SPIDERS, MITES, AND MYRIAPODS
In this chapter we shall take examples from two Classes--the
Arachnoi´dea and the Myriap´oda. To the first-named Class belong also
the Scorpions, which, with the Book Scorpions, the Harvestmen, and some
others, may be neglected here. This will leave us only the Spiders and
Mites to deal with.
Every one knows a Spider when he sees one, though not every one can
give offhand a definition that shall include the whole Order. Let us
endeavour to express their characters in simple terms, keeping in mind
the definition of head, thorax, and abdomen in Chapter II. In Spiders
the head and thorax are joined together in one unsegmented portion,
called the cephalothorax, and this is connected with the abdomen, which
is also unsegmented, by a more or less slender stalk or peduncle. So
that while Insects have three regions, Spiders have but two. We may
express the difference thus:
Insects head, thorax abdomen.
\---v---/[image]
Spider cephalothorax abdomen.
The cephalothorax bears six pairs of appendages (Fig. 48A).
Taking these in order, there are--
1. A pair of falces (_an_), representing the antennae of
insects. These have a movable claw-like joint at the extremity,
perforated so as to convey into a wound the secretion from the
poison-glands.
2. A pair of five-jointed pedipalps (_p_), sometimes called
maxillary palpi, from the fact that the parts whence they spring
correspond to the maxillae of insects (Fig. 21A,
_d_). In the female the last joint terminates in a claw; in
the male this joint is specially modified for sexual purposes
(Fig. 48B).
3. Four pairs of walking legs, of which only the coxae
(_c_) are shown in the diagram. The two front legs are
often used as organs of touch.
Breathing is carried on by chambered air-tubes or lung-sacs, and in all
our British Spiders (with the single exception of the native Trap-door
Spider, which, by the way, does not make a trap-door) there are also
air-tubes resembling those of Insects. The lung-sacs open to the
external air by stigmata (_st_) on the abdomen, while the ordinary
air-tubes open near the spinnerets (_sp_), the organs employed in
the production of the silky threads from which are formed webs, nests,
egg-bags, and ropes. By means of these threads, spiders drop from their
webs to the ground, construct flying bridges from one point to another,
and even in some sort travel through the air.
Spiders live upon the juices of their prey, which are drawn up into the
stomach by means of a suctorial apparatus at the end of the gullet.
The young, when they leave the egg, resemble their parents in all
respects but size--that is, they undergo no metamorphosis.
The eyes of spiders are simple, and six or eight in number. These vary
much in size and relative position, and furnish characters of great
importance in classifying species. Those of the Hunting Spiders, which
make no web, but pursue or spring upon their prey, are usually arranged
in three rows; while those which make webs for the capture of prey have
the eyes in a double row. In all cases they are on the front part of
the upper surface of the cephalothorax.
[Illustration:
FIG. 48.--_A._ Scheme of under surface of Wolf
Spider (female). _B._ Pedipalp of male (enlarged). For
other references, see text.]
The Common Garden Spider (_Epei´ra diade´ma_) is a very good
subject, and there can be no difficulty in procuring any number of
specimens. The smallest garden will be sure to yield a plentiful
supply; and even if we have no garden, a very superficial search
among the hedgerows will give us as many as we can possibly want.
Every one knows this spider, and the beautifully regular web which
it makes. First of all, what one may call the outline of the web
is spun--strong threads stretching from point to point, to which
are attached lines radiating from a common centre. This may be
represented diagrammatically by drawing a circle and producing radii
from the centre to the circumference in all directions, or by making a
rectangular figure and drawing lines to the boundaries from the point
where the diagonals intersect. It must not, however, be supposed that
the outline of the web is of a regular form. In this respect the spider
adapts itself to circumstances, and spins a greater or less number of
supporting threads, as may be necessary.
[Illustration: FIG. 49.--Garden Spider and Web.]
Then the spiral is made (Fig. 49) from the centre to the circumference.
The first few turns are of the same character as the radial lines; but
all the rest of the short lines forming the spiral, and connecting
the radial lines, are coated with a viscid secretion, which is the
essential part of the snare; for the victims are really limed like
birds on a twig, not entangled in the threads. It is well to test the
character of the different parts of the spiral, not only by touch,
which is convincing enough, but with pocket lens. Our low powers will
not give such results as are shown in Fig. 50, but we shall have no
difficulty in distinguishing the sticky parts from those not coated
with the secretion.
[Illustration: FIG. 50.--_A_ Adhesive Threads of
Spiral. _B._ Non-adhesive Radial Threads.]
It may be doubted whether all the Spiders of this genus make the
spiral in the same way, for observers differ in their description of
what they have seen. Some say that a non-adhesive spiral from centre
to circumference is first made, and that the spider then moves ‘in a
closer spiral from the circumference inwards, biting away the former
spiral, replacing it by another, which is viscid and adhesive[29].’ Dr.
Butler, on the other hand, who ‘watched spiders for months together,
petting, feeding, and trying experiments with them every morning,’
after describing the making of the first and non-adhesive part of the
spiral, says, ‘This line is not carried to the boundary, but at some
distance from the centre a second is commenced, formed of extremely
viscid silk, upon which the gummy secretion is distinctly visible,
with the aid of a lens, in the form of closely approximated globules
of amber-coloured glue. It is said that when the viscid lines are
completed the spider cuts away the unadhesive lines, but this I have
never observed[30].’ My own observations lead me to believe that Dr.
Butler is correct in his description. Though I have often watched,
I have never seen a spider ‘biting’ away any part of its web, nor
would the falces appear to be adapted for such an operation. They are
piercing, not cutting, weapons.
The spider may be watched at leisure, if put into a bottle of moderate
size, the top of which should be covered with muslin or calico to
prevent escape. Here it is fairly easy to bring the pocket lens into
play, and to distinguish the different parts of the animal. The eyes,
and their arrangement, should be particularly noticed.
[Illustration: FIG. 51.--Anchorage of Web.]
Blackwall, in support of the position that in making their webs
spiders are guided by touch rather than sight, says, ‘Various species,
when confined in spacious glass jars placed in situations absolutely
impervious to light, construct nets which do not exhibit the slightest
irregularity of plan or defect of structure[31].’ My specimens have
always been kept in the light, and in small bottles rather than
spacious jars, but I have never seen spiders attempt to make a
geometrical web under such conditions.
A bottle which for some months served as a prison-house for a Garden
Spider now stands on my writing-table. Its sides are marked by hundreds
of ‘anchorages’--but the threads cross and recross, forming in some
parts a kind of sheet, and in others a tangled mass. Some of these
threads must have been covered with viscid secretion, for flies were
limed, and so fell a prey to the spider. Their dried skins are dotted
about among the threads, and the spider itself perished long ago from
cold. But I keep the bottle as a curiosity, to show that these spiders
do not always make geometrical webs.
When one has a Garden Spider in a bottle, it may be observed to
practise a curious and very effective method of disabling its prey.
If a bluebottle or any other large fly be dropped and entangled among
the threads, the spider will envelop it in a sheet of web. This is
how Blackwall describes the operation: ‘Causing the victim to rotate
by the action of the third pair of legs and the palps, the first pair
of legs also being frequently employed in a similar manner, they
extend the spinners laterally, and applying to them alternately the
_sustentaculum_ of each posterior leg, they seize and draw out
numerous fine lines in the form of a fillet, which they attach to their
revolving prey, and thus involve it in a dense covering of silk from
one extremity to the other. By means of this stratagem they are capable
of overcoming formidable and powerful insects, such as wasps, bees, and
even large beetles[32].’
The operation does not occupy much time; in a very few minutes the fly
is swathed in a silky covering as completely as an Egyptian mummy in
its linen folds. Of course resistance, much less attack, is out of the
question, and when it is thus rendered powerless for harm the spider
proceeds with its meal.
The _sustentac´ulum_--or support--is a strong movable spine near
the end of the tarsus, on the under side of each of the last pair of
legs. These spines act in opposition to the claws, and thus these
animals are enabled to hold with a firm grasp such lines as they have
occasion to draw from their spinners with the feet of the hind legs,
and such also as they design to attach themselves to.
With regard to this method of swathing prey, Hudson[33] says of
an Argentine spider, ‘that its intelligence has supplemented this
instinctive procedure with a very curious and unique habit. The
Pholcus, in spite of its size, is a weak creature, possessing
little venom to dispatch its prey with, so that it makes a long and
laborious task of killing a fly. A fly, when caught in a web, is a
noisy creature, and it thus happens that when the Daddy longlegs--as
Anglo-Argentines have dubbed this species--succeeds in snaring a
captive, the shrill outrageous cries of the victim are heard for a
long time--often for ten or twelve minutes. This noise greatly excites
other spiders in the vicinity, and presently they are seen quitting
their webs and hurrying to the scene of conflict. Sometimes the captor
is driven off, and then the strongest or most daring spider carries
away the fly. But where a large colony are allowed to continue for a
long time in undisturbed possession of a ceiling, when one has caught
a fly he proceeds rapidly to throw a covering of web over it, then,
cutting it away, drops it down and lets it hang suspended by a line at
a distance of two or three feet from the ceiling. The other spiders
arrive on the scene, and after a short investigation retreat to their
own webs, and when the coast is clear our spider proceeds to draw up
the captive fly, which is by this time exhausted with its struggles.’
In this connexion Hudson notes that spiders are attracted by the sound
of the vibration of a string or wire, thinking it made by an insect
that has been caught; and he says that the stories of tame spiders
are founded on a misunderstanding of the motive that brings the
animal down. We may well doubt if spiders are attracted by music, but
that some spiders possess a sense of hearing, or something analogous
thereto, seems to be proved by the existence of stridulating organs in
certain members of the group.
[Illustration: FIG. 52.--Foot of Garden Spider.]
[Illustration: FIG. 53.--Spinnerets of Garden Spider.]
Two other points remain to be noted. The feet should be examined,
for their structure throws some light on the way in which the Garden
Spider runs so securely to and fro on the radial lines. One of the
legs of a dead specimen should be detached, and its armature of spines
and hairs noted. The foot is armed with three stout claws, which
are pectinated--that is, toothed like a comb, and beneath them are
smaller ones, sometimes described as toothed hairs. It will be easy
to understand what a secure foothold these claws give the animal as it
runs backwards and forwards over the radial lines; for if the web be
shaken by the wind or designedly its owner can take a firm hold with
each foot, and thus have eight separate points of attachment. An inch
hand lens is quite sufficient to give a general idea of the hairy and
spinous clothing of the legs and the position of the claws; but to see
the teeth a higher power--a half-inch--will be necessary.
It is a good plan to choose a freshly killed specimen for the
examination of its spinnerets. If the spider is laid on its back in a
glass dish, gentle pressure on the abdomen away from the cephalothorax
will cause the material secreted by the spinning-glands to ooze out.
This, however, cannot be done if the specimen has been for some time
in spirit. We shall be able to make out six tubes (four of them larger
than the other two) grouped round the anal aperture; but, for the
present, we must take for granted the fact that these are made up of
a number of smaller tubes, so that the end of each spinneret is not
unlike the rose of a watering-can. A good half-inch will give some
indications of this rose-like appearance.
The Jumping Spider (_Sal´ticus sce´nicus_) belongs to a family
the members of which make no web or snare for the capture of prey, but
trust for their food-supply to their skill in stalking flies and other
insects, which they capture by a sudden spring. It is from this habit
that the type-genus and the family have received their scientific and
popular names.
The common British species is extremely abundant, and wherever flies
are plentiful these spiders will not be very far off. This is the case
not only in the country, but also in London and other large towns. It
is a noticeable spider from its coloration--black marked with white.
The eyes are eight in number; though the artist in our illustration
has only represented four. The centre two of the front are the largest
of all, and have been compared not inaptly to bull’s eyes. Two others
are placed on each side of the cephalothorax, so that the eyes form,
roughly, three sides of a square, and the central eyes in the lateral
lines are the smallest. We may represent them thus [image], while the
eyes of the Garden Spider are disposed in this fashion [image].
[Illustration: FIG. 54.--Jumping Spider. (Line shows
natural size.)]
[Illustration: FIG. 55.--Falces of Male Jumping Spider.]
It requires some little practice to detect the eyes of spiders and to
remember their position in the different genera, but by examining every
spider met with, and consulting some handbook to the group, one will
soon be able to determine the commoner British species.
With a couple of needles we may remove the falces (Fig. 55) for
examination; and there is no difficulty in mounting them, as shown in
the cut, on a piece of card, so that they may be compared with the same
weapons in other species. For example, the falces of the Garden Spider
differ from those of the Jumping Spider in that they are short and
stout, and the teeth on the basal joint are more in number.
The following account of the habit of this spider in capturing prey is
from Kirby and Spence’s _Introduction to Entomology_ (ed. 1870, p.
453):--‘When these insects spy a small gnat or fly upon a wall, they
creep very gently towards it with short steps, till they come within a
convenient distance, when they spring upon it suddenly like a tiger.
Bartram observed one of these spiders that jumped two feet upon a
humble-bee. The most amusing account, however, of the motions of these
animals is given by the celebrated Evelyn in his _Travels_. When
at Rome, he often observed a spider of this kind hunting the flies
which alighted upon a rail on which was its station. It kept crawling
under the rail till it arrived at the part opposite to the fly, when
stealing up it would attempt to leap upon it. If it discovered that
it was not perfectly opposite, it would immediately slide down again
unobserved, and at the next attempt would come directly upon the
fly’s back. Did the fly happen not to be within a leap, it would move
towards it so softly, that its motion seemed not more perceptible
than that of the shadow of the gnomon of a dial. If the intended prey
moved, the spider would keep pace with it as exactly as if they were
actuated by one spirit, moving backwards, forwards, or on each side
without turning. When the fly took wing, and pitched itself behind the
huntress, she turned round with the swiftness of thought, and always
kept her head towards it, though to all appearance as immovable as one
of the nails driven into the wood on which was her station: till at
last, being arrived within due distance, swift as lightning she made
the fatal leap and secured her prey.’
This spider employs a clever precaution against falling, when about to
spring upon its prey. It applies the end of the abdomen to the surface
on which it stands, and opening its spinnerets, makes an ‘anchorage’
(Fig. 56). Then, by the act of springing, it draws from the spinnerets
a line attached to the spot from which it started. This line is strong
enough to support the spider in case it misses its prey.
[Illustration:
FIG. 56.--_A._ Foot or Jumping Spider. _B._
Scopula. (Much enlarged.)]
The foot of the Jumping Spider is extremely interesting, and shows
a very ingenious arrangement, by means of which the animal can run
with difficulty on smooth polished upright surfaces, or retain its
footing when it alights on such surfaces after a spring. We can see
this arrangement in Fig. 56. Just behind the claws is a bundle of
coarse hairs, technically called a _scop´ula_, or little brush.
From these hairs adhesive matter flows, and in this fashion the spider
literally sticks on. With this brush of hairs may be compared the
tarsal cushions of many flies, and the adhesive hairs of Dytiscus
and other beetles (see Chap. II)--though these last have a different
function.
Diving Spiders (_Argyrone´ta aquat´ica_) are especially suitable
for our purpose. They are very common in most ponds, and in some
places are so abundant as to be almost a nuisance to the collector who
is in search of other things. Three of them are represented in Fig. 57.
One is swimming; another is just entering the bell-shaped web under
water; and the third is just climbing out of the water on to the broad
floating leaves of the water crowfoot.
[Illustration: FIG. 57.--Diving Spiders.]
De Geer’s account of these spiders is extremely interesting[34], and
we may verify it for ourselves, for these creatures may be kept without
any trouble. They certainly agree, when a number are kept in captivity,
much better than do other spiders. De Geer kept several in the same
aquarium, and says that when they met they felt each other with their
palps, and opened their falces, but he never saw them do any harm to
each other. I have kept them under similar conditions with the same
result.
He tells us that these spiders spin under water a cell of strong,
close, white silk, the shape of which he compares to a skull-cap, the
half of a pigeon’s egg, or a diving-bell. In September, 1736, he first
became acquainted with these creatures, and kept one in an aquarium for
four months. It made its cell against the side of the aquarium, and the
top of it rose above the surface of the water. (This was due to its
being inflated. The web was not spun above the surface.) The walls of
the cell were very thin, but it was filled with air, and the spider was
resting inside, head uppermost, with its legs pressed against the body.
About three months later he found that the mouth of the cell was
closed, and the spider was comfortably settled in its winter quarters.
When pressure was applied the cell burst and the air escaped, mounting
up to the top in bubbles. The spider made its way out, and took an
Asellus that was offered, and made a meal of it.
De Geer then came to the conclusion that these cells under water were
constructed for the purpose of affording the spider a retreat when the
water was frozen over, so that they could not come to the surface
for a supply of air. He found, however, by observation, that they
were also made in summer by both sexes. In a cell of this kind the
female deposits her eggs--from eighty to a hundred in number, enclosed
in a cocoon of white silk--and keeps guard over them, with her head
defending the entrance to the cell.
He succeeded in finding out the method by which the Diving Spider fills
its cell with air. He noticed that when the creature was moving about
in the water, its body was covered with a layer of air, and that this
air was renewed from time to time when the animal came to the surface
and raised its abdomen above the water. Loaded, so to speak, with air
in this fashion, the spider descended, and entered the cell backwards,
leaving an air-bubble. Having repeated this several times, at last she
removed all the water from the cell, introducing in its place an equal
quantity of air.
It is very easy to watch the Diving Spider making its dwelling under
water, and filling it with air. First of all the web is woven in a
bell or thimble shape between the sprays of water-weed, or against the
bottom or side of the aquarium. It is curious to notice how practices
that must be necessary when the creature is at liberty are continued
in captivity where they are useless. A web constructed in running
water, or even in a pond or ditch, is liable to be swept away or to be
emptied of air by a very slight current, so its owner has recourse to
a system of guys and supporting threads, which are not required when
the spider is safely housed in a small aquarium. Nevertheless, the guys
are made. In an eight-ounce bottle I have now a male Diving Spider,
which has lived there for about seven months. Its cell is made between
the whorls of a spray of milfoil, and guy threads have been carried to
no less than five whorls--two above and three below the opening. Now
that it is filled with air, the cell gleams in the water like a great
bubble of quicksilver. The air may be expelled by shaking or tilting
the bottle, and if the web be not damaged the spider will generally
refill it with air, though sometimes it prefers to make a new dwelling.
Fig. 58 shows the cell of a Diving Spider; the white lines represent
supporting threads attached to the water plants.
In examining dead specimens we shall find that, contrary to what is
usual, the male exceeds the female in size. I have a slide of a male,
with the legs spread out before and behind, and the measurement from
the claws of the first pair of legs to those of the fourth pair is 1¾
inches. The body is ¾ inch long. The whole surface is more thickly
clothed with hair than is the case with other spiders, and the reason
for this is obvious. This hairy body-covering serves to carry down into
the water a layer of air, and the fringe of hair on the legs makes them
efficient swimming organs.
[Illustration: FIG. 58.--Cell of Diving Spider.]
The Order of Mites will yield us subjects for our pocket lens. Mites
are related to Spiders, but form a distinct Order. Like the Spiders,
some are aquatic, though the most of them live on land. Many are
parasitic, during the whole or part of their lives subsisting on
the juices of their hosts: the food of others consists of organic
_débris_, and these seem to be of benefit to man, since they act
as scavengers. If we turn to page 96 we shall there find noted the
points of difference in the arrangement of the main divisions of the
body in Insects and in Spiders. In Mites the distinction between the
cephalothorax and abdomen is lost, and the body is more or less oval or
globular. In the perfect forms there are eight legs; but some, in their
earlier stages, have only six. The mouth may be adapted for biting,
though it is usually suctorial. In the Cheese Mites and some others
breathing seems to be carried on through the skin, for there are no
air-tubes; but in most Mites air-tubes, with two stigmata, are present.
If we take a dip with the collecting-net in almost any pond we shall
be pretty sure to capture some specimens of Water Mites of the genus
Hydrach´na, easily recognizable by their bright coloration, their eight
swimming legs thickly fringed with hair, and their unceasing activity.
They may be kept in a bottle, or other small vessel, with some
water-weed, and will forage for themselves. In Fig. 59 we shall see
the points we have to look for in examining a Water Mite with a pocket
lens. There should be no difficulty in making out in the specimen all
the details shown in the cut.
It may be that they will breed: if so we should avail ourselves of
the opportunity of watching their development. Their life-history is
somewhat curious, and is specially interesting from the fact that
while Swammerdam had some faint perception of the true meaning of what
he saw, De Geer, writing a hundred years later, drew entirely wrong
conclusions from similar observations. It was left for Dugès to clear
up the matter in the _Annales des Sciences Naturelles_, 1834.
Before summarizing the account of the French naturalist it may be well
to quote what Swammerdam and De Geer have said on the subject:--
‘There is nothing more remarkable in this insect [the Water Scorpion]
than that it constantly appears covered with a prodigious number of
nits of different sorts and sizes, though perhaps we may with more
reason consider them as so many little creatures, which live and grow
by sucking the Scorpion’s blood. These are somewhat of an oblong
figure, approaching to round, and have a shining, and as it were
bloated, surface, without any of the rings observable in most insects.
The neck is oblong and shaped like a pear, with the small end sticking
in the Scorpion’s body. The colour of this insect is a mean between
that of vermilion and purple; and when it is pretty well grown there
appears within it an elegant transparent spot or particle (Fig. 59).
[Illustration: FIG. 59.--Red Water Mite (nat. size, and
under surface magnified).]
‘This spot or particle induced me to consider with more attention
this minute and hitherto unregarded insect, and even to undertake the
dissection of it. But who would imagine that on this examination it
should prove a perfect and surprising insect? This is, however, a
certain fact; and thus in that infinite variety of works, by means of
which God is pleased to make Himself known to us, we ever meet with new
matter of admiration and astonishment.
‘This little creature being extracted from the shell that covered it,
looks like a young spider before it has left its egg. On the fore part
is the head (Fig. 43D, _a_) and on its head are the eyes, _b_: under
the eyes are placed its little legs elegantly coiled and folded, _c_
_c_; but they appear much more distinctly on turning the insect on its
back, _d_ _d_; and in this situation also it best appears with what art
these legs are laid up in the shell, and all are covered with hair.
The colour of this little creature is, as I have already observed, a
mean between that of vermilion and purple; and this colour shows itself
through the coat or shell, which is transparent. I cannot determine to
what species of insects this is to be referred; nor can I say to what
size it grows, or by what kind of creature it is thus deposited on the
Water Scorpion in the form of an egg, there to receive life and growth.
Nevertheless, I cannot but look on the discovery I have made as very
interesting, since it proves that there are in the nature of things
eggs which acquire a sensible growth by an entraneous nourishment,
unless perhaps some naturalist should choose to consider this as a
complete insect, rather than as an egg; nor shall I strenuously oppose
his opinion, seeing that, in all cases, the egg is in reality no
other than the insect itself, which remains in that state till it has
acquired sufficient strength to break its prison, and live without such
a covering[35].’
Having quoted Swammerdam, let us see what De Geer has to say on the
subject:--
‘On the body and legs of many aquatic insects, such as Dytiscus and
Water Scorpions, may be frequently seen little oval, seed-like bodies,
of a bright red colour, firmly attached, and, as it were, implanted
in the skin, by a little stalk. I have had Water Scorpions with the
upper surface so covered with these red bodies that there was scarcely
a vacant space on the skin. They are most frequently to be seen in
the spring; but the insects on which there was such a great number
did not live long with me. Having crushed some of these seed-like
bodies, I found them filled with red liquid matter. I am convinced, by
experiment, that they are the eggs of Water Mites, since there came out
of them little red Mites with round bodies and long legs, which swam
about with great swiftness.
‘These red Water Mites, then, attach their eggs to the bodies and legs
of larger aquatic insects, and there they remain till the young are
hatched. And since we find eggs of many different sizes, we may be
sure that they grow and increase in size, doubtless owing to a certain
nutritive juice which passes from the body of the insect into the egg.
Hence it is, as I have seen myself, that Water Scorpions loaded with
these eggs become weak and feeble, because they are obliged to furnish
their hangers-on with nourishment from their own bodies. Moreover,
these eggs appear to cause the Water Scorpions some irritation or
uneasiness, since I have often seen them rub with their feet those
parts of the body where the eggs were; and perhaps they did this with
the view of rubbing them off, but they were too securely fastened[36].’
Dugès watched the development of the common Red Water Mite (Fig. 59),
and tells us that towards the end of May the females deposit their eggs
in the leaves of pondweed, which they puncture with their beaks. The
larva (Fig. 60), red in colour, with six legs, is free-swimming, and
has a large beak, which looks like a great head, and terminates in a
narrow mouth. It is not known how long this larval stage continues;
but in the next stage (Fig. 61) the Mite becomes parasitic on aquatic
beetles and bugs, fastening its beak into the body of its host,
from which it derives its nourishment. The legs and palps are often
retracted or absorbed, so that it is not difficult to understand
how it was Swammerdam and De Geer took these parasitic nymphs to
be the eggs of the Mite. During their parasitic condition they
increase considerably in size, at last emerging as adult eight-legged
free-swimming Mites. It was just before the emergence of the Mite
that Swammerdam examined the parasitic nymph, for he figures the
‘insect,’ which he extracted from the egg, as having eight legs (Fig.
43D).
[Illustration: FIG. 60.--Larva of Water Mite.]
[Illustration: FIG. 61.--Nymph of Water Mite.]
I once found a Water Mite in the body of a Dytiscus[37]. I was
breaking up the beetle, and had removed the elytra and the wings. I
only wanted the external skeleton; so a slit was made between the
plates of the dorsal and ventral surface, and the intestines removed.
The Mite was embedded in the fat-body. I could find no mention in the
literature of Beetles or Mites of any similar occurrence; and should
scarcely have mentioned it here, had I not been unexpectedly confirmed
by my friend Mr. G. E. Mainland, F.R.M.S., who once had a similar
experience, and who kindly allows me to quote from a letter he sent me
on the subject:--
‘I am sorry to say I can find no documentary evidence as to the
Arachnid I found embedded in the tissues of Dytiscus, but a good deal
has come back to my recollection. On removing the right elytron and
slitting up the dorsal surface, I found it in the tissue close up to
the thorax. I cannot recollect what ultimately became of it, after I
had shown it to friends at the Hackney Microscopical Society.... I know
that I carefully measured the Hydrachna (which was abnormally large)
and its relative size to that of its host, and made a comparison in a
lecture (to the Senior Band of Hope at St. Michael’s, Hackney) of a man
with a creature as large as a guinea-pig under his shoulder-blade.’
The occurrence of the Mite _inside_ the Beetle was, of course,
quite exceptional. It probably found its way in through one of the
abdominal spiracles.
The Beetle Mite (_Gam´asus coleoptrato´rum_) (Fig. 62) is
extremely common, and is parasitic on the Dung Beetle and on the Humble
Bee, so that in order to examine the parasite we must capture the host.
There can be no difficulty about this, for Dung Beetles and Humble Bees
are plentiful enough. This Beetle Mite, apparently, does not infest
other species of beetles. I have kept the Devil’s Coach Horse in a
bottle with the common Dung Beetle for some months, and though the
latter swarmed with these parasites, they never left their host for
the other beetle. Even when removed by means of a small brush from one
beetle to the other, they left the Devil’s Coach Horse of their own
accord, and soon made their way back to the Dung Beetle.
[Illustration: FIG. 62.--Beetle Mite.]
These parasites, with their host, came into my possession in a strange
way. A friend, who knew my hobbies, told me that he had managed to
procure for me some young beetles just born. I ventured to suggest that
beetles were not born as beetles, but in quite a different shape. My
suggestion was received unsympathetically, and I was told that I should
alter my opinion when the creatures were sent me. But I did not. The
box contained a Dung Beetle, over which were swarming scores of these
little Mites, and I had some difficulty in convincing the gentleman
who sent them to me that these Mites were not the young of the beetle.
We should compare this Beetle Mite with the Water Mite, and notice the
difference in the mouth parts and the legs, which have a large pad
between the claws.
We may find another Beetle Mite, closely allied to this species, on the
Devil’s Coach Horse, and some of its near relations. This Mite was also
known to De Geer[38], whose remarks upon it are worth quoting, in a
condensed form.
He found a beetle covered with these Mites, and on examining them with
a hand lens saw that they were attached to their host by a long thread
or stalk, which came from the posterior end of the body. Several Mites,
he tells us, were joined together by one thread which fastened them all
to the beetle; and he came to the conclusion that the parasites were
nourished at the expense of the beetle, the thread serving to convey
the juices of its body to them.
‘It is very singular,’ he says, ‘to see living insects planted on the
body of larger insects, from which they draw their subsistence by means
of a thread or stalk.’ And then he goes on to compare these ‘vegetative
Mites,’ as he calls them, with the ‘eggs’ of the Water Mites, which
he found on Dytiscus and the Water Scorpion. The thread exists, and
the Mites are attached by it to their host, but they do not draw
nourishment through it from the beetle, for it is composed of their
excrements.
The Myriapods are worm-shaped creatures, breathing by means of
air-tubes, and furnished with a number of limbs closely resembling
each other. There are two groups: the Centipedes and the Millepedes.
The former have the body flattened, with one pair of appendages to each
segment, the first pair being modified into piercing poison-organs, and
they feed on living prey. The body of the Millepedes is round, with two
pairs of appendages to each segment; they have no poison-organs, and
their food consists chiefly, if not entirely, of vegetable matter.
There seems to be some doubt, however, as to whether _Ju´lus_, one
of the commonest Millepedes, does not occasionally indulge in animal
food. In _Nature Notes_ (Jan. 1896) there was a review of the
_Cambridge Natural History_ (vol. v). The reviewer, in a brief
summary of Mr. Sinclair’s part of the book (the Myriapods), said,
after describing the Centipedes: ‘The millepedes, on the contrary, are
sluggish vegetarians, with hard, cylindrical bodies, &c.’ On this a
correspondent wrote in the March number: ‘Some time ago my attention
was attracted to a large earthworm, writhing and twisting about on
the garden path, as though in pain, or through having received some
injury. On examining it more closely to ascertain the cause of its
unusual movements, I found that a millepede had fastened itself to the
side of the worm, and appeared to be boring or eating its way into
the body, whilst the most violent efforts on the part of the worm
were ineffectual in shaking off its antagonist. If the millepede is
a vegetarian, what could be its object in attacking so harmless and
defenceless a creature as the earthworm? The above, which I take to
be a millepede, is the black or dark-coloured creature “with hard,
cylindrical body” ordinarily found coiled up in a spiral under stones
or rubbish.’
The editor, as a matter of course, referred the matter to the writer
of the review. His reply was as follows: ‘If there is no mistake
about the identity of the aggressor in the account cited above, the
observation is one of considerable interest; for, so far as we are
aware, it is the only case on record of a millepede being guilty
of such conduct. But were it not for the positive statement that
the species was the dark-coloured creature with a hard, cylindrical
body, which is ordinarily found coiled up in a spiral under stones
or rubbish--a description which exactly applies to the millepedes of
the genus _Julus_--we should have concluded without hesitation
that the struggle in question was merely one of those that habitually
takes place between the centipedes of the genera _Litho´bius_ or
_Geoph´ilus_ and the earthworms upon which they feed.’
Both the Centipedes and Millepedes are shy, light-shunning animals,
and if we turn over some stones in the garden or in a walk through the
fields we shall probably find specimens enough to serve us in getting
some idea of both groups.
The Centipedes are sometimes called ‘Hundred-legs,’ but this implies
the possession of many more legs than the creatures really have. In
Norfolk and Suffolk the people call them ‘Forty-legs,’ and this is much
nearer the mark.
_Litho´bius forfica´tus_, about an inch long and rufous brown in
colour, is extremely abundant under stones and the bark of trees, and
in cellars and outhouses. These animals run with great rapidity when
disturbed, so that one needs to be on the alert to seize them when
they are driven from the places in which they lurk. The body has nine
principal and six subsidiary or smaller rings, and there are fifteen
pairs of walking legs, besides the first pair, which are modified to
serve as poison-organs. De Geer says that he never dared to pick up
these Centipedes with ungloved hands, because he had seen a fly, which
had been bitten by one, die on the spot, ‘which seems to be a sign that
their bite is venomous.’ He examined their modified legs with a good
microscope, but could not distinguish any opening. There is, however,
an opening, as De Geer suspected, though he could not distinguish it;
it lies near the point, and we may also trace the canal through the
claw down to the poison-glands which lie, one on each side, at the base
of the claws. The mouth parts resemble those of insects, and may be
dissected out in the same way. When this Centipede walks, says De Geer,
it does not use the last four pairs of legs, but drags them after it;
but when it walks backwards, which it does as well as forwards, it then
makes use of these four pairs of legs in the same way as the others. If
we keep Lithobius alive we shall see that it can walk backwards, though
it can scarcely be said to go as well one way as the other. From the
same old writer some useful hints as to the method of keeping these
animals may be gathered. Those that he kept in a vessel without any
moisture soon died, and were quite dried up in twenty-four hours, which
will teach us to keep them in a vessel with damp earth, shaded from
light and heat.
Dr. Sharp[39] gives some interesting details about the breeding habits
of Lithobius, and describes, for the first time, the uses of the two
hooks on the under surface of the body of the female.
He experimented with Centipedes and Millepedes. Keeping them in large
shallow glass vessels, the bottom of which was covered with a layer
of earth, he fed the specimens of Lithobius on insects and worms, and
sometimes on raw chopped meat, but they did not thrive on this as they
did on prey which they could kill for themselves.
[Illustration: FIG. 63.--_A._ _Lithobius
forficatus._ _B._ Mouth parts seen from below (After
Graber.)]
Lithobius, he tells us, lays but few eggs compared with the number
deposited by Julus. Each egg, as it leaves the oviduct, is received
by the hooks mentioned above, and by means of them it is rolled on
the soil till a covering of earth adheres to the viscous material
with which the egg is coated. The male considers the eggs special
delicacies, and devours them whenever he has the opportunity. It is to
prevent this that the female covers them with earth, so that the male
may not recognize them.
Geophilus is a much longer animal than Lithobius, for its body may
consist of from 80 to 180 rings. The species have no eyes. Several of
them are common, especially in the south of England, and possess, as
do many genera of the same family, the property of phosphorescence,
whence have arisen the stories of ‘luminous earthworms’ current from
time to time. _Geophilus crassipes_ is the form most frequently
captured when displaying its light. It is from one inch to two inches
long, of a reddish-orange colour, and somewhat worm-like in shape. Mr.
Pocock, of the British Museum (Natural History), says: ‘The property
of luminosity lies in an adhesive fluid secreted by glands which open
upon the lower surface of the body, and the power of discharging or
retaining the fluid appears to be entirely under the centipede’s
control. The phenomenon is observable during the autumn months, from
about the middle of September to the end of November, and although its
significance is not clearly understood, it is generally believed to be
connected with the pairing of the sexes[40].’
During a visit to the seaside it is well to look out for specimens of
a marine Centipede, which is, apparently, not very common. It is said
to occur ‘under stones and sea-weeds on the shore at or near Plymouth’;
and in 1895 I had the good fortune to meet with one at Bexhill.
This Centipede does not live in the sea, but will survive prolonged
immersion in salt water. It is far too valuable for us to pick to
pieces, so that, if we should have the good fortune to meet with one,
we should carefully examine it, making what notes are necessary, and
then pickle the specimen and send it to the British Museum, Cromwell
Road, S.W. The tube should be labelled with the place and date of
capture, and it should be stated on the label whether the animal was
taken above or below high-water mark.
The Common Millepede--often miscalled the wireworm--is readily
distinguished by the absence of poison-claws, and its cylindrical
worm-like appearance. De Geer, who of course adopted the Linnaean
definition of ‘Insects,’ says of the Millepedes of the genus Julus, to
which our Common Millepede (_Ju´lus terres´tris_) belongs: ‘They
form, as it were, the last link of the chain which unites the class of
Insects to that of Worms, for the body is elongated and cylindrical;
and though they have a great number of feet, these are so short, that
when these animals walk, they seem rather to glide along after the
fashion of legless worms[41].’
[Illustration: FIG. 64.--The Common Millepede.]
Dr. Sharp says that these animals do very well in confinement, and he
found that sliced apples and grass formed the best food for them. He
watched the process of nest-making and egg-laying, and these creatures
are so abundant, and the necessary appliances so simple, that we may
follow his example and see it for ourselves. His arrangements were the
same as for Lithobius, and he saw the female make a hollow sphere of a
bit of earth, stuck together by the secretion from the salivary glands,
and smooth on the inside. A small hole was left on the top, and through
this she passed in from 60 to 100 eggs, closing the aperture with earth
moistened with the salivary secretion. The eggs were hatched in about
twelve days. The young of all the Myriapods when they leave the egg
have but three pairs of legs, but the number of limbs and segments is
increased at each successive moult.
Having watched our Millepedes in confinement, it will be well to take a
preserved specimen and examine it carefully with the pocket lens, so as
to compare it with the Centipede; then to compare both with the common
earthworm, and to note the points of likeness and of difference. The
dark spots on each segment in the illustration show the stigmata.
[Illustration: FIG. 65.--Segments of Millepede
(magnified).]