THE OLFACTORY Rt'CEPTORS OF THE BLOWFLY 107 



the tip. The distribution of the third type is not known because it has been 

 seen in only a few electronmicrographs. 



One or more of these three types of sensillum is obviously olfactory. All 

 possess many features in common. These are : a hollow cuticular peg all 

 or part of whose wall is extremely thin and very possibly perforated ; a 

 cuticular sheath or tube, the scolopoid sheath, extending from the basal 

 region of the peg a variable distance down into the underlying tissue ; 

 one to four deep-lying bipolar neurons whose dendrites pass into the 

 scolopoid sheath thence into the lumen of the peg. Since there are many 

 hundred-fold more cell bodies associated with the pegs than there are 

 axons in the antennal nerve, and since synapses do not occur in the antenna, 

 it follows that there is extensive fusion of axons. Axonal fusion is a 

 common feature of the insect sensory system. 



THIN-WALLED PEGS IN PITS (PLATES I AND li) 



These pegs, characteristic of the more proximal antennal pits, are delicate 

 rounded, structures averaging 13// in length. In cross section they are 

 circular except when cut slightly obliquely. The average diameter is 

 1.5-1.8//. The cuticular wall at its thinnest point averages 0.08//. 



The number of neurons associated with each peg varies from one to 

 two. Their cell bodies lie 15 or more microns below the base of the pegs. 

 Their dendrites upon approaching the basal region of the peg enter the 

 scolopoid sheath, which appears to fuse with the walls of the peg as it 

 extends into it. Within the peg the dendrites completely fill the lumen. 



A conspicuous feature of the cuticular walls of the peg is the indenta- 

 tions in its surface. Each peg possesses from 180 to 360, which constitute 

 from 7-14 per cent of its surface. These are minute circular pits whose 

 orifices are narrower (0.035//) than the basal chamber (0.088//) (Plate II, 

 Fig. 1). It is difficult to ascertain whether or not these pits are indeed 

 perforations, although this is the most likely interpretation. In any event, 

 the electron-dense outer layer of the cuticle does not enter the openings. 

 At the bottom of each opening is a material that is not part of the cuticle 

 and seems to be continuous with the sheets and nets of tissue filling the 

 lumen. At the base of the openings, however, and only there, the material 

 occurs in the form of fine parallel strands. In cross sections these appear 

 as round tubules. 



THICK-WALLED PEGS IN PITS (PLATE I, FIG. 4) 



These differ from the foregoing principally in possessing a thicker wall 

 at the base and being more acutely pointed. Characteristically they are 

 found in the more distal antennal pits. They also are innervated by one 

 or two cells. When two neurons are present, the scolopoid sheath is 

 compartmentalized to accommodate the two dendrites (Plate I, Fig. 4). 



