has three advantages. It enables the reader to find the locality quickly on the
O.S. map; it facilitates cross-reference to the accompanying distribution maps;
and, thirdly, it will be of service to the Biological Records Centre if they are
ever able to produce dot distribution maps for the microlepidoptera. Imprecise
records from places situated in more than one square are assigned to the more
southerly or westerly. An exception is made with Harwood's records from
Colchester which are allocated to TM 02 in preference to TL 92 because he
appears to have done most of his collecting east of Colchester.
The maps
It is often said that distribution maps show the distribution of collectors
rather than of species. To this it may be added that they also show the particular
interest or specialisation of the collector, who inevitably will record certain
groups of insects more fully than others. A third factor is the instability of the
insects themselves. Those with a specialised habitat such as a salt-marsh are the
most stable and may persist in the same spot almost indefinitely if allowed to do
so. Others are more adaptable and nomadic; if robbed of one habitat, they
colonise another, even if this involves some modification in behaviour such as
adaptation to an urban environment. Other insects change their ground in
response to stimuli which are hard to analyse. This may be simply extension or
contraction of range; here the former will be more reliably reported than the
latter, just as it is easier to record the first swallow of spring than the last of
autumn. Movement is not always centrifugal or centripetal; it may be linear,
e.g. from one wood to another. There are regularly holes in the pattern of
distribution, belying labels such as 'common everywhere', so popular with the
writers of local lists. These 'holes' may be of a temporary nature, when it is
customary to attribute them to parasites or disease; however, as I have ob-
served in Essex, only one species may be absent from an association of insects
all subject to the same diseases and parasites, this showing that some other
factor is at work. A map cannot distinguish between a 'hole' and a lacuna in
recording.
Essex collectors have lived and operated almost exclusively in the four
corners of the county and this is reflected in the recording. In order to lessen the
consequent imbalance, my wife and I have ranged over the whole county in the
last few years. For example, in 1979 we recorded in each of the 57
ten-kilometre squares at least twice and in several many more times, concent-
rating on the less well known areas. But the second of the factors I mentioned
above came into play: my main interest is in the leaf-miners and I recorded
them much more comprehensively than, say, the Pyralidae. A further problem
arises over common species. Past writers naturally enough state that they are
widespread without giving any localities that can be entered on a map; as a
result, a common species may have fewer dots than a rare one. The macro-
lepidoptera have many recorders, most of whom send their data to the Bio-
logical Records Centre at Monk's Wood. There is no such service yet for
microlepidopterists, who are few in number and understandably remiss in
sending their records to an individual in place of an institution.
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