20 
Transactions of the Society. 
probably serves both purposes ; tbe terminal joint is articulated to the 
end of the penultimate, and folds down like tbe blade of a pocket- 
knife, or like tbe falces of spiders ; but the latter close sideways, 
whereas those of the Acarus-palpus turn downward. This form is 
found in the Water-mites only, and serves, inter alia , for holding-on 
purposes. Lastly we come to the singular “ antenniform palpi,” which 
are only found in the small sub-family of the Bdellinae ; here the 
organs are extremely long and of substantial thickness ; they are not 
carried like other palpi, but are raised above the rostrum and usually 
point forward and upward, quite in the position of antennae, which 
they greatly resemble in appearance. It is difficult to say what 
are the functions of these organs ; they are not kept in motion as the 
tactile apparatus of an Acarus usually is, and they are not raptorial ; 
the mandibles perform that office. They may be auditory. 
The mandibles themselves are the next mouth-organs to demand 
our attention, and they are very important ; they are the great 
feeding-instruments ; and the commonest form of them in the 
Acarina is that of a hard, chitinised chela, the movable arm of which 
is the lower arm, and works perpendicularly up and down. This is 
the almost universal form in the three great families of the Oribatidae, 
the Gamasidae, and the Tyroglyphidae ; and yet it is an extremely 
uncommon form in other orders of Arachnida ; Galeodes has it, and 
some few others ; but as a rule where the mandible is chelate the 
chela works sideways ; and in large groups, such as the Spiders, the 
terminal joint is articulated to the end of the penultimate ; and even 
then it works sideways. Another curious fact connected with this 
form of mandible is that in each of the three great families before 
named there is one single genus of a few species which has not the 
chelate mandible : the explanation of this is the same in each instance, 
viz. that the movable arm of the chela has become obsolete, and the 
fixed one has been lengthened, thus transforming * a seizing and 
crushing instrument into a sawing or piercing one. The genus in the 
Oribatidae is Serrarius ; it consists only of two species, and is very 
remarkable, because the creatures are so precisely like others in external 
appearance that they were unhesitatingly classed in one of the larger 
genera, and it was even doubted whether they were good species. 
When I was writing my work on the British Oribatidae for the Bay 
Society I drew the mandible of each creature ; on dissecting out that 
of the only British species of what is now this genus I discovered, 
to my astonishment, that it was totally different from every other 
known mandible in the family : it is a sawing organ. The exceptional 
genus in the Gamasidae is Stylochirns, and here the mandible is a long 
piercing-organ ; the second arm of the chela has not entirely dis- 
peared, there is just a relic of it left ; but both arms are fixed without 
any movable articulation. There is only one knpwn species. The 
unique genus of Tyroglyphidae is Histiostoma ; there are several 
species, and they are very strange creatures. Many years ago Megnin 
