378 ME. A. D. MICHA.EL ON HTPOPI, 



have endeavoured to ascertain ; but first let me say a few words 

 as to what a Hypopus is like. 



There are several forms of Hypopus, differing more or less from 

 each other, but to take a typical kind, such as Dufour's fero- 

 niarum, it is an extremely minute Acarid, rarely more than 

 •25 millim. in extreme length, vrhich presents, at first sight, some- 

 what the appearance of a miniature Limulus without a telson, 

 its whole dorsal aspect being entirely covered by a nearly hemi- 

 spherical, chitinous carapace, concealing cephalothorax and ab- 

 domen, and coming to the ground all round when the creature is 

 quiet. The two front pairs of legs and the long setae at the ends 

 of the fourth pair project somewhat when the animal is walking, 

 but can be entirely withdrawn under the carapace, and usually 

 are so. The mouth-organs are very rudimentary, consisting, it 

 is asserted, of a mere hole, covered and closed by a flap, and from 

 which projects a tube of moderate length, getting finer towards 

 the distal end, where it is said to be closed, and which tube ter- 

 minates in two long bristles, which do not pass within the tube, 

 and cannot, it is said, be regarded as maxillae. The ventral sur- 

 face is covered with a soft integument, and bears numerous 

 suckers, chiefly near the posterior part ; by these suckers the 

 creature adheres to the polished, chitinous surfaces of Insects, 

 Gramasids, &c. 



The three front pairs of legs terminate in a double claw, and 

 caruncle or sucker; the fourth is without either, and terminates 

 m very long bristles, like the hind legs of Sarcoptes scabiei, but 

 larger in j)roportion ; this is probably what deceived Grerlach, and 

 led him to consider it an Itch-mite. The chitinous carapace is 

 frequently sufliciently transparent to allow the form of the crea- 

 ture to be seen through it. 



For some two or three years I had carefully watched Tyro- 

 glyplii in confinement in small glass cages, under favourable and 

 unfavourable conditions, but my efforts were chiefly directed to 

 Tyroglyplitis sh-o and lotiyior, and I did not succeed in getting any 

 hypopial forms from them, nor in seeing any thing that would 

 elucidate the question. In 1881, however, I came across a quan- 

 tity of Megnin's Tyroglyp)Tius mycopliagus ; I soon found that, 

 with this species, there was not any difficulty whatever in re- 

 peating his experiments. The nymphs readily turned into 

 Sypopi, and the Hypopi returned to the form of nymphs of the 

 Tyroglyplms, in each case by an ecdysis ; and I was able to secure 



