226 A. D. MICHAEL ON THE ACAR1NA. 



after the same number of changes of skin as carries the male to 

 perfection that the female enters the nubile stage. 



It may be mentioned that, previous to the change to the repro- 

 ductive stage, the vulva may sometimes be distinctly seen through 

 the nubile skin, and that eggs almost ready for deposition may also 

 be seen in the same manner. I have myself observed apparently 

 fully-formed eggs in what looked like the nymphs of some of the 

 Oribatidce, a fact which renders it most difficult to know which are 

 nymphs and which are not, unless one knows the life-history of the 

 species well. 



The result of all this may be shortly stated to be that, whether 

 the form be properly called nymph or nubile female, copulation 

 takes place in most of the Acarina in a different stage from egg- 

 bearing and by a different external female organ, and that what is 

 ordinarily called the vulva is an egg-laying organ only, and that the 

 female copulative organ is posterior, and also that the two stages, 

 viz., nubile and reproductive, are often so different that they would 

 not be recognised as being the same creature. It may possibly be 

 worth consideration, whether any instances of suppressed agamo- 

 genesis in other animals might be explained by the fecundation of 

 the female in a stage when she does not show apparent external 

 sexual organs. 



From MM. Robin and Megnin's works I have given, Fig. 1, a 

 drawing of the posterior part of the male, P. glandarinus , showing 

 the penis, i. Fig. 2 is the posterior portion of the nubile female 

 with which coition is effected, showing the cylindrical projections o. 

 Fig. 3 is the posterior part of the reproductive female, showing the 

 vulva, /, and the opaque portion of the sternite, k, and even a 

 cursory glance will show the great difference from the nubile form. 

 M. Megnin is of opinion that, in all Acari, coition takes place with 

 a female in the nubile stage before the development of an external 

 vulva, and he expressly mentions that this is the case in the genera 

 Tyroglyphus and Glyciphagus, and this statement is made in the 

 joint paper by Messrs. Robin and Megnin. On the other hand, 

 Robin, in a former paper on Tyroglyphus siculus, elaborately des- 

 cribes the copulation of the adult female, and I have myself fre- 

 quently watched the copulation of Glycipltagus plwniger y which has, 

 in every case, taken place with the adult female, having an external 

 vulva, but clearly at the anal region of the body, not at the con- 

 spicuous vulva with the horse-shoe sternite. I am moreover of 



