21 
The President's Address. By A. D. Michael. 
observed that in the immense cellars below Paris, which are devoted 
to the cultivation of mushrooms, those which had been left a little 
too long, so that decay had commenced, were covered with a thin film 
of liquid ; and that in this film waded swarms of an opaque white 
Acarus, usually almost, but not quite, immersed. The mandibles of 
this Mite were sharply serrated but not chelate ; they were darted 
in and out alternately with extreme rapidity as the creature slowly 
waded along ; a large flagellum on each side of the maxillary lip 
beat equally rapidly, and produced a current which transferred the 
contents of the cells that the mandibles had sawn through into the 
mouth of the destroyer. Canestrini had found an allied creature 
before Megnin did, but without observing the peculiar habits of life 
above referred to, and had called it Histiostoma. The species are 
widely spread, and are a very common accompaniment of the early 
stages of vegetable decay. 
During the past year (1895) these Acari have acquired a new and 
wholly unexpected interest from a paper in the Danish language, 
published by A. S. Jensen of Copenhagen. In 1863, Leuckart described 
how the Horse-leech ( Aulastomum gulo ) laid its eggs in a capsule 
which it buried in the banks of the stream or pond which it inhabited. 
In 1885, Bergh investigated and described the somewhat complex 
metamorphoses which the leach-larva undergoes within the capsule ; 
he also mentioned that he found Mites in the capsule. Jensen 
has now carefully traced the life-history of this Mite, which he finds 
is a Histiostoma ; it appears from his researches that from the egg 
of the Acarus within the capsule emerges the usual hexapod larva, 
which soon changes its skin and becomes an octopod nymph which 
gradually devours the larval leech and all the albuminoid contents of 
the capsule. The nymph then passes into the hypopial stage, in which 
it can support greater exposure to changes of temperature, &c., and 
can attach itself firmly to insects and other creatures ; the empty 
capsule breaks and frees the Hypopus, which avails itself of its power 
of fastening on to other creatures to obtain a conveyance to a fresh 
capsule ; it then leaves the temporary host, mounts upon the new 
capsule, casts the hypopial skin, becoming a fully grown nymph of 
the ordinary sort, which penetrates through the wall of the capsule 
and assumes the adult form inside ; there reproduction takes place, 
and the same life-circle commences anew. The author thinks that 
the nymph uses its short, powerful front-legs to dig through the 
wall ; but I do not gather that he has seen the process. It is quite 
probable that these legs may assist in opening the slit, but, judging 
from analogy, which is perhaps dangerous, I should think it probable 
that the principal instruments used in cutting through the wall of 
the capsule are the same which I have often watched in action, cutting 
through vegetable cells ; namely the sharp serrated mandibles with 
their rapid backward and forward motion, reminding one of the action 
of a key-hole saw. 
