Savory - Arachnida 1977

208 Ill. PROLES ARACHNES
saprophages whilst others are graminivores and some parasitic. Amongst
the parasites are many skin and feather mites of birds, skin parasites of
mammals and respiratory and visceral parasites of vertebrates.
Astigmatid mites are mostly slow moving and weakly sclerotized.
They range in length from 200 to 1,500 /km and respire through their
integument. The group is divided into some 40 families.
An important astigmatid mite is Acarus siro which feeds directly on
stored grain. It is of economic significance since it destroys the wheat
germ. Of the parasitic mites the ear mite of dogs and cats, Otodectes
cynotis, is important as is the feather mite of domestic fowl and some other
birds, Megninia cubitalis. Sarcoptes scabiei is a mite which attacks the skin
of man as well as a number of other animals. It causes a condition known
as scabies or sarcoptic mange. Other mites which affect man are the
house dust mite, DermatophagoideJ pteronyssinus, which has been implicated
as the cause of house dust allergies, and Dermatophagoides scheremetewskyi
which causes eczema seborrhica.
Cryptostigmata
Commonly known as oribatid mites or beetle mites, because of their
superficial resemblance to these insects, this is a cosmopolitan group of
heavily sclerotized mites which range in length from 200 to 1,500 fkm.
Members ofthe 100 or so families ofcryptostigmatids are mainly fungivorous,
saprophagous or algivorous and are common in soil, leaflitter
and under stones and bark.
Spiracles are absent in these mites although some families possess
tracheae associated with paired dorsal pseudostigmata and with the
bases of the first and third legs.
Species of the genus Oppia are common inhabitants of forest humus
and grasslands. A common example is Oppia ornata found in mixed oak
and beech humus. Another species of this genus, 0. minuta, is an important
vector of the sheep tapeworm Monie::_ia expansa, as too is Galumna
vzrgznzenszs.
DISTRIBUTION
The Acari are ubiquitous, for their small size, their modes of life and
their ability to withstand adverse conditions all help them to achieve
universal distribution. The most northerly land in the world, the north
coast of Greenland, numbers mites among its fauna and they have also
been found not only on the fringe of the Antarctic, but also on the
Shackleton Glacier in 86° 22'S. Mites attain great altitudes, and are,
for example, plentiful in the fleece of the sheep which live at a height of
26. THE ORDER ACARI 209
above 4,500 m on the plateaux of Turkestan. At much greater heights
than this they are to be found drifting in the aerial plankton.
PALAEONTOLOGY
The oldest known fossil mite belongs to the Devonian age, and was found
in the Old Red Sandstone of Aberdeen. Its name is Protacarus crani, and
to some extent it resembles the recent Eupodoidea of the Prostigmata,
but has also some primitive characters.
Oligocene amber contains a fair number of isolated genera. Some of
these appear to be extinct, but some, especially among those belonging
to Cryptostigmata, belong to recent genera, and cannot actually be
distinguished from living species.
From peat, galls have been obtained some of which may well have
been caused by members of the Prostigmata.
CLASSIFICATION
The classification of the Acari has passed through the stages common to
all large groups of animals, the known species of which are constantly
increasing in num her. At present nearly 2,000 genera and some 20,000
species have been described and it is estimated that up to 500,000 more
species may exist.
A brief look at some earlier systems of classification is useful as it
helps in the understanding of present schemes.
The system proposed by Banks in 1905 was in general used for over
20 years, and for its simplicity has much to recommend it. He divided
the order Acari into eight super-families as follows:
(1) Eupodoidea (5) Gamasoidea
(2) Trombididoidea (6) Oribatoidea
(3) Hydrachnoidea (7) Sarcoptoidea
( 4) Ixodoidea (8) Demodicoidea
Oudemans in 1906 forwarded a different classificatory system based
mainly upon the number of respiratory plates (spiracles or stigmata). He
divided the class Acari into five sub-classes:
( 1) Astigmata
(2) Lipostigmata
(3) Zemiostigmata
(4) Octostigmata
( 5) Distigmata
Later, in 1923 Oudemans proposed a new system in which the then
order Acari was divided into six sub-orders: