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ified legs, or gonopods, that transmit the sperm. The 'intercalary' stage opens the way
to repeated reproduction by males without which there would soon be a great imbal-
ance of sexes among mature individuals. A few species avoid this particular problem
by reproducing parthenogenetically; males are either rare or absent altogether.
F IG. 28
'Flat-backed' millipede Polydesmus angustatus. (Photograph J. Grant.)
Estimating the numbers of such creatures is easy enough, if laborious, when they
can be washed out of a friable soil over sieves and floated off on a solution of mag-
nesium sulphate which has a high density (like the Dead Sea). With soils containing
a lot of organic matter, this method is not possible, and it is usual to use a heat and
humidity gradient to expel them from soil cores. However, this relies on a thorough
understanding of behavioural responses to such factors, and experiments show that
there are often subtle changes in activity even between different juvenile stages of the
same species. Pitfall traps are good at catching the adults of many litter-inhabiting
millipedes but the young stages of Polydesmus species, for instance, are rarely caught
this way.
Garden compost heaps and piles of leaves in city parks make good hunting
grounds for millipedes, woodlice, pseudoscorpions and other litter-loving inverteb-
rates. Some exotic species have been added to the British list in recent years from such
habitats, including Unciger foetidus and the blind Cylindroiulus vulnerarius. They
have probably been introduced from the Continent, or even further afield, on the roots
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