Savory - Arachnida 1977

248 IV. DE ARACHNOLOGIA
so commonly condemned as is a cog in the \vheel and gives
things a push in some direction. 'Ve do not know enough to say precisely
where or which way, but it is certain that the natural world \Vould
be somehmv different if the harvestman were not present. Therefore,
let it be admitted that there is probably au indirect economic importance
for all Arachnida, each one of which bears its share in this process.
The following are examples of direct economic importance for Arachnida
other than Acari:
Limulus has been used in America as food for both pigs and
poultry and in some areas it is believed that this food makes the hens
lay. Females arc preferred on account of the eggs they carry. Thai
(Siamese) hunt the species Tachypleus for the sake of its eggs and
a female has been sold for the equivalent of 5p. The eggs are said to
look like caviare and to taste like potato.
Scorpions of several genera e.g. Buthus, Centruroides, Centrurus
and Tityus, are known to be distinctly poisonous if not dangerous
to man. Lay people in certain countries fear, avoid, or become mentally
sick even from the stings of non-poisonous
(iii) Spiders of several genera e.g. Chiracanthium, Latrodectus and
Loxosceles, are definitely known to inflict poisonous bites on man.
Arachnid venom is given a fuller treatment elsewhere in this book.
; Finally, although it is not apparent or well known, spiders of
certain genera e.g. ~ephila and Latrodectus, are in a small way beneficial
to man. The silk of the former has been woven into cloth for
ornamental tapestries and that of the latter has been used as cross-hairs
in precision instruments e.g. surveying instruments, gun-sights and
periscopes.
In the final analysis, however, the Arachnida whose known influence
on human affairs is sufficiently important and far-reaching to justify
this chapter are the Acari. They include species that attack growing
crops, stored crops and manufactured produce; species which are
parasitic on man and his domesticated animals; species which are
hosts or bearers of pathogenic organisms and transmit them to man and
his domesticated plants and animals; and species that attack,
or transport themselves upon species that are beneficial or injurious to
man and his industries.
The number of mite species is very great and their influence is chiefly
felt through the numbers of individuals which spring into existence
wherever and whenever conditions favourable to their multiplication
obtain. However, certain families contain species that are most
frequently of economic importance. The work of the economic acarologist,
like that of his brother, the economic entomologist, has consisted
largely of biological and ecological studies on these animals, to deter-
30. ECONOMIC ARACHNOLOGY 249
mine the point at which they may be most successfully attacked or used.
Mites do not collaborate in this process with much enthusiasm, for as
Shipley said oflice "When you want them to live, they die and >vhen you
want them to die, they live and multiply exceedingly".
The following is a brief account of a representative selection of acarid
Since the subject is large and complex, no attempt at encyclopaedic
completeness has been made in this limited space.
Among those mites that attack and injure the growing crops of man,
those of the families Tetranychidae, Tenuipalpidae, Tarsonemidae,
Eriophyidae and Acaridae are the most important. The Tetranychidae,
also known as red-spiders or spider mites, are probably the most frequently
encountered mite enemies of the home gardeners. Although
small, they multiply so rapidly that they easily become a serious
economic menace. Tetranychus urticae (and T. cinnabarinus in greenhouses)
in England and Europe attack cucumbers, tomatoes, hops,
strawberries, carnations, peaches and many other vegetable, fruit and
ornamental crops. Trouble begins with the rising spring temperatures
which awaken the hibernating pests and continue throughout the
summer as successive mite generations feed and lay their eggs upon the
stems and leaves of the plants. In October, the females are fertilized,
change from dull green to brilliant scarlet, become gregarious, live in
communal webs and finally hibernate or lay over-wintering eggs,
completing the cycle of seasonal life.
Two other important pests of fruit crops should also be mentioned.
They are Panonychus ulmi, the apple red-spider which is found on apples,
plums, pears and raspberries, and Bryobia ribis found on gooseberries.
These are leaf-feeders that suck the sap from the under-side of the
In general they pass the winter as eggs.
In America, and on other crops, these and other genera of spider
mites often become numerous and seriously injurious. For instance
Tetranychus pacificus, Bryobia praetiosa, Panonychus citri, Eotetranychus sexmaculatus
and other species may damage and defoliate apple, plum,
almond and other fruit trees on the Pacific coast. \Vhereas Eutetranychus
banksi, P. citri and E. sexmaculatus are the species most important on
Florida citrus trees, B. praetiosa also seriously damages grass and clover
crops and in autumn often enters houses in large, nuisance numbers.
Flat mites of the Tenuipalpidae are very small, flattened, red or red,
black and green mites that seriously injure citrus, avocado and other
fruits in the tropical and sub-tropical regions of the world. Perhaps the
most important, especially in the Americas, is Brevipalpus cal!fomicus
which is known to attack papaya, guava, citrus and avocado. In Florida,
this mite is known to transmit or cause the disease condition known as
Florida leprosis on citrus trees.