24 
Transactions of the Society. 
large species and also some smaller Gamasids and placed them in a 
cell together. I used to feed them with small insects or parts of 
insects. When there were plenty of insects, so that each Gamasid 
could, so to speak, feed at a separate table, all was well ; but when a 
single small insect was given the Holotaspis did not at all approve of 
the smaller Gamasids sharing his meal and used to drive them away, 
and the small ones could be seen waiting round and watching like a 
set of little hungry jackals until the Holotaspis had finished and gone 
away, when they rushed in and demolished the scraps. Now how 
did the blind Holotaspis know that the smaller Gamasids were there, 
and how did the lesser fry know when the Holotaspis had finished 
and departed ? Is it smell ? I can hardly think so. How could 
smell enable them to catch Thysanuridse ? Is it the tactile sense ? 
We know that eyeless Infusoria swim about by hundreds in a small 
quantity of water and do not collide : we can conceive that each may 
make some current in the water which may affect the tactile sense of 
its neighbours, and warn them to keep out of the way ; but how 
shall we understand this in the case of Acari standing in air ? Or do 
they see in some way unknown to us ? Or, finally, have they some 
sense which we do not possess or comprehend ? It is said that it is 
impossible for a man really to comprehend a sense which he him- 
self does not possess. I think I remember a statement in one of the 
books of the writer who assumed the nom de plume of “ The Old 
Shikari,” to the effect that an Italian guide’s warning of “ Non e 
possibile, signore” had sent him over some crazy bridge south of 
the Alps ; and it may be that even the sixth sense impossibility will 
be overcome some day. 
The parasitism of Acari is also curious and interesting, because it 
is of so many sorts. Most people think that almost all Acari are 
parasitic ; this is an entire mistake ; by far the greater number are 
not anything of the kind. But amongst those that are we find almost 
every sort of parasitism, including some sorts rare or unknown among 
other creatures; we have temporary parasites and permanent para- 
sites, parasites during youth only as in the Hydrachnidse, parasites at 
all ages such as JDemodex ; parasites which derive their whole 
nourishment from the host, and if let alone will eventually kill it, 
such as Sarcoptes and the Cytolichus found on the serous membranes 
of the common fowl ; parasites that have their food provided by the 
host, but in a way that is beneficial to the host itself, such as the 
Analgesinse which serve to keep the feathers of birds clean without 
injuring them. We have mutualists, commensalists ; parasites such 
as the Hypopi, about which I shall have something more to say, 
which only get up to have a ride, and use their host as we use a cab 
or an omnibus, and no more eat the host than we eat the cab ; 
parasites such as the Uropoda vegetans of de Geer, which also seek 
conveyance only and attach themselves by the matter passing from 
the anus generally to the legs of ground beetles, so that when the 
