with 180 to 350 openings. These may actually be holes in the 



