1914. J NATURAL SCIENCES OF PHILADELPHIA. 313 



which the hairs have been removed. When more carefully observed 

 under a high-power lens, a striking difference in external form is 

 seen. The pores appear as small bright spots when a strong trans- 

 mitted light is used. Each bright spot has a dark boundary or pore 

 wall (fig. 3, PorW). Near the centre of this boundary is a trans- 

 parent spot, the pore aperture, which may be round, oblong, or 

 slit-like. On the wings the pore apertures are always round or oblong 

 (fig. 5, PorAp) and never slit-like as on. the legs (fig. 3, PorAp). At 

 the lowest focusing level any pore aperture, however, is perfectly 

 round. The boundary (fig. 3, PorB) of the pore is usually bordered 

 by a band of darker chitin. 



(6) Internal Structure. — All the olfactory pores studied are inverted 

 flask-shaped structures in which the bottoms of the flasks are chitin- 

 ous layers (fig. 6, ChL). These layers of chitin contain the pore 

 apertures and they form external coverings for the pores. In a 

 typical pore as found in the tibia of Formica, the neck {NkFl) of the 

 flask is wide and the mouth (Mo) is flaring. About two-thirds of 

 the space at the bottom of the flask is occupied by a chitinous cone 

 (Con). The cone generally stains less deeply than the surrounding 

 chitin (Ch), but is not separated from it. The apex of the cone is 

 hollow and extends to the neck of the flask. The sense cell (SC) 

 lies beneath the mouth of the flask. It is bipolar, long, and slender, 

 and comparatively large. The sense fiber (SF) of this cell is en- 

 larged at the apex of the cone. Its peripheral end runs into the 

 hollow of the cone, pierces the bottom of the cone, and enters the 

 lowest portion of the pore aperture. The nerve fiber (fig. 13, NF) 

 of the sense cell joins a nerve cord. The nucleus (SCNuc) with its 

 nucleoli (SCNucl) is always conspicuous. 



Now since the anatomy of a pore is understood, the external 

 appearance of a pore may be explained. The dark border of chitin 

 (fig. 3, PorB) around the boundary of the pore is due to the thick 

 chitin (fig. 13, aa) at the mouth of the flask. The boundary (flg. 3, 

 PorW) is the same as the greatest width of the flask. The bright 

 area inside the boundary is caused by the light having to pass through 

 only the chitinous cone (fig. 13, Con) and the chitinous laj^er (fig. 13, 

 ChL). The aperture appears transparent because the sense fiber 

 (fig. 13, SF) and all the other tissues have been removed by the 

 caustic potash treatment. 



Sections through the tibiae of pupal muddobbers throw some light 

 on the origin of the anatomy of a pore. Quite often very large cells 

 (Plate XII, fig. 38, SC) may be seen among the small hypodermal cells 



