Article beginning on page 159.
Psyche 9:159-162, 1900.

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PSYCHE:.
THE DISTRIBUTION OF HOLARCTIC COLLEMBOLA. BY JUSTUS WATSON FOLSOM, CHAMPAIGN, ILL. [Annual address of the retiring president of the Cambridge Entomological Club, January 11, 1qo1.1 No less than one hundred and fifty-
two species ol Colleinbola are known to
occur in North America, of which thirty- eight, .it least, are shared with Europe. Their specific identity has been estab-
lished, not simply from descriptions,
but by a. thorough comparison of speci-
mens. Twenty-five per cent of the
Nearctic species, then, are also Palae-
arctic, Specimens of the latter, as a rule, average slightly larger than of the form- er, but that is all.
I doubt if as large a proportion of
Holarctic species is recorded for any
other order of animals ; moreover, this
proportion is increasing, with the com-
parison of additional specimens.
Apparently restricted to the eastern
part of the United States are several
species which may have been recently
introduced ; such are Neafzwa mscorum,
A'ho-rwa armata, Orchesella n'Itct~'s, Sirs bn-ski, Tomocerus vu&a?vs and trihfi-
ferzts ,
The following occur abundantly
throughout Europe and the United
States : Aphorum incrmis, Podti-ra apa-
tical .Ach.o~~;des armatm, Isotumz ftmef(~å´~-k 7)iridis and pahtstris, also ~ ~ O ? ? ? O ~ ? å ´ } J ~ ?tzrt/t?/asciata. These are the more ob- trusive members of the order and, hav-
ing been widely collected, furnish
valuable data for the study of distribu- tion. The list will doubtless be in-
creased when more & known about
the Collembola of the western states;
as it is, the forms next named, which
range throughout Furope, arc known to
occur in most of the States cast of the
Rocky Mountains : Am?+idtr gm~~ftå´ri(z
Isotoma &?m, Lepid(q~r/m cynnrus,
Sirs nz~ro~n(~t"l&/(r, Sm/~;///m+us apnfi- as, n2a Zmgreni, hortensis and degms.
In the arctic regions the less special-
ized C:ollemhola Ilot~rish, some species becoming' larger than in middle Europe.
Thanks to several expeditions, our
knowleclgc of (lie arctic range of many
species is definite. Of the sixty known
species of arctic mid subarctic Collem-
bola, at least twenty are broadly dis-
tributed in the United States. and all
but a few occur in northwn and middle
Europe; in fact, but three or four are
peculiar to the arctic region.
In Greenland, Spitzbergen, Franz




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Josef Land, Nova Zen~bia, Siberia and
Alaska are found many of the species
common to Europe and the United
States, for example, Mauura 7?ZZiSC07WfZ, Amirida panaria, AcJiorzites armatzds,
Isotomapalusfris and viridis. Hitherto,
Tomocerus nigw of Europe, T. arcticifs
of Siberia and T. america~zr~s of Cali-
fornia have been regarded as distinct
species. The Harriman expedition,
however, brought from Alaska numerous
specimens which form perfect inter-
gradations between the tllree forms
named.
There is, then, not only a remarkable
agreement in structure between European
and North American Collembola, but
also, for many species, continuity of clis- tribution.
The specific identity of so many Hol-
arctic species, an identity which would
not be expected upon a priori grounds,
may easily be accounted for. The
Collembola and Thysanura, the most
generalized of l~exapods, present very
few adaptive characters as contrasted
with other insects. The Collembola
feed upon organic debris and are mostly
confined to moist and decayingvegetable
matter or to water surfaces. Like the
worms, the simplicity of their external
organization is to be attributed to the
uniformity of their environment. The
simple conditions of food, moisture,
temperature, etc., which a Collembolan
requires, may be found almost anywhere
in the Holarctic region ; so that, after all, it is difficult to understand how, under such circumstances, any decided modi-
fication of even varietal value could
occur. The species of continuous Hol-
arctic distribution are, in every case, only such as can exist in a comparatively
simple environment, -the more spe-
cialized species and genera are not found to any extent in the arctic region.
The wide distribution of Collembola
is surprising, for they appear to have no means of self dispersal ; they lack wings and probably always did, as none are
found in the embryo ; their feeble walk- ing and leaping could procure only a
limited local distribution ; a dry spot is an effective barrier to most Collem-
bola, which require an atmosphere sat-
uratecl with moisture. The insects may
possibly be blown about to some extent,
but their eggs are probably not, as they are laid in the soil, under bark, or in other concealed places.
As for accidental means of dispersal,
there may he many. Several species
not indigenous have been found ingreen-
houses among exotic plants. Isotoma
firnetaria, abundant in rich soil, fre-
quently occurs on potted plants. I used
to find f','/;fomobrya mulfifasciaia and other species among early strawberries
which had been brought to Massachu-
setts from the south. The influence of
man, then, in transporting Collembola
upon plants, fruits or vegetables is not inconsiderable.
The lower animals perhaps assist now
and then. Moniez found a species of
Entomobrya by hundreds in the fresh
ncs~ of ;i huh, nl~erc tlicy were feeding upon the lining of Feathers ; Wahlgren




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February, qoi] 3's I'cHJ?. 161
records three species of Collembola
from the mossy nests of sea gulls ;
whether birds carry these insects among
their feathers, or not, is not known.
Mr. S. R. Williams gave me a specimen
of Entomotryagriseo-olizmta and a Thrips both of which he found deep in the fur
of a mole.
These accidental, or occasional, means
of dispersion may be recognized without
being assigned too much importance.
Certainly, human intervention cannot
be heldresponsiblefor the distribution of the arctic forms by which the European
and North American faunae are linked
together.
By far the most important agent of
dispersion is running water. During
most of the year, some species of Col-
lembola are to be found on our streams
and ponds and, in some months, as many
as a dozen kinds at once. Afew of these, such as Podzn-a aquatics, htoma palus-
tris, Swinthurus aquaficq ma/mgf,eni
and spinatus possess structural adapta-
tions for their semiaquatic life, but many other species are met with which, though normally terrestrial, are quite at home
on the surface of fresh water, which can- not wet them, and in which they can-
not sink; they leap upon the surface
film with ease.
Such species undoubtedly owe their
broad distribution mainly to streams of
fresh water.
The snow fleas, which attract interest
by their sudden appearance in immense
numbers, are distributed by the same
means. In thc latter part of winter
they may be found, more or less be-
numbed, under the loose bark of trees,
especially pine, oak and maple, or else
about the roots among dead leaves, in
which situations their eggs are laid.
Rendered active by the first warmth of
spring, the little creatures wander out
and son~etimes darken the snow by their
numbers. Rivulets of melted snow or
of rain water carry them to the brooks
whence they are borne to the rivers and
scattered no one knows how far.
Many other species are washed from
stream to stream in the same way;
floating logs, branches and roots must
often transport Collembola, especially of the genus Isotoma, a large proportion
ot which live under loose bark.
In fact.
I believe that the most important agents of dispersion for inland Collembola are
fresh water streams.
Marine currents, also, are of vast im-
portance in this respect.
Wahlgren re-
cords nine species of Collembola from
barren rocky islands off the Swedish
coast and properly maintains that they
were distributed solely by sea water.
These are Aniirida viarifima and tzdl-
be@, Anitrophoriis larZXs, Xenyl/a å´aim. &a and htimicola, Achorutes Tiaticvs,
Isofma ~'iridis, Entomobrya lan//~ino.?a and Sira h~iski.
It is a significant fact that almost all those named are Holarctic.
The species of Anurida, indeed, are
restricted to salt water. A. mari//?/ia is common on the coast of Massachusetts,
where it occurs in colonies between tide marks. At low tide the insects walk




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162 PSYCHE. [February, qoi
about and feed upon dead mollusks ; as
the tide rises they hide under stones and become submerged.
Now this species occurs not only on
the coasts of New England, Long Island,
Florida, and probably of 'intervening
places, but also along the entire western coast of Europe ; its distribution by
marine currents, therefore, cannot be
doubted.
Isotoma liesseisi, a marine species
found first at Polaris Bay, is not rare on the coast of Massachusetts and has late- ly been found in Spitzbergen. Xenylla
humicola has a similar distribution.
Aclwruter tuZ/he@ (ditbius Tull.) in-
habits Siberia, Nova Zembla, Franz
Josef Land and Spitzbergen, and
also
occurs on the salt marshes of eastern
Massachusetts.
Achorutes viatz'ais not only ranges
over the Palaearctic region, including
the arctic islands, but has been found
in California and even in Tierra del
Fuego.
Finally, Achorutes armatus, which
occurs throughout the Holarctic region,
has been taken in South America, New
Zealand and Sumatra.
More cosmopolitan forms are known.
but none of the other faunal regions
share their species to the extent to
which the Palaearctic and Nearctic do.
I have shown that a number of gene-
ralized species of Collembola inhabiting both Europe and the United States are
practically continuous in their distribu- tion between the two places. The fact,
however, that most Holarctic species
are discontinuous must be accounted
for and the explanation of the fact is
important for its general bearing.
Nearly all the Holarctic species of
the Arctic regions proper belong to the
most generalized families, i. e., Aphor- uridae and Poduridae, and can live in
an environment of extreme simplicity
and rigor, needing but the scantiest of
vegetation and being quite tolerant of
cold ; in temperate regions they are
noticeably the most active species of
the order during the winter.
The Kntomobryiclae and Sminthuridae,
on the other hand, require more warmth
and certainly a more luxuriant vegeta-
tion than arctic regions afford. Now
these most specialized families, adapted to a more complex environment, contain
many species which, although identical
in middle Europe and the United
Slates, do not exist, it may safely be
said, in the intervening regions, where, most probably, they formerly must have
occurred. The inference is obvious,
then, that a higher temperature and
a more luxuriant vegetation than at
present once prevailed in the arctic
zone. This is no news, of course, -
but the additional evidence is worth
something.




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