w line of severe
cold between climates otherwise very similar.[926]

_Migratory instincts._--Nearly all the insects of the United States and
Canada, differ specifically from the European; while those of Greenland
appear to be in a great measure identical with our own. Some insects are
very local; while a few, on the contrary, are common to remote
countries, between which the torrid zone and the ocean intervene. Thus
our painted lady butterfly (_Vanessa cardui_) re-appears at the Cape of
Good Hope and in New Holland and Japan with scarcely a varying
streak.[927] The same species is said to be one of the few insects which
are universally dispersed over the earth, being found in Europe, Asia,
Africa, and America; and its wide range is the more interesting, because
it seems explained by its migratory instinct, seconded, no doubt, by a
capacity, enjoyed by few species, of enduring a great diversity of
temperature.

A vast swarm of this species, forming a column from ten to fifteen feet
broad, was, a few years since, observed in the Canton de Vaud; they
traversed the country with great rapidity from north to south, all
flying onwards in regular order, close together, and not turning from
their course on the approach of other objects. Professor Bonelli, of
Turin, observed, in March of the same year, a similar swarm of the same
species, also directing their flight from north to south, in Piedmont,
in such immense numbers that at night the flowers were literally covered
with them. They had been traced from Coni, Raconi, Susa, &c. A similar
flight at the end of the last century is recorded by M. Louch in the
Memoirs of the Academy of Turin. The fact is the more worthy of notice,
because the caterpillars of this butterfly are not gregarious, but
solitary from the moment that they are hatched; and this instinct
remains dormant, while generation after generation passes away, till it
suddenly displays itself in full energy when their numbers happen to be
in excess.

Not only peculiar species, but certain types, distinguish particular
countries; and there are groups, observes Kirby, which represent each
other in distant regions, whether in their form, their functions, or in
both. Thus the honey and wax of Europe, Asia, and Africa, are in each
case prepared by bees congenerous with our common hive-bee (_Apis_,
Latr.); while, in America, this genus is nowhere indigenous, but is
replaced by Melipona, Trigona, and Euglossa; and in New