596 BRITISH 0RIBAT1D.E. 



been able to ascertain quite to my own satisfaction 

 whether the vagina ends blindly or has an open end 

 facing the genital plates ; I am inclined to think that 

 the former is the correct view, at first at all events. 

 This condition is shown at PL LIII, fig. 4. In 

 the same manner with the nymph of the male, the 

 testes and vesicula seminalis are apparently more or 

 less formed, but I have not observed any intromittent 

 organ, and the whole system is far smaller and more 

 doubtful and difficult to trace than that of the female. 



It may be remarked that the external genital plates 

 are developed most distinctly in all the nymphs, but 

 they are decidedly smaller than the corresponding 

 plates of the adult ; and probably in the female would 

 be too small to be functional ; of course they are not 

 usually hard. 



Respiratory Organs. — It is in this system that the 

 greatest diff'erence between the nymphs and larvos, and 

 the imagos exists ; and it is of a nature which possibly 

 might not be anticipated, because in Insects a widely 

 different, and even opposite, arrangement, usually 

 occurs. Thus everyone who has dissected a silkworm, or, 

 indeed, almost any Lepidopterous larva, knows well how 

 very powerful and abundant the tracheas are, far more 

 so than in the imagos of the species ; but this does not 

 in any way hold good with the larv« and nymphs of 

 the OribatidcB. The whole tracheal system of the adult 

 has been completely absent from every nymph and 

 larva of the family which I have dissected, nor has its 

 place been supplied by any other internal respiratory 

 system whatever that I have been able to discover ; 

 the breathing, or rather aeration of the blood, or peri- 

 visceral fl^uids, appears to take place by the general 

 body-surface, as in the Ti/rogli/phidce, Sarcoptidce, and 

 other groups of the Acarina, which constitute the 

 Acari proper of many authors. This, perhaps, is not to 

 be wondered at when we remember that, although the 

 adults of OrihatidcB have hard chitinous cuticles not in 

 any way adapted to serve for the transmission of gases 



