232 THE PYGIDIUM OF THE COMMON FLEA. 



But perhaps the most pleasing structure, which is also one of 

 those surprising things in Nature, is the delicate dome which arches 

 above each follicle with its enclosed apparatus, and through whose 

 central eye the long tapering filament projects. If this domoid 

 membrane has not previously been observed, I am at a loss to 

 account for the oversight. It is almost conspicuous ; it demands 

 only a slightly upward focus to define its rotundity and its circular 

 eye ; if the long bristle is followed from tip to base, the dome can 

 scarcely fail to make itself seen. 



This over-arching membrane is set down above the follicle, 

 and is a little greater than it in diameter, the greatest width of the 

 dome being about i/i,8oo inch, with about 1/2,000 inch to that 

 of the depression which it covers. In the diagram it is shown as 

 it appears in surface view, with a regularly spherical outline, the 

 summit somewhat flattened and the region about the central eye 

 conspicuously depressed. In some specimens, probably in all, 

 when seen in profile, the dome is widened above, and the convex 

 walls slope inward to the base. But as to the existence of the 

 domoid membrane there is not the slightest doubt. It is present 

 above every pygidial areola, adding much to the interest of the 

 orgaHj somewhat to its complexity, and happily adding emphasis 

 to the general belief that the pygidium is an auditory organ. The 

 function of this membranous dome can be readily inferred. 



It possesses, however, some minute appendages whose function 

 is not so easily conjectured. Around the eye of the dome, as 

 shown in the diagram, is a single row of from eight to ten projec- 

 tions which, for want of a better name, may be called setae. These 

 are positively not the tips of the flame-shaped hairs, and they are 

 as positively not minute folds in the membrane of the dome, but 

 strictly independent organs, minute enough to satisfy the most 

 exacting microscopist who likes to work with high powers. In 

 estimated length they vary from 1/15,000 to 1/20,000 inch. I have 

 no micrometer ruled fine enough to measure such setae, but the 

 estimate seems to be at least within the actuality. 



These small objects arise from the surface of the dome in a 

 regular row, and take varying positions, being sometimes perpen- 

 dicular at the edge of the eye, at times leaning over the margin ; 

 in other instances they appear further down the slope, standing 



