ARTICULATA (ARTIC´ULUS, A JOINT).
The animals belonging to this subkingdom are specially distinguished by
the body and limbs being jointed: as familiar instances, may be
mentioned the lobster, the wood-louse, spiders, insects, and worms.
Taking the class Crustacea, to which the two first animals belong, we
find interesting microscopic forms in the subclass Entomos´traca
(ἔντομον, insect; ὄστρακον, shell).
ENTOMOSTRACA.--The animals contained in this Order are met with in every
pool or pond, some of them inhabiting the sea. They are mostly minute,
yet visible to the naked eye, forming specks swimming actively or
leaping through the water; hence some of them have been called
water-fleas. The body of the animal is protected by a shell or
car´apace, which in some consists of a single piece (Pl. IX. fig. 30),
while in others it consists of two similar parts or valves (fig. 31), in
the latter case the joints of the body being indistinctly visible. The
head is furnished with usually two projecting feelers or antennæ
(_anten´na_, a sail-yard), one of which is uppermost or superior (Pl.
IX. figs. 30, 31, 34 _a_), the other lowermost or inferior (figs. 30 and
34 _b_); and these are often used for swimming. The antennæ are jointed,
and sometimes beautifully plúmose (_pluma_, a feather) or feathery, _i.
e._ furnished with rows of long and very slender filaments. There are
several pairs of jointed legs, some of which serve as jaws (foot-jaws),
while others are finely filamentous to serve for swimming and as
respiratory organs (branchial feet). The four species figured are very
common.
_Cypris tristriáta_ (Pl. IX. fig. 34) is found in ponds and ditches. The
carapace is bivalve, or has two valves, which are convex and oval; and
it is of a greenish colour, with three irregular dark stripes behind.
The superior antennæ (_a_) are jointed and finely feathery, the inferior
antennæ (_b_) having a tuft or pencil of fine filaments arising from
their anterior margin. The eye is single. The animal swims steadily and
freely through the water.
The eggs of _Cypris_ (Pl. IX. fig. 35) are often found in glasses of
water containing the animals. They are rounded or oblong, of a red
colour, glued together by an amorphous jelly, and adherent to pieces of
stick or the sides of the glass. They are enclosed in a thick shell,
which exhibits a cellular appearance in the surface view, and is
striated in the side view; so that the structure of the shell is
prismatic, as in that of the oyster. When the eggs escape from the
shell, they present the appearance represented in fig. 35 _a_, the body
of the young animal being enclosed in a transparent envelope, one end of
which forms a blunt protrusion; there is also a separate slender process
enclosing the superior antennæ. After a time, the envelope is cast off
(fig. 35 _b_), when the animal begins its active stage of life. The
cast-off envelopes (fig. 35 _c_), with the protruded portions wrinkled,
are often found in the sediment of water containing the animals. The
structure of these ova is that of what are called winter ova, which
agree with the resting-spores of the lower plants or the Algæ.
_Cy´clops quadricor´nis_ (Pl. IX. fig. 30) is another common species. In
this the body is closely surrounded by the jointed shell, as in a
lobster. The superior antennæ (_a_) are very long and many-jointed, each
joint having short bristles arising from it, while the inferior antennæ
are short and four-jointed. There is no separate head, this being
united to or consolidated with the first joint of the thor´ax or chest,
the head and thorax together comprising four joints. The remaining
joints enclose the belly, or abdómen, which has the appearance of a
tail; but the tail is constituted by the two last parallel pieces, which
are furnished with fine feathery filaments.
The female is most commonly met with, and is easily known by having the
egg-pouch, or ovary (_o_) external on each side, and filled with eggs or
ova. The little _Cyclops_ is readily recognized by its form and jerking
motion through the water.
_Daph´nia púlex_ (Pl. IX. fig. 31) is a very common Entomostracan, and
is very well adapted to illustrate the structure, on account of its size
and transparence. In this animal the body is loosely connected with a
bivalve shell, which, on careful examination, is seen to be reticulated
or marked with net-like lines. The superior antennæ (_a_) are very
small, placed under a small beak, and have at the end a minute tuft of
hairs. The inferior antennæ (_b_) resemble arms, being large and
branched; and by means of them the animal rows itself through the water.
The structure of the eye is curious, consisting of a number of round
lenses aggregated together, the fine muscular threads by which it is
moved being easily distinguished with a high power. The legs are
flattened, and furnished with elegant feathery sétæ (_seta_, a bristle),
serving as gills or branchiæ. They are constantly in motion, fanning the
water so as to change incessantly the portion with which they are in
contact. About the middle of the back is placed the little transparent
heart, with its colourless blood, which may be distinctly seen beating,
or contracting and dilating, in the living animal; and between the back
of the animal and the shell are seen the ova, which remain there until
they are hatched.
The genera and species of the Entomostraca are very numerous. Those
mentioned above will serve to illustrate the general structure of the
order. To distinguish the man´dibles (_mandib´ula_, a jaw) or proper
jaws, the foot-jaws, and the branchial legs, the animals must be
dissected in water with the mounted needles. The very delicate feathery
filaments of the branchiæ may be best observed when these organs are
dried on a slide.
The Entromostaca may be kept alive in a jar of water with water-plants
for a long period. They may be removed from the water for examination by
the dipping-tube, and are best observed in a live-box.
ARACH´NIDA (ἁρἁχνη, spider) is the Class of spiders, scorpions, and
mites.
_Aranéida._--This Order contains the more highly developed forms of the
Class, among which are the common spiders of houses and gardens; and
some of their structures are very curious and interesting.
The head of spiders is united or fused with the thorax, forming one
piece, which is called the _ceph´alothorax_ (κεφαλἠ, head, θὡραξ,
chest).
The claw-jaws, or _man´dibles_, are terminated by a curved and pointed
claw, with which the spiders hold their prey. It is traversed by a
slender canal, containing a slender tube or duct leading from a
poison-gland, and opening near its point; and when the insect prey is
transfixed by the mandible, the poison is pressed out and enters the
wound.
Near the root or base of the mandibles on each side is a jointed feeler,
or pal´pus; but spiders have no anten´næ. The eyes are simple, forming
separate round shining dots, and are called _ocel´li_ (_ocel´lus_, a
little eye); they are usually placed on the top of the head, and are
often arranged in a geometrical form, as a triangle, &c.
The legs are four pairs; they are hairy, and terminated by two or three
claws, which are fringed with minute teeth, or pec´tinate. These claws
serve to comb the fibres of the web, just as we comb our hair with a
common comb.
The _spinnerets_, with which spiders form their web, are very curious
organs. They are situated at the under and hind part of the body, and
consist of two or three cones, or papillæ, on each side. On the summits
of these papillæ are very numerous bristle-like tubes, through which the
secretion of certain glands passes; this secretion, when hardened by
exposure to the air, forms the fibres of the web.
On carefully examining a spider’s web, the radial fibres, or those which
pass from the centre to the circumference, will be found to be smooth,
these fibres serve to fix the web; while the cross fibres are covered
with numerous viscid globules, which serve to attach flies or other prey
to them. This difference of the fibres is best observed with a
hand-lens.
ACARÍNA, or the Order of Mites.--Here belongs cheese-mite, _Ac´arus
domes´ticus_ (Pl. IX. fig. 36). Its body is somewhat milky white, oval,
and furnished with feathery hairs. When viewed from beneath, there is
seen a transverse line, indicating the separation of the thorax from the
abdomen; and another line in front of this, with four minute tubercles,
from each of which arises a hair. The head is pointed and beak-like,
forming a _ros´trum_ (_rostrum_, a beak), consisting of two mandibles
pressed together; these can only be seen to be separate when dissected
apart with the mounted needles. Each mandible is chélate (χηλἠ,
forceps), or has the form of a lobster’s claw; and beneath the two
mandibles is a flat membranous under lip or labium, consolidated on each
side with a palp. The legs are four pairs, as in all the Arachnida; they
are pinkish, 6-jointed, and terminated by a leaflike sucker and a minute
claw.
The males (fig. 38) are smaller than the females, the fore legs being
much stouter, and furnished with a blunt tooth (fig. 38 _a_). The eggs
can often be distinguished within the body of the female (fig. 36);
they are oval and granular.
Another species of _Acarus_, _A. sac´chari_, is found abundantly in
ordinary moist sugar. If a little of the sugar be placed in a
wine-glass, some water added, and the mixture be stirred until the sugar
is dissolved, the _Acari_ will be found both in the sediment and
floating on the surface.
A somewhat larger member of the order occurs as a parasite upon a
species of Dung-beetle (_Geotrúpes stercorárius_) which is vulgarly
known as the Lousy Watchman. The beetle is black, shaded with purple,
about three-quarters of an inch long, and is found under cow-dung. The
mites cling pertinaciously to the under parts of the beetle, and can
easily be seen with the naked eye. They are whitish, with the mandibles,
the sucker, and two claws very distinct; and the palpi are unattached to
the labium, or free. These mites form the species _Gam´asus
coleoptrator´um_. Another species, _Gamasus telárius_, is the red spider
of the greenhouse.
_Trombid´ium fuliginósum_ (Pl. IX. fig. 37 _d_) is a common red spider
of gardens. It is of a scarlet colour, appearing velvety from the
presence of a dense coat of feathery hairs (fig. 37 _f_). The palpi of
this mite are large, free, the last joint but one (Pl. IX. fig. 37 _a_)
being furnished with a claw, while the last joint is obtuse, and
resembles a lateral appendage. The mandibles (fig. 37 _b_) are furnished
with a sharp curved claw. The legs are long, especially the anterior
pair, and terminated by two claws, with a delicate sucker-like appendage
(fig. 37 _c_).
Another species of _Trombidium_, _T. holoseric´eum_, greatly resembling
the last, is also found in gardens. It may be easily distinguished from
the last by the club-shaped hairs (fig. 37 _e_) existing upon the body.
The harvest-bug, which causes such irritation of the legs of persons who
frequent corn-fields in the autumn, is also a species of
_Trombidium_--_T. autumnále_.
PLATE X. [page 133.]
INSECTS.
Fig.
1. _Atropos pulsotorius_: * natural size.
2. _Aphis_ of Geranium: _a_, foot; _b_, anal tube; _c_, antenna.
3. Scales on wing of Apollo-butterfly.
4. _Lithobius forcipatus._
5. _Lithobius forcipatus_, head of: _a_, antennæ; _b_, mandibles; _c_,
labial palpi; _d_, labium.
6. _Dytiscus marginalis_, head of larva.
7. Young larva of _Dytiscus marginalis_.
8. Pupa of Gnat (_Culex pipiens_).
9. Larva of Gnat.
10. Head of Gnat, male.
11. Head of Gnat, female.
12. _Coccinella 7-punctata_ (large Lady-bird), labium of.
13. _Coccinella 7-punctata_, mandible of.
14. _Coccinella 7-punctata_, labrum of.
15. _Coccinella 7-punctata_, antenna of.
16. Coccinella 7-punctata, maxilla of: _a_, palp; _b_, _c_, lobes of
maxilla.
17. Head of _Musca domestica_ (House-fly): _a_, antenna; _b_, labial
palpi; _c_, proboscis.
18. Head of _Stomoxys calcitrans_: 18 _a_, antenna.
19. Scales of insects: _a_, scale of _Podura_; _b_, of House-moth
(_Tinea vestianella_); _c_, of _Podura_; _d_, of _Lepisma_; _e_, hair of
_Podura_; _f_, scale of Cabbage-Butterfly (_Pontia brassicæ_).
20. Head of Cabbage-Butterfly: _a_, antennæ broken off; _b_, palp; _c_,
tongue (antlia); _d_, club of antenna.
21. Head of human Flea (_Pulex irritans_), female (♀): _a_, palpi; _b_,
maxillæ.
22. Flea of the Rat (_Pulex muris_), male (♂).
23. _Pterostichus (Steropus) madidus_; 23 _a_, antenna.
24. Part of leg of _Pterostichus madidus_: _a_, tibia; _b_, tarsus.
25. Labrum of _Pterostichus madidus_.
26. Mandible of _Pterostichus madidus_.
27. Labium of _Pterostichus madidus_: a, mentum; b, labial palp.
28. Maxilla of Pterostichus madidus: _a_, claw; _b_, _c_, maxillary
palps.
29. Proboscis of House-fly.
30. Larva of Flea.
31. Larva of _Chironomos plumosus_.
32. Foot of House-fly.
33. Eye of House-fly.
34. Leg of Ant (_Formica fusca_); 34*, pectinate process.
[Illustration: Plate X.
W Bagg sculp
London: John Van Voorst.]
_Preparation._--The organs of the mouth, &c., of the Spiders are easily
prepared for examination, by carefully pulling them off with forceps or
the mounted needles, then drying them under pressure between two glass
slips, macerating in turpentine, and mounting in balsam. Those of the
Acarina should be dissected out with the needles, after the body has
been crushed in a drop of water on a slide, and the internal substance
has been gently washed away with a hair pencil. They may then be dried
on a slide, with a cover laid on, and turpentine applied to the edge of
the cover, balsam being added when most of this has evaporated. The
various parts may also be mounted in chloride of calcium or glycerine.
INSECTS.--The members of the class of Insects are extremely interesting
to the microscopic observer, not only on account of the beautiful
structures which they present, but also from these being comparatively
large, usually coloured, and easily distinguished under the lower
powers. Hence they form admirable objects for study to those who are but
little accustomed to the use of the microscope.
MYRIAP´ODA (μὑριος, myriad, ποῦς, foot).--This Order contains those
insects which are popularly known as the hundred-legs and millepedes; by
many zoologists they have been arranged in a distinct class.
The most common member of this Order is _Lithóbius forcipátus_ (Pl. X.
fig. 4), which is found under stones, in cellars, and among
garden-rubbish. It is of a yellowish-brown colour, with long,
many-jointed, gradually tapering or setáceous (_séta_, a bristle)
antennæ (figs. 4 _a_, 5 _a_), and two large and powerful mandibles (fig.
5 _b_) resembling those of the spiders. It has also a broad, notched,
and toothed lower lip, or labium (fig. 5 _d_), above which are two
toothed jaws, or maxillæ, and two lip-feelers, or labial palpi (_c_).
The eyes consist of a group of ocelli on each side (_e_). The body is
protected by alternately larger and smaller dorsal plates, which are
fifteen in number; and there are fifteen pairs of legs, which are
terminated by a single claw. On the sides of the body will be found some
oval dark-looking bodies, fringed with hairs; these are the spir´acles
(_spirac´ulum_, a breathing-hole) or breathing-pores. They form the
orifices of certain branched and transversely striated tubes, which are
distributed throughout the body; the tubes are called _trácheæ_
(_trachea_, the windpipe), and their walls contain an elastic spiral
fibre which keeps them open. These parts of the insect can only be
distinctly seen when the body has been slit up on the under side; and,
after washing away the animal matter with water by the aid of a hair
pencil, pressed between two slides with a clip, dried, soaked in
turpentine, and mounted in balsam.
THYSANÚRA (θὑσανοι, fringe, οὑρἀ, tail).--The insects belonging to the
genus _Podúra_, of this Order, are very minute and difficult to examine;
but they are specially interesting, on account of the structure of their
scales. They are common in gardens and cellars, under flower-pots, &c.,
and are about one-tenth of an inch long. They are of a brownish or
silvery-leaden colour, wingless, with six legs, and when touched they
leap like a flea. The leaping motion is produced by the action of the
tail, which is forked and bent under the body.
The body is usually covered with minute scales (Pl. X. fig. 19, _a_,
_c_), and these are used as test-objects. The structure of the scales
varies in the different genera and species; those usually used (fig. 19
_a_) are stated to belong to _Podúra plum´bea_; it appears, however,
that this is not correct. The scales sold as test-objects under this
name are covered with minute and short raised lines (fig. 19 _a_),
arranged in irregular but somewhat parallel wavy rows. It requires a
good microscope and a high power to show them distinctly, and they
should appear perfectly black and separate. The little lines are much
coarser in some scales than in others; so that there are easy and
difficult scales, as they are called.
The _Poduræ_ may be caught by holding a sheet of paper near their haunts
and disturbing them; and when they have jumped upon the paper, a slide
laid upon them and gently pressed will remove some of the scales for
examination.
The scales should be mounted as dry transparent objects; for if wetted,
they become very transparent, and the markings appear removed, which
however is not really the case.
The scales of _Lepis´ma saccharína_ (Pl. X. fig. 19 _d_), a member of
this Order, were formerly used as test-objects; but they are too easily
made out to serve for this purpose with modern microscopes. The insect
is not common. The scales (fig. 19 _d_) exhibit continuous nearly
parallel longitudinal lines or ribs.
ANOPLÚRA (ἄνοπλος, unarmed, οὑρἀ, tail).--This third Order of insects
consists of the Lice of the Mammalia and birds. They are minute,
resembling mites to the naked eye, but may be at once distinguished from
them by the distinct head and thorax and the presence of six instead of
eight legs.
Some of them are suctorial, _i. e._ have a short and slender tube, with
which they suck the blood of the animals of which they are parasites;
while others are mandibulate, or have mandibles, and also maxillæ, their
food consisting of portions of feathers, hairs, and scurf. The legs are
usually short and stout, and the claws large and powerful, to enable
them to hold firmly to the hairs, &c. The Anoplura are most abundant on
dirty and diseased animals.
SUCTOR´IA.--The fourth Order of insects consists of the genus
_Púlex_,--_Pulex irrítans_ being the human flea. Other species are
found upon different animals, as upon the dog, the rat (Pl. X. fig. 22),
the fowl, the pigeon, &c.
The head and the dark eye (fig. 21, head of the human flea) are very
evident. The antennæ or head-feelers are very minute and difficult to
find, being sunk in a little pit or fossa behind the eye; they may,
however, generally be recognized by the detection of the last joint,
which is pectinate or cut like a comb. The body, including the head,
consists of thirteen joints, one for the head, three for the chest or
thorax, and nine for the belly or abdómen,--it being understood that by
“joint” is meant a segment, and not the line of junction of two
segments. The indication of these joints is afforded by the horny
integument, which consists of a corresponding number of rings, forming
in fact the skeleton of the animal. This in the flea, as in all insects
and other Articulata, is external or cutaneous (_cútis_, skin), and
consists of the hardened skin, the peculiar animal substance of which it
is composed being called chitine (χιτὠν, tunic). These chitinous rings
overlap, and are composed of a dorsal or upper, and a ventral or lower
half; and near the middle of each is a row of hairs directed backwards.
Along the sides of the body of the insect may be seen a row of dots;
these are the spiracles or orifices of the breathing-tubes (tracheæ).
The legs are many-jointed, long, furnished with numerous spines, and
terminated by two slightly curved claws, each with a little blunt tooth
at its base. The claws are not perfectly smooth on the inside, but are
covered with slightly raised lines, like a file, so that a better hold
can be taken of bodies.
But the most interesting parts of the flea are those of the mouth, with
which it punctures the skin and sucks the blood. There are nine of
these; and they are best seen when the head of the flea is pulled off
with the mounted needles, and the parts spread out and mounted in
balsam, a high power being used to examine them. One of them forms a
long and slender bristle (seta) or tongue, furnished with distant minute
teeth. On each side of this is a flattened seta, with two rows of teeth
on the edges; these are the lancets, and when not in use these organs
are inclosed in two jointed sheaths. Outside these are two
representatives of jaws, or maxillæ (_b_), each having a jointed feeler
or palp (_a_) arising from it; the probable use of the palps being to
feel the position of the skin, so that the animal may be able to adjust
the lancets at a proper distance for puncture.
The eggs of the flea are often visible within the body of the parent;
and when this is crushed, they are more distinctly seen, of various
sizes, and contained within a long tube, which is the ovary or egg-bag.
The eggs are laid by the animals upon carpets, woollen garments, or in
the cracks of dirty floorboards; they are just perceptible to the eye as
white oblong specks, and they may always be found on the rug when a cat
is kept in the house. When hatched, they give rise to a minute white
worm-like maggot, or larva (fig. 30), having a 12-jointed body, with two
rudimentary antennæ and two slightly curved hooks appended to the last
joint. The mouth-organs of the larva are adapted for biting, and not for
sucking, as in the perfect animal, the jaws or maxillæ being distinctly
toothed. When they have acquired full growth, which takes place in about
twelve days in warm weather, they spin around themselves a little silky
cocoon, and become transformed into a chrysalis or pupa; and from this,
in about a fortnight, the perfect insect escapes.
DIP´TERA (δἰς, twice, πτερὀν, wing).--This, which forms the fifth Order
of Insects, consists of the two-winged insects, or flies, as the
house-fly, the blue-bottle, the gnats, &c.
MUS´CIDÆ.--The house-fly, and the blue-bottle or meat-fly, are both
species of the genus _Mus´ca_, belonging to this family, the former
being _Musca domes´tica_, the latter _Musca vomitor´ia_. Both these
insects are seen to be wonderfully constructed when minutely examined,
and they possess considerable resemblance in general structure.
On examining the head of the house-fly (Pl. X. fig. 17) under a low
power, and as an opake object, the observer will be struck with the
remarkable appearance presented by the two eyes, which are large, placed
one on each side of the fore part of the head, and composed of very
numerous little eyes closely packed together, or they are compound, as
it is called. The use of this compound structure is evidently to enable
the little animal to see in all directions without moving the head and
eyes. Each little eye has a lens to bring the rays of light emanating
from objects to a focus upon a nerve. The packing of the eyes together
gives rise to their angular form or their straight sides, each of the
little surfaces or facets being hexagonal, or bounded by six sides (fig.
33). In front of and between the eyes are seen the two small antennæ;
these have three joints, the third of which is larger than the rest
(fig. 17 _a_), and arising from near its base is a feathery bristle or
seta; these structures are best seen when the antennæ are pulled off
with a pair of forceps and mounted separately. Below the antennæ, and
extending downwards and forwards is the proboscis, or tongue, as it is
called, which can be entirely retracted within a pit in the fore part of
the head, or protruded at the will of the animal. This is a very
beautiful and complicated instrument, and is best examined when spread
out and separately mounted (Pl. X. fig. 29). It consists of a fleshy
tube, dilated at the end into two lobes, which are flattened beneath to
form a sucking-disk. The end is furnished with two solid horny lateral
branches to keep it expanded, and with two longitudinal tubes beneath,
running parallel, from the outer sides of which arise numerous nearly
parallel branches. These tubes and their branches are incomplete
beneath, and consist of imperfect rings, otherwise greatly resembling
tracheæ. On each side of the proboscis is a lip-feeler or labial palpus,
for the organ represents the labium of other insects. All these parts
are better seen in the proboscis of the blow-fly than in that of the
house-fly, on account of their larger size. The head, the thorax, and
the abdomen are very distinct in the fly, being separated from each
other by well-marked constrictions.
The legs are composed of five parts, each having a separate name. The
first piece or joint, which is that attached to the body, is called the
hip, or coxa; the next is a very small, somewhat triangular piece, and
is the trochan´ter; next comes the long and stout thigh, or fémur; this
being succeeded by the tib´ia, which, as in most insects, is furnished
with strong spines at the end; and lastly is the foot, or tar´sus,
consisting of five joints, the three last of which are represented in
the figure (Pl. X. fig. 32). At the end of the fifth or last joint of
the tarsus (fig. 32) (for it must be noted that the joints of the limbs
of insects are always numbered in order of distance from the body) are
two soft little cushions, or pulvil´li, which are covered on the under
surface with numerous hair-like bodies dilated at the end, acting as
suckers in enabling the fly to adhere to smooth surfaces. In addition to
these organs are two curved claws, and between them a sharp straight
spine.
The eggs of the fly are deposited upon heaps of decaying animal and
vegetable matters, as dungheaps, &c. The blow-fly deposits its eggs in
the same situations, but especially those where the animal matters are
most abundant: every one knows the eggs as deposited upon tainted meats,
when they are called flyblows. When the eggs are hatched, the _larvæ_
or maggots make their appearance. The larvæ of the blow-fly are well
known to the angler, who uses them for bait, and calls them gentles.
These larvæ exhibit some interesting points of structure. The jointed or
ringed condition of the body is distinct to the naked eye. The head is
provided with two rudimentary palpi, placed each upon a rounded papilla;
also with two brown curved and horny hooks or jaws. On the posterior end
of the body are two brown spots, which consist of spiracles, and have
three sieve-like oblong orifices; and at the anterior margin of each
segment of the body are very numerous little short spines, with the
points directed backwards. These answer the purpose, to some extent, of
legs; for when the larva is moving, and has forced itself through the
matter in which it burrows, the little spines prevent the body being
forced backwards as the head is pushed forwards and meets any
resistance.
To examine the structure of these larvæ, the gentles should be killed by
immersion for a time in warm water, and then dried by touching them with
blotting-paper. The hooks and palpi can be seen by holding the body in
the forceps as an opake object. To observe the spiracles, the end of the
body should be cut off, and the animal matter washed away in a
watch-glass with water and a hair pencil, then spread out, dried between
two slides, and mounted in balsam.
The larvæ of the house-fly and of the blow-fly very closely resemble
each other, so much so that the former are generally overlooked; hence
it is often wondered where flies come from, although they are so
numerous in every house. When the larvæ of these flies are fully
developed, they gradually assume a brown colour, the organs of the head
are retracted, and the skin becomes dry and hard. This is the state of
_pupa_, or chrysalis; and while remaining in this state of rest, the
development of the wings, legs, &c., takes place; so that when the
insect emerges from the shell of the chrysalis, it has attained its
highest state of development, and forms the _imágo_ or perfect insect.
It may be remarked here, that when insects pass through the three states
of larva, pupa, and imago, they are said to undergo complete
metamorphosis. There is not a more curious object than that presented by
the young fly contained in its case, as seen on carefully cutting away
portions from one end of the case of the chrysalis. The body and head
are quite white, with beautiful blood-red eyes.
Most persons must have noticed another kind of house-fly, having the
wings more widely separated than in the common fly, and moving more
slowly through the air. This is _Stomox´ys cal´citrans_. The proboscis
of this fly (Pl. X. fig. 18) differs from that of the house-fly in being
longer, more bent, and but little expanded at the end; also in being
provided with two long setæ, one forming a slender sharp lancet, the
other being somewhat stouter, and forming its sheath. The fly is thus
enabled to pierce the flesh and suck the blood.
CULIC´IDÆ.--We will now say a few words about the gnats, which are old
favourites for microscopic examination. The three states of larva, pupa,
and imago must be considered separately.
The imago or perfect form is well known. The males are easily
distinguished from the females by the difference in structure of the
antennæ, which in the males (Pl. X. fig. 10, head) are very beautifully
plumose or feathery, whilst in the females (fig. 11) the hairs are very
short--the long proboscis, or rostrum, or bundle of biting-organs
forming the striking feature; this difference is immediately evident to
the naked eye.
The antennæ of the males (fig. 10 _a_) consist of numerous small joints,
from each of which arises a ring of long hairs, giving the appearance of
a tuft on each side when flattened for view as a transparent object
under the microscope; at the end are two longer joints, the first having
a small ring of shorter hairs.
The proboscis of the female is a very complicated organ, consisting of
six separate bristle-like pieces or setæ, all nearly of the same length.
Two of these are somewhat curved near the end, and provided on the
inside with fine teeth; another pair consists of very thin
lancet-pointed instruments; then comes another lancet-pointed seta, very
sharp at the end, and traversed by a canal; next, a stouter and
darker-looking tube, slit up underneath, which serves to contain the two
toothed setæ; and lastly, a stout and broader sheath, also slit up
throughout its length beneath, in which all are packed. This sheath has
two lobes at the end, and has some resemblance to the proboscis of the
fly, of which organ it is the representative.
The gnat lays its eggs in water. The eggs are longish oval, with a kind
of neck at the upper end; they are glued together side by side in large
numbers, and form a boat-like mass, floating on the surface of the
water. The larvæ (Pl. X. fig. 9) are very commonly found skipping
through the water, or hanging, as it were, by the tail from the surface.
The head is very broad; and arising from each joint of the body are
tufts of hairs. Near the end of the body is a tube which communicates
with the tracheæ; and when the animal is quiet, this tube is brought to
the surface; so that when the animal appears hanging to the surface, it
is breathing. In addition to this method of respiration, there are other
respiratory organs attached to the last joint of the body, consisting of
leaf-like plates; hence these larvæ have an aquatic as well as an aërial
respiration: the respiratory or branchial plates also serve as a tail to
aid in swimming. Running down the back of the larva will be seen a thin
delicate vessel, dilated opposite the thorax, and beating or contracting
at regular intervals. This is the dorsal vessel, the dilatation
representing the heart of the higher animals; and it serves to propel
the colourless blood throughout the body.
After several moultings or castings of the skin, to allow of the growth
of the insect (for insects grow only in the larval state), the larva
becomes transformed into the pupa (Pl. X. fig. 8). In this state the
animals still move about in the water, and are often found suspended
from its surface by two respiratory tubes, which, however, are not
connected with the tail, but arise from the thorax; and the various
parts of the perfect insect may be seen through the case or skin, within
which they are closely packed.
When the pupa has attained its full development, the perfect insect
emerges from it, leaving the water to fly about and seek food.
TIPU´LIDÆ.--In the water of ponds and pools a young larva (Pl. X. fig.
31) will often be met with, which is that of _Chiron´omus plumósus_, a
largish gnat-like insect, belonging to the daddy-long-legs family
(Tipulidæ). This larva exhibits the usual thirteen segments, including
the head. In the young state the body is nearly colourless; but in the
mature larva it is of a blood-colour, and about an inch in length.
Beneath the first joint of the body are two foot-like processes covered
with hair; and at the end of the body are also two processes, surmounted
with hooks. The three last joints of the body are also furnished each
with a pair of fleshy processes, those of the first pair being very
short.
HYMENOP´TERA (ὑμἠν, membrane, πτερὀν, wing).--This, which forms the
sixth Order of insects, contains the bee, the wasp, the ant, &c.
In the Hymenoptera there is a curious contrivance for linking the two
wings on each side, so that they may form a single piece in the flight
of the insect. It consists of a row of hooks, placed upon the anterior
nerve of the hind wing, which play upon the folded-in corresponding edge
of the fore wing; the hooks, sliding along this edge, allow of freedom
of motion, although still holding the two wings together. This structure
may be well seen in the wings of the Humble Bee when mounted in balsam.
The sting of the wasp and bee is also a singular organ. In both insects
it is much alike, consisting of a sheath, slit up beneath, in which are
contained two long setæ, or lancets, with bent-back (recurved) teeth
near the end. These setæ are inserted into the flesh during the act of
stinging, and at the same time the poisonous secretion from two glands
is forced into the wound, which causes the severe pain resulting from
the sting.
In the wingless neuters of the common ant, attached to the end of the
tibia (Pl. X. fig. 34 _c_) is a beautiful pectinate process, somewhat
resembling a comb (fig. 34 _a_).
LEPIDOP´TERA (λεπἰς, scale, πτερὀν, wing).--This Order contains the
butterflies and the moths, the entire bodies of which are covered with
minute scales. When the insects are handled, these scales adhere to the
fingers as a fine dust; and on pressing a slide against the insects,
they may be removed for examination. They consist of a very slender and
short quill, by which they are attached, and a flattened plate of
various forms (Pl. X. fig. 19, _b_, _f_); it is, however, generally
narrower near the quill, and expanded towards the free end, where it is
often cut into lobes or toothlike segments. The scales are usually
covered with continuous longitudinal lines or ridges, with granules of
colouring-matter (pigment) situated between the two thin layers of which
the scales consist. In some of them the form is that of a filament,
either simple or branched at the end (fig. 19 _e_), when they resemble
minute hairs. In the males of the large white Cabbage Butterfly
(_Pi´eris brass´icæ_), certain of the scales of the wings are covered
with longitudinal rows of very minute dots (fig. 19 _f_), and have
little tassel-like bodies at the end. The males may be distinguished
from the females by the front wings having no black spots, while those
of the females have two upon each wing. When the scales are examined as
they exist upon the wings of the Lepidoptera, they are found to be
imbricated (Pl. X. fig. 3) or overlapping each other like the tiles on
the roof of a house.
The Lepidoptera suck the honey of flowers by means of a spiral tongue
(Pl. X. fig. 20 _c_) or ant´lia (_antlia_, a sucking-tube); this
consists of two halves, which represent the maxillæ of other insects;
and their margins are fringed with little tassel-like bodies, probably
organs of taste. The antennæ (fig. 20 _a_) are many-jointed, clubbed at
the ends (_d_) in the butterflies, and simple in the moths. The palpi
(fig. 20 _b_) are short and densely covered with scales. In the large
eyes the facets are very distinct and suitable for examination.
The larvæ are well known as caterpillars. They have six legs, as in the
perfect insects, but rudimentary and with single claws; also some
additional pairs of pro-legs, as they are called, with a crown of hooks,
towards the hind part of the body. The spiracles of caterpillars are
very favourable for observation.
NEUROP´TERA (νεῦρον, nerve, πτερὀν, wing).--This Order contains the
Dragon-flies (Libellúlidæ), the Day-fly (_Ephem´era_), &c., in which the
wings are usually so large and so beautifully netted. The species
figured (Pl. X. fig. 1), which is wingless, is very common in old books
and in collections of dried plants. It is whitish, mite-like, with
setaceous many-jointed antennæ, 3-jointed tarsi, and very broad thighs
(femora). Its name is _At´ropos pulsator´ius_.
HEMIP´TERA (ἤμισυς, half, πτερὀν, wing).--This Order contains the bugs
and other noxious insects. Those which we shall notice are the species
of _A´phis_, commonly known as Plant-lice and Green-fly, which are found
too frequently upon unhealthy plants. The species figured (Pl. X. fig.
2) is that of the geranium (_Pelargonium_). The head is small and
notched or emarginate. The body is oval and furnished behind with two
prolonged tubercles, these being covered with scales, giving them a
somewhat striated appearance (fig. 2 _b_). The antennæ are 6-jointed
(_c_), the second joint being very small, the last joint long, excavated
on one side, and ringed: these organs are reflexed over the body in the
natural state. There are two compound eyes, and three simple eyes or
ocelli, forming a triangle on the top of the head. The proboscis or
rostrum is bent under the body, 4-jointed, and contains three setæ, two
of them forming very slender lancets. The legs are long; the tarsi (fig.
2 _a_) 2-jointed, the first or basal joint being very minute, and the
last furnished with two claws.
In a colony of these insects, some are winged and some wingless; those
without wings being usually in the larva state, the pupæ having
rudimentary wings, and the males and females usually perfect wings.
_Aphis brass´icæ_ is the destructive turnip-fly.
COLEOP´TERA (κολεὀς, sheath).--This, which is the last Order to be
noticed, contains the Beetles, so easily recognized by their hard and
horny fore wings or wing-cases. The parts of the mouth in these insects
are exceedingly well adapted for examination; and as they are not fused
or consolidated with each other, they serve to illustrate the typical
constitution of the organs as existing in these animals.
_Pteros´tichus_ (_Ster´opus_) _mad´idus_ (Pl. X. fig. 23) is common in
cellars and gardens among vegetable rubbish. The body of this beetle is
shining black, the head projecting; the antennæ (fig. 23 _a_) are
filiform, and compressed towards the end. The thorax is somewhat
rounded, with a deep rough pit and a longitudinal stria at each
posterior angle. The wing-cases, or elytra, are longitudinally striated;
the wings, which in most beetles are concealed beneath the elytra when
the insects are at rest, being absent. The tibiæ of the fore legs are
notched on the inside (fig. 24 _a_), a fringe of hairs being situated in
the notch; the tarsi are 5-jointed (fig. 24 _b_), the first four joints
being triangular, the last elongate and terminated by two curved claws.
In the male the three first joints of the tarsi of the anterior legs
(fig. 24 _b_) are dilated and heart-shaped.
The parts of the mouth (which are named after an analogy with those of
the higher animals) consist of the following pieces:--An upper lip, or
_lábrum_ (fig. 25 _a_), which is squarish (quad´rate) and slightly
notched; a quadrate lower lip, or _lábium_ (fig. 27), with a process on
each side, and two 4-jointed lip-feelers or labial palpi (_b_); and
below the labium is a chin, or _mentum_ (fig. 27 _a_), with a projecting
bifid tooth: these parts form the roof and the floor of the mouth. Next
come the _man´dibles_ (fig. 26), one on each side, which are stout,
curved, and pointed; beneath which are the _maxil´læ_ (fig. 28), also
one on each side, and provided with a fixed claw (_a_), ciliated on the
inside, and furnished with two pairs of jaw-feelers, or _maxillary
palpi_, the inner (_b_) being 2-jointed, while the outer (_c_) are
4-jointed. It will be noticed that the jaws work laterally, or from side
to side, and not perpendicularly as in the higher animals.
These parts may be found in most beetles which the observer may submit
to examination, being however somewhat modified in different genera. We
may consider those existing in the Lady-birds, or species of
_Coccinel´la_, by way of comparison. The body in these insects is very
convex, and the head sunk deeply in the thorax. The antennæ (fig. 15)
are short, clubbed (clávate), and compressed. The thorax is short and
lúnate, or half-moon shaped. The mandibles (fig. 13) are curved, bifid
at the apex, and with a tooth on the inside near the base. The labrum
(fig. 14) is transverse, or broader than long. The labium (fig. 12) is
furnished with two palpi, which are 3-jointed. The maxillæ (fig. 16) are
two-lobed, the lobes (_b_, _c_) being ciliated, and the 4-jointed palpi
(_a_) have the last joint large and hatchet-shaped.
Coleopterous insects undergo complete metamorphosis, the larvæ being
commonly known as grubs. The larvæ of the aquatic beetles will often be
met with in the water of ponds or ditches, especially that of the common
large water-beetle (_Dytis´cus marginális_), or water-boatman as it is
called (Pl. X. fig. 7), and in various stages of growth. The structure
of the mouth-organs (fig. 6), which are, however, imperfect or
rudimentary in some parts, can be readily made out; and their names may
easily be found by comparison with what has been stated in regard to the
organs of the perfect beetle.
_Examination, &c._--The means of catching insects will readily occur to
the reader. A bag-net made of a curved piece of cane, to which is fitted
a bag made of net, will serve to catch those which trust to flight for
escape from their enemies, such as the Lepidoptera; and these may be
killed by firm pressure of the thorax between the finger and the thumb.
The running insects, as the beetles, may be caught in a spoon or with
forceps; and they may be killed by immersion in boiling water or in
camphorated spirit. In an excursion, most insects may be carried in a
well-corked bottle containing a little wool and a lump of camphor, which
stupifies them. When the insects are dead, the limbs should be extended
into the natural position by means of pins, the insect being transfixed
by a pin run through the thorax or one of the elytra and extending into
a sheet of cork. To preserve them, they may be kept in a box, the
bottom of which is covered with sheet cork, into which the pins are
stuck.
The smaller beetles, &c., which cannot be transfixed with a pin, may be
mounted as opake objects upon slips of card, the legs &c., being
carefully spread out, and gummed in position with a strong solution of
gum-tragacanth in boiling water. Many of the smaller Curculion´idæ or
diamond-beetles, in which the labium forms a rostrum or beak, with
elbowed or half-bent antennæ, form beautiful opake objects when thus
mounted, on account of the brilliant scales with which they are covered.
There are two ways of examining insects--either in the entire state as
opake objects or the separate parts mounted as transparent objects. In
the former case the pin with which the insect is transfixed should be
stuck into a slide made of cork, and this laid upon the stage, or the
pin may be held by the forceps. In this way, with the use of the side
condenser and a low power, the general form and arrangement of the parts
of the insect can be made out. The more minute details must be searched
for in the individual organs which have been picked off with forceps,
and mounted in balsam.
If it be required to submit the parts of a dried insect to examination,
this must be previously soaked in warm water for a time, as the legs,
&c., become very brittle when dry, and are thus easily injured.
ROTATOR´IA (_róta_, a wheel) or ROTIF´ERA (_rota_ and _fero_, to
bear).--The animals contained in this class are minute, being just
distinguishable to the naked eye as white specks. They are common in
long-kept infusions and among _Confervæ_ in the water of pools and
ditches. Their body is usually longer than broad, often presenting
indications of rings; and at or near the posterior end is frequently
found a prolongation resembling a tail, but terminated by two short
moveable thumb-like processes, rarely a sucker, which enable the
animals to cling to objects. The most characteristic organ, however, is
a kind of rounded or oval disk, placed at the anterior end of the body,
and furnished with cilia. When these are in active motion, the organ
appears as a revolving wheel, whence the name of Wheel-animalcules, by
which they are sometimes designated. The wheel-organ enables the animals
to swim through the water, and also brings their food to the mouth by
the currents which it produces. It is usually cleft into two or more
lobes, and can be retracted, as is commonly the case when the animals
are disturbed.
In many of these animals the body is more or less covered by a horny
shell or carapace; and in some it is fixed at the bottom of a tube,
within which it can be withdrawn. On the anterior part of the body are
frequently seen two or more red spots, which represent eyes. The
alimentary canal is mostly distinct, being indicated by the colour of
its contents, and it is lined with cilia. Towards its front portion is a
gizzard (Pl. XI. fig. 2 _a_) containing teeth, which are sometimes
attached to a jointed jaw-like framework; these are usually in active
motion. No heart or blood-vessels have been observed in the Rotatoria;
but on each side of the body in many of them is a long wavy tube,
containing at intervals minute ciliated bodies, the cilia propelling the
water through the tubes, and so exerting an aërating or respiratory
function. The reproduction of the Rotatoria takes place by the formation
of ova, which may often be distinguished within the body of the parent.
_Rot´ifer vulgáris_ (Pl. XI. fig. 2) is a common species. It has a
spindle-shaped body, which is capable of contraction almost into a ball.
The front or head end is sometimes protruded (fig. 2), at others
retracted and obscured by the exserted disk (fig. 2*); and beneath it is
a tentacle-like organ, supposed to represent an antenna (fig. 2 _b_).
The position of the jaws is indicated at _a_. The alimentary canal is
seen running down the body; and two ova exist, one on each side of it,
these being often recognizable by the existence of the eyes and jaws. At
the end of the body are two lateral processes, and a tail-like piece,
which can be withdrawn or protruded and is furnished with two moveable
portions or toes.
PLATE XI. [PAGE 150.]
ROTATORIA, INFUSORIA, &c.
Fig.
1. _Anguillula_ (_Dorylaimus_), species of.
2. _Rotifer vulgaris_: _a_, jaws and teeth; _b_, antenna; 2*,
wheel-organ expanded.
3. _Pterodina patina._
4. _Floscularia ornata._
5. _Hydra viridis._
6. _Arcella vulgaris._
7. _Arcella aculeata._
8. _Arcella aculeata_, shell with animal.
9. _Arcella dentata._
10. _Amœba diffluens._
12. _Actinophrys sol._
13. Sponge, fibres of; 13 _a_, _b_, _c_, spicules of Sponge.
14. _Sertularia pumila_, polypidom.
15. _Sertularia pumila_, polypidom with polypes.
16. _Monas lens._
17. _Cercomonas globulus._
18. _Cercomonas crassicauda._
19. _Heteromita ovata._
20. _Anthophysa Mülleri._
21. _Dinobryon sertularia._
22. _Trachelomonas volvocina._
23. _Chætoglena volvocina._
24. _Euglena viridis._
25. _Astasia hæmatodes._
26. _Enchelys nodulosa_; _a_, undergoing transverse division.
27. _Oxytricha gibba_; 27 _a_, side view.
28. _Paramecium aurelia_: _a_, contractile vesicle; _b_, a gastric
sacculus.
29. _Amphileptus fasciola._
30. _Colpoda cucullus_: _a_, contractile vesicle.
31. _Nassula elegans_: _a_, vesicle; 31 _b_, encysted form.
32. _Coleps hirtus._
33. _Vaginicola crystallina._
34. _Vorticella convallaria_: _a_, stalk spirally contracted; _b_, body
undergoing longitudinal division.
35. _Vorticella convallaria_ encysted and discharging the young brood.
36. _Vorticella convallaria_, body with nucleus (_a_).
37. _Chilodon cucullulus._
38. _Stentor polymorphus_: _a_, body extended; _b_, body contracted;
_c_, bodies aggregated around a globule of jelly; _d_, bodies adherent
to the side of a glass.
39. _Alyscum saltans._
40. _Podophrya fixa_, or the _Podophrya_-form of _Vorticella_.
[Illustration: Plate XI.
W Bagg sculp
London: John Van Voorst.]
_Pterodína pat´ina_ (Pl. XI. fig. 3).--This species has a shell or
carapace on the back, a two-lobed rotatory organ, two eyes, and a
slender wrinkled tail ciliated at the extremity. The curved alimentary
canal, and the two strong muscles inclined at an angle, are easily
distinguishable.
_Flosculária ornáta_ (Pl. XI. fig. 4) is a very beautiful member of the
Rotatoria, and is found adhering to _Confervæ_ and other water-plants.
The body is club-shaped, and contained in a transparent tube, the ringed
narrower portion being fixed to its base. The rotatory organ is divided
into five or six lobes, furnished with long, slender, radiating
tentacular filaments; these are not vibratile like ordinary cilia, but
can be slowly moved. In the contracted state, the filaments form a
pencil-like bundle.
_Examination, &c._--The Rotatoria are best examined in the living state,
the drop of water in which they are viewed being very small, so that
their movements may be impeded; and while they are struggling to escape,
the various parts of the body will come into view. Their preservation
has been attempted by drying on a slide; but when dead they become so
contracted and altered, that it is difficult to make out their
structure. Should the observer wish to record any observations on their
reproduction or habits, it will be well to preserve a specimen of the
jaws and teeth, as the species might be with certainty identified by
careful examination of their minute structure.
ENTOZÓA (ἑντὀς, within, ξῶον, animal).--This class consists of the
parasitic worms, as the Tape-worm (_Tænia_), the Thread-worm and
Round-worm (_As´caris_), which live within the bodies of man and
animals. It also includes the microscopic eel-like animalcules (species
of _Anguil´lula_) which are found in sour paste (_A. glútinis_), in
vinegar (_A. acéti_), and in blighted wheat (_A. trit´ici_). Some of the
species of allied genera are met with in damp moss and in the débris or
fragments of vegetable substances decaying in water. The general
appearance of the microscopic species is that of a minute colourless
eel, writhing in the water (Pl. XI. fig. 1). Their internal organs are
difficult to distinguish. The alimentary canal is usually evident, and
dilated into a kind of stomach, containing near its commencement some
rod-like or otherwise-formed teeth. In the species figured there are two
apparently tubular lancets, which are capable of protrusion, and
evidently serve to wound the prey.