Residents or Migrants?

TRANSACTIONS. RESIDENTS OR MIGRANTS ?

BY E . P . WILTSHIRE, B . A . ,

F.R.E.S.

ON one point, the residential status of their specimens, local Entomologists are noticeably more sensitive than Ornithologists.

The latter seldom resent the imputation of foreign origin in a specimen : perhaps because Bird-migrations are far better publicised, since a higher proportion of birds than of Insects appear to migrate and, indeed, fewer Ornithologists collect specimens. Local Entomologists however have been prone to resent the suggestion that a certain Butterfly or Moth in their district is an immigrant from overscas. As the study of Lepidoptera-migrations gains more adherents, this anti-migrant complex in Entomologists is disappearing ; but one may ask how and why they acquired it.

Firstly the complex arises out of natural conservatism, for the realisation that Insects migrate widely is comparatively recent.

Secondly the Naturalists may be well acquainted with the early stages of the species in question and they may, from its known residential capacity, rather naturally but incorrectly infer that no specimen taken within the same area that it breeds in, is aught but indigenous. A third and less obvious explanation is that, since most of them only collect British Lepidoptera, it has become a point of honour not to include Continental specimens in their cabinets. In the case of rarities that do not breed in Britain, the immigrant status of specimens caught here is obvious and therefore not resented ; and some collectors while paying extravagant prices for specimens alleged to have been caught in Britain spurn indistinguishable and far cheaper ones admitted to have been caught abroad. In the case of the true British residents, the collector is subconsciously subject to the same anti-foreign prejudice. His local patriotism may also make him consider it a reproach to his district that a certain species should be adventitious rather than indigenous there. Such partiality is very human, but may in certain cases lead to misinterpreting the facts. The follovving view of the subject of immigrants and residents is accordingly otfered as a useful corrective to local sensibilities and also as in accord with the facts.

We can group animals together according to their geographical headquarters; or we can use a quite different Classification and subdivide them into Migrants and Non-migrants, admitting that the latter outnumber the former in Lepidoptera, and that these are

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RESIDENTS OR MIGRANTS ?

but two extreme types which rather grade into one another. Why some Insects migrate while others do not is still a mystery ; but it seems that species capable of a profound diapause (Hibernation or sestivation) are less migratory than those with a short life-cycle.

It is significant that Birds are almost the only Vertebrates that neither hibernate or sestivate, and in them migration is most highly developed. There is no uniformity in the migration-patterns even of the migratory Insects of one climatic or geographical region, for each species is a law to itself in this respect.

If we use the above two ways of classifying British Butterflies, we find that, by the second, the Migrant class will include the Painted Lady (V. cardui) and the Large White (P. brassicae), while the Non-migrants will include the Purple Emperor (A. iris) and the Holly Blue (L. argiolus) ; but by the first way, viz., the zoogeographical, brassicae, iris and argiolus would be grouped together as Euro-Siberian, i.e. as having their headquarters in the mis-called " Temperate " zone of Europe and Asia, and cardui as SubTropical or Tropical. An immigrant into Britain is thus either migrating within its geographical headquarters (e.g. brassicae) or penetrating outside its headquarters (e.g. cardui).

In the latter case, permanent residence is improbable, if not impossible ; in the former case, the immigrants can, and probably do, interbreed with the resident stock. Of course, if the Insect is a non-migrant, it can only appear in a district indigenously or by importation, and the latter mode is exceptional. Recent studies however have shown that a number of British resident Lepidoptera also migrate widely, e.g. the Large White (P. brassicae), the Yellow Underwing (T. pronuba), and the Diamond-back (P. maculipennis).

In some cases, the question of continued survival in Britain is still unresolved ; other cases are of an intermediate and puzzling nature, owing to periods of scarcity and abundance. The general tendency is for migration to be proved in species in which it was not previously suspected, but in one case at least the reverse has occurred : the Brighton Wainscot (O. musculosa) is now known to inhabit the cornlands and downs of Wiltshire, after long being regarded as a rare immigrant; as to its migratory capacity, recent studies have given a negative answer.

It is natural to wish to know where a specimen one catches or buys came from, but it will now be clear that in many cases certainty is impossible. If the species is one that both resides locally and immigrates from abroad, the indigenous status of a specimen caught in Suffolk is no more than probable ; and, failing proof that it is a local resident, a specimen of one of the more mobile species is not evenprobpblyhome-bred. But the Naturalist, with the wider view of the migrant-resident question, will find it of less moment to ascertain the local origin of a specimen than the local status of the species to which it belongs.

FOREIGN OFFICE, WHITEHALL ;

22 June,

1944.