Savory - Arachnida 1977

84 II. DE ARACHNIDIS
much of the zone. Here the chief features of living must be the scarcity
offood, the lack of water, the heat of the sun by day and the difference
between day and night temperatures.
The large size mentioned above must be regarded as one form of
adaptation to high temperatures. It is in itself a water-conserving
mechanism, exploiting the low ratio between surface area and volume,
and it is supplemented by the nature of the respiratory system. The
book-lungs, which form the primitive respiratory organs of Arachnida,
are most numerous in the scorpions and in the thcraphosid spiders:
they can function efficiently in connection with a less elaborate circulatory
system than that which must exist in an arachnid with a localized
respiratory system.
The large desert Solifugac represent an alternative arrangement.
They have no book-lungs, but only tracheae. Their tracheal system is,
however, more elaborate than that found in any other order, and thus
permits the great activity and high speeds that are so characteristic of
this order. A tracheal system of respiration is, in fact, an advance on
the book-lung, and has been very successful in other orders.
The sporadically distributed species of the first three orders mentioned
in category (ii) contain the most strongly cryptozoic, photophobic,
stereotropic and hygrophilic of all Arachnida. l\Iost of them are slight
in build, slender in dimensions, delicate in constitution; and they
arc among the most typical of all Arachnida. They contrast most
emphatically with the great multitude of temperate zone species, which
include, among others, the web spiders that live exposed lives in the
open.
The very large third group, inhabiting both north and south temperate
zones and reaching its limits towards the polar regions, illustrates
very clearly the relationship between physical conditions, body size
and the respiratory system.
Book-lungs are in the majority limited to a single pair served by a
more complicated arterial system, but very often they arc supplemented
by tracheae. This is true of the majority of spiders, which also present
differences in the position of the spiracular opening. There can be little
doubt that tracheae help to economize internal moisture, and that in
small spiders this function is improved by a thicker sclerotization of the
exoskeleton.
The orders Opiliones and Pscudoscorpiones show tracheal respiration
only, and in the most active genera of the former there are supplementary
tracheae with spiracles in the legs.
The fourth category, the polar Arachnida, is also well characterized.
There is a sharp distinction between the Arctic and Antarctic, due
to the isolation of the Antarctic continent and the tempestuous nature
9. ZOOGEOGRAPHY: DISTRIBUTION 85
of the oceans that surround it, an isolation that affects the character
of other groups in the polar faunas.
The first record of an Antarctic arachnid was the discovery of the
mite Penthaleus belli under moss at Cape Adare during the Southern
Cross expedition of 1899-1900. During Shackelton's ~imrod expedition
( 1907-09) several species of mites were found to be abundant in Coast
Lake, near Cape Royds on Ross Island. Exoskeletons of others were
found in other lakes, but none were seen alive. A mite, however, hatched
among vegetable matter brought from Deep Lake after the return of the
expedition to England. These Ross Island mites were never named in
the scientific reports of the expedition, but they are of interest as being
among the first Arachnida to be recorded from the far south. Since
then, mites and accompanying spring-tails have been found at stations
from sea-level to heights of 1,830 m in latitude 77°South, where the
temperature may fall in the winter to --65°C, and as far inland as the
86th parallel. Mites appear to be more resistant than insects and so
able to survive by living under stones where the temperature remains
high enough to allow metabolism to continue.
No spiders or members of any other order have been found on the
Antarctic continent, and the most southerly spider is most probably
either Rubrius subfasciatus from Cape Horn or Notiomaso australis from
South Georgia in 55°S.
Within the Arctic Circle are lands of comparative fertility, supporting
the Esquimaux and the Samoyeds, with their herds, and producing
flowering plants. The 80th parallel of north latitude passes through the
island of Spitzbergen, and a very considerable number of spiders and at
least one harvestman have now been recorded from this island and
elsewhere, chiefly by the pioneering researches of Braendegaarde. The
farthest north land is the northern coast of Greenland, which reaches
latitude 83°N, only 644 km from the Pole. This region was explored in
1917 by the Fourth Thule expedition led by K. Rasmussen, and their
report stated that "Spiders and Earth-mites also support life in these
high latitudes". At least 50 species of spiders have been found in
Greenland, and 15 species are known from the island of Spitzbergen.
In comparison with these results it is interesting to recall that in 1924
spiders of the family Salticidae were found at a height of 6, 700 m on
Mount Everest, and were then described as holding the proud position
of being the highest permanent inhabitants of the world. They must,
however, be supposed to share this honour with some insects, probably
spring-tails, which, though undetected at the time, provide them with
food.
Two other aspects of distribution remain to be considered, the
unexpected and the cosmic.