On May 13, 1883, I chanced to enter a meadow a few miles above
Washington, on the Virginia side of the Potomac, at the head of a small
stream emptying into the river. It was between two hills, at an
elevation of 100 feet above the Potomac, and about a mile from the
river. Here I saw many clayey mounds covering burrows scattered over the
ground irregularly both upon the banks of the stream and in the adjacent
meadow, even as far as ten yards from the bed of the brook. My curiosity
was aroused, and I explored several of the holes, finding in each a
good-sized crayfish, which Prof. Walter Faxon identified as _Cambarus
diogenes_, Girard _(C. obesus_, Hagen), otherwise known as the burrowing
crayfish. I afterward visited the locality several times, collecting
specimens of the mounds and crayfishes, which are now in the United
States National Museum, and making observations.
At that time of the year the stream was receding, and the meadow was
beginning to dry. At a period not over a month previous, the meadows, at
least as far from the stream as the burrows were found, had been covered
with water. Those burrows near the stream were less than six inches
deep, and there was a gradual increase in depth as the distance from the
stream became greater. Moreover, the holes farthest from the stream were
in nearly every case covered by a mound, while those nearer had either a
very small chimney or none at all, and subsequent visits proved that at
that time of year the mounds were just being constructed, for each time
I revisited the place the mounds were more numerous.
[Illustration: Fig. 1 Section of Crayfish burrow]
The length, width, general direction of the burrows, and number of the
openings were extremely variable, and the same is true of the mounds.
Fig. 1 illustrates a typical burrow shown in section. Here the main
burrow is very nearly perpendicular, there being but one oblique opening
having a very small mound, and the main mound is somewhat wider than
long. Occasionally the burrows are very tortuous, and there are often
two or three extra openings, each sometimes covered by a mound. There is
every conceivable shape and size in the chimneys, ranging from a mere
ridge of mud, evidently the first foundation, to those with a breadth
one-half the height. The typical mound is one which covers the
perpendicular burrow in Fig. 1, its dimensions being six inches broad
and four high. Two other forms are shown in Fig. 2. The burrows near the
stream were seldom more than six inches deep, being nearly
perpendicular, with an enlargement at the base, and always with at least
one oblique opening. The mounds were usually of yellow clay, although in
one place the ground was of fine gravel, and there the chimneys were of
the same character. They were always circularly pyramidal in shape, the
hole inside being very smooth, but the outside was formed of irregular
nodules of clay hardened in the sun and lying just as they fell when
dropped from the top of the mound. A small quantity of grass and leaves
was mixed through the mound, but this was apparently accidental.
The size of the burrows varied from half an inch to two inches in
diameter, being smooth for the entire distance, and nearly uniform in
width. Where the burrow was far distant from the stream, the upper part
was hard and dry. In the deeper holes I invariably found several
enlargements at various points in the burrow. Some burrows were three
feet deep, indeed they all go down to water, and, as the water in the
ground lowers, the burrow is undoubtedly projected deeper. The diagonal
openings never at that season of the year have perfect chimneys, and
seldom more than a mere rim. In no case did I find any connection
between two different burrows. In digging after the inhabitants I was
seldom able to secure a specimen from the deeper burrows, for I found
that the animal always retreated to the extreme end, and when it could
go no farther would use its claws in defense. Both males and females
have burrows, but they were never found together, each burrow having but
a single individual. There is seldom more than a pint of water in each
hole, and this is muddy and hardly suitable to sustain life.
[Illustration: Fig. 2 Crayfish Mound]
The neighboring brooks and springs were inhabited by another species of
crayfish, _Cambaras bartonii_, but although especial search was made for
the burrowing species, in no case was a single specimen found outside of
the burrows. _C. bartonii_ was taken both in the swiftly running
portions of the stream and in the shallow side pools, as well as in the
springs at the head of small rivers. It would swim about in all
directions, and was often found under stones and in little holes and
crevices, none of which appeared to have been made for the purpose of
retreat, but were accidental. The crayfishes would leave these little
retreats whenever disturbed, and swim away down stream out of sight.
They were often found some distance from the main stream under rocks
that had been covered by the brook at a higher watermark; but although
there was very little water under the rocks, and the stream had not
covered them for at least two weeks, they showed no tendency to burrow.
Nor have I ever found any burrows formed by the river species _Cumbarus
affinis._ although I have searched over miles of marsh land on the
Potomac for this purpose.
[Illustration: Fig. 2 Crayfish Mound (shorter)]
The brook near where my observations were made was fast decreasing in
volume, and would probably continue to do so until in July its bed would
be nearly dry. During the wet seasons the meadow is itself covered. Even
in the banks of the stream, then under water, there were holes, but they
all extended obliquely without exception, there being no perpendicular
burrows and no mounds. The holes extended in about six inches, and there
was never a perpendicular branch, nor even an enlargement at the end. I
always found the inhabitant near the mouth, and by quickly cutting off
the rear part of the hole could force him out, but unless forcibly
driven out it would never leave the hole, not even when a stick was
thrust in behind it. It was undoubtedly this species that Dr. Godman
mentioned in his "Rambles of a Naturalist," and which Dr. Abbott _(Am.
Nal.,_ 1873, p. 81) refers to _C. bartonii_. Although I have no proof
that this is so, I am inclined to believe that the burrowing crayfishes
retire to the stream in winter and remain there until early spring, when
they construct their burrows for the purpose of rearing their young and
escaping the summer droughts. My reason for saying this is that I found
one burrow which on my first visit was but six inches deep, and later
had been projected to a depth at least twice as great, and the
inhabitant was an old female.
I think that after the winter has passed, and while the marsh is still
covered with water, impregnation takes place and burrows are immediately
begun. I do not believe that the same burrow is occupied for more than
one year, as it would probably fill up during the winter. At first it
burrows diagonally, and as long as the mouth is covered with water is
satisfied with this oblique hole. When the water recedes, leaving the
opening uncovered, the burrow must be dug deeper, and the economy of a
perpendicular burrow must immediately suggest itself. From that time the
perpendicular direction is preserved with more or less regularity.
Immediately after the perpendicular hole is begun, a shorter opening to
the surface is needed for conveying the mud from the nest, and then the
perpendicular opening is made. Mud from this, and also from the first
part of the perpendicular burrow, is carried out of the diagonal opening
and deposited on the edge. If a freshet occurs before this rim of mud
has had a chance to harden, it is washed away, and no mound is formed
over the oblique burrow.
After the vertical opening is made, as the hole is bored deeper, mud is
deposited on the edge, and the deeper it is dug the higher the mound. I
do not think that the chimney is a necessary part of the nest, but
simply the result of digging. I carried away several mounds, and in a
week revisited the place, and no attempt had been made to replace them;
but in one case, where I had in addition partly destroyed the burrow by
dropping mud into it, there was a simple half rim of mud around the
edge, showing that the crayfish had been at work; and as the mud was dry
the clearing must have been done soon after my departure. That the
crayfish retreats as the water in the ground falls lower and lower is
proved by the fact that at various intervals there are bottled-shaped
cavities marking the end of the burrow at an earlier period. A few of
those mounds farthest from the stream had their mouths closed by a
pellet of mud. It is said that all are closed during the summer months.
How these animals can live for months in the muddy, impure water is to
me a puzzle. They are very sluggish, possessing none of the quick
motions of their allied _C. bartonii,_ for when taken out and placed
either in water or on the ground, they move very slowly. The power of
throwing off their claws when these are grasped is often exercised.
About the middle of May the eggs hatch, and for a time the young cling
to the mother, but I am unable to state how long they remain thus. After
hatching they must grow rapidly, and soon the burrow will be too small
for them to live in, and they must migrate. It would be interesting to
know more about the habits of this peculiar species, about which so
little has been written. An interesting point to settle would be how and
where it gets its food. The burrow contains none, either animal or
vegetable. Food must be procured at night, or when the sun is not
shining brightly. In the spring and fall the green stalks of meadow
grasses would furnish food, but when these become parched and dry they
must either dig after and eat the roots, or search in the stream. I feel
satisfied that they do not tunnel among the roots, for if they did so
these burrows would be frequently met with. Little has as yet been
published upon this subject, and that little covers only two spring
months--April and May--and it would be interesting if those who have an
opportunity to watch the species during other seasons, or who have
observed them at any season of the year, would make known their results.