1919] Mclndoo: Olfactory Sense 79 



aperture (Ap) which appears as a transparent spot; the aper- 

 ture is a minute opening passing through the thin chitin inside 

 the walh 



Sometimes it is almost impossible to distinguish a secondary 

 seta from a pore, but in almost all cases the chitin inside the 

 wall of such a seta is lighter in color than that inside the wall of 

 a pore, and this fact may help to distinguish a seta from a pore 

 in case the seta has been pulled out; this comparison may be 

 seen by referring to Figures 25 and 28, both of which structures 

 come from the same region on the epicranium. 



In size the pores do not vary greatly, as may be observed by 

 looking at Figures 10 to 28. The smallest one (Fig. 10, Mxj) 

 lies on the terminal segment of the maxillary palpus and the 

 largest one (Fig. 14) in the army worm lies on the anal proleg; 

 the former is scarcely discernible and seldom has a border. The 

 one (Fig. 10, Mxi) nearest it is always slit-shaped and al^o in 

 some species can scarcely be seen. The other two {Mxg and 

 Mxh) on this segment are easily seen in good mounts, and have 

 distinct borders. 



3. Internal Anatomy of Pores or Punctures. 



A reference to Figures 29 to 48 shows that the internal 

 structure of these organs is like that of those in other insects, 

 and consequently only a brief description of the various parts 

 of them will suffice here. Lying in the thick hypodermis 

 (Fig. 29, Hyp) is the large sense cell {SC) whose peripheral end 

 (SF) passes through the pore canal {Can), pierces the chitinous 

 cone (Fig. 30, Con) and then stops in the bottom of the pore 

 aperture (Ap) where it seems to come in direct contact with the 

 external air. The chitin covering the cone may be dome- 

 shaped, with the dome either lying in a depression (Fig. 29) or 

 elevated above the surrounding chitin (Fig. 33) ; or it may form 

 a depression whose bottom is pierced by the pore aperture 

 (Fig. 36) ; or it may lie on the same level with the surrounding 

 chitin (Fig. 31). 



Since the chitin was so thick, not a single section showed all 

 the details of one of these organs, because the microtome knife 

 never passed properly through the structure; but Figures 29 

 and 30 combined give a good idea of their anatomy, which is 

 very different from the internal anatomy of a large hair (Fig. 34) 

 which is formed by a large hair-mother cell (IlrMC) sending 



