THE PRESIDENT'S ADDRESS. 217 



Tyroglyphus-\\ym\)\i, leaving nothing but the skin. Finally, 

 Haller came nearest to the truth when he said " Hypopial form is 

 a travelling-dress." Some years ago, during a short stay in a 

 country house, I was struck by the fact that almost every fly, or 

 other small moving creature, which came out of the cucumber-bed 

 was laden with Hypopi to sueh an extent that I called the flies " the 

 emigrant ships," and yet Tyroglyplius was swarming and evidently 

 thriving in the hot-bed. I easily satisfied myself that the Hypopus 

 really was an immature stage of the Tyroglyplius, but the circum- 

 stances did not seem to admit of Megnin's explanation. I was 

 thus led to undertake a series of experiments, too long to be 

 detailed here, by which I think I proved that Hypopus is a stage 

 occupying the period between two ecdyses of the Tyroglyphus- 

 nymph, and which, although it does not occur in all individuals, is 

 a perfectly normal condition, not induced by any adverse circum- 

 stances, but forming a provision of nature for the distribution of 

 the species far more efficient than any adopted by the Tyroglyplius 

 form, which would die if carried by the fly into hot sunshine or 

 warm dry places. Here, then, we have an instance of a parasite, 

 if that name can really be applied to it, which resides temporarily 

 upon the host, but requires nothing from him except conveyance 

 to " fresh fields and pastures new ; ' the host is really the 

 emigrant ship, as I used to call it. Another curious acarine para- 

 site, whose parasitism is of a somewhat allied nature, is Uropoda 

 vegetans. This creature looks rather like a large Hypopus at 

 first sight, but even a cursory examination will show that it is a 

 very different being. Its mouth organs, instead of being rudi- 

 mentary, are more highly developed and complex than those of 

 any of the Acarina, except its brother Gamasids. It possesses 

 tracheae which are absent from Hypopus, but like that creature it 

 only uses its host to convey it to suitable food. It is predatory, 

 living upon very minute insects and Acari. The small quantity of 

 fecal matter, which passes from the minute round anus of this or 

 other closely-allied species, soon hardens on exposure to the air, 

 and by this it is attached to the host ; the attaching matter, of 

 course, increases in quantity, but it does so by adding to the length, 

 not the thickness, of the stalk. Thus the Uropoda is at last attached 

 to the host by a long thin stalk, which keeps host and parasite 

 some distance apart. The host is most commonly a beetle, and 

 the Uropoda, when attached to one of the beetle's legs, has an ex- 



