The Hog Louse 713 



At the posterior pole of the egg is a very characteristic structure (Plate 

 LXVI, 34), to which Graber (1872:165) gave the name " Eistigma." The 

 earliest description of this structure is that of Leuckart (1855:139, 141), 

 who observed it on the eggs of Pediculus capitis and Haematopinus 

 siiis, and it was seen also by Landois (1864:15) on the egg of Phthirius. 

 Gross (1906:372) has given the first detailed description of it and figured 

 its structure. The egg stigma forms a roundish swelling on the chorion 

 and is pierced by numerous thin-walled canals, which narrow toward 

 their inner ends and converge to one side. Gross studied its formation 

 in detail, and in young stages found the egg follicle closed bj' a plug 

 extending far into the interior of the yolk, but as growth proceeds the 

 plug becomes leveled. The nuclei are small and the inner ends of the 

 cells are drawn to a point. These inner ends are cut off from the cells 

 in a characteristic manner and the nucleus is drawn to the outer wall, 

 '\\-hile between them is a zone of protoplasm in which cell boundaries 

 can no longer be recognized. Between the detached inner pieces begins 

 the deposition of chitinous substance, and this appears as fine striae, 

 while at the exterior, where the deposition has become more advanced, 

 the thin, chitinous lamellae have lost their color. Then are formed in 

 the region of the stigma the endochorion and the exochorion. The stigma 

 is now completely developed. The point formed by the pointed ends of 

 the cells still remains attached inside, cell walls can be recognized, and 

 it can be seen that each cell forms a canal. 



No satisfactory biological interpretation has been found for the egg 

 stigma, a structure found in no other insect order. Earlier authors 

 advanced three views, none of which has proved satisfactorJ^ Leuckart 

 (1855:139) and Melnikow (1869:154) regarded it as an attachment disk; 

 but if this were its function, why should it be pierced by canals in most 

 cases? Kramer (1869:462) regarded it as the true micropjde; but in 

 some species, for example Nirmus, the canals do not pass to the inner 

 ends of the cells. Graber (1872:163) interpreted it as a means of aeration 

 for the eggs and so named it; but in most cases it is covered over by 

 secretion. The closing of the pores by seci-etion would be essential in 

 Pediculus, since, according to Sikora (1915:. 530) and Nuttall (1917 d: 148), 

 the embryo escapes from the egg by pumping air through its alimentary 

 canal in order to increase the pressure in the egg and force open the 

 operculum. 



