Common Objects of the Microscope
TextCHAPTER VII
Antennæ, their Structure and Use—Eyes, Compound and Simple—Breathing Organs—Jaws and their Appendages—Legs, Feet, and Suckers—Digestive Organs—Wings, Scales, and Hairs—Eggs of Insects—Hair, Wool, Linen, Silk, and Cotton—Scales of Fish—Feathers—Skin and its Structure—Epithelium—Nails, Bone, and Teeth—Blood Corpuscles and Circulation—Elastic Tissues—Muscle and Nerve.
We now take leave of the vegetables for a time, and turn our attention to the animal kingdom.
On Plate
VI
. may be seen many beautiful examples of animal structures, most of them being taken from the insect tribes. We will begin with the antennæ, or horns, as they are popularly termed, of the insect.
The forms of these organs are as varied as those of the insects to which they belong, and they are so well defined that a single antenna will, in almost every instance, enable a good entomologist to designate the genus to which the insect belonged. The functions of the antennæ are not satisfactorily ascertained. They are certainly often used as organs of speech, as may be seen when two ants meet each other, cross their antennæ, and then start off simultaneously to some task which is too much for a single ant. This pretty scene may be witnessed on any fine day in a wood, and a very animated series of conversations may readily be elicited by laying a stick across their paths, or putting a dead mouse or large insect in their way.
I once saw a very curious scene of this kind take place at an ant’s nest near Hastings. A great daddy long-legs had, unfortunately for itself, settled on the nest, and was immediately “pinned” by an ant or two at each leg, so effectually that all its struggles availed nothing. Help was, however, needed, and away ran four or five ants in different directions, intercepting every comrade they met, and by a touch of the antennæ sending them off in the proper direction. A large number of the wise insects soon crowded round the poor victim, whose fate was rapidly sealed. Every ant took its proper place, just like a gang of labourers under the orders of their foreman; and by dint of pushing and pulling, the long-legged insect was dragged to one of the entrances of the nest, and speedily disappeared.
Many of the ichneumon-flies may also be seen quivering their antennæ with eager zeal, and evidently using them as feelers, to ascertain the presence of the insect in which they intend to lay their eggs; and many other similar instances will be familiar to anyone who has been in the habit of watching insects and their ways.
It is, however, most likely that the antennæ serve other purposes than that which has just been mentioned, and many entomologists are of opinion that they serve as organs of hearing.
Fig.
15
, Plate VI., represents a part of one of the joints belonging to the antennæ of the common house-fly; it is seen to be covered with a multitude of little depressions, some being small, and others very much larger. A section of the same antenna, but on a larger scale, is shown by Fig.
16
, in order to exhibit the real form of these depressions. Nerves have been traced to these curious cavities, which evidently serve some very useful purpose, some authors thinking them to belong to the sense of smell, and others to that of hearing. Perhaps they may be the avenues of some sensation not possessed by the human race, and of which we are therefore ignorant. Fig.
17
represents a section of the antennæ of an ichneumon-fly, to show the structure of these organs of sense.
We will now glance cursorily at the forms of antennæ which are depicted in the Plate.
Fig.
1
is the antenna of the common cricket, which consists of a vast number of little joints, each a trifle smaller than the preceding one, the whole forming a long, thread-like organ. Fig.
2
is taken from the grasshopper, and shows that the joints are larger in the middle than at either end.
Figs.
3
and 5 are from two minute species of cocktailed beetles (
Staphylínidæ
), which swarm throughout the summer months, and even in the winter may be found in profusion under stones and moss. The insect from which Fig.
5
was taken is so small that it is almost invisible to the naked eye, and was captured on the wing by waving a sheet of gummed paper under the shade of a tree. These are the tiresome little insects that so often get into the eye in the summer, and cause such pain and inconvenience until they are removed.
Fig.
4
shows the antenna of the tortoise beetle (
Cássida
), so common on many leaves, and remarkable for its likeness to the reptile from which it derives its popular name. Fig.
3
is from one of the weevils, and shows the extremely long basal joint of the antennæ of these beetles, as well as the clubbed extremity. Fig.
7
is the beautifully notched antenna of the cardinal beetle (
Pyrochróa
), and Fig.
11
is the fan-like one of the common cockchafer. This specimen is taken from a male insect, and the reader will find his trouble repaid on mounting one of these antennæ as a permanent object.
Fig.
12
is an antenna from one of the common ground beetles (
Cárabus
) looking like a string of elongated pears, from the form of the joints. The reader will see that in beetles he is sure to find eleven joints in the antennæ.
Fig.
10
is the entire antenna of a fly (
Syrphus
), one of those pretty flies which may be seen hovering over one spot for a minute, and then darting off like lightning to hang over another. The large joint is the one on which are found those curious depressions that have already been mentioned. Fig.
8
is one of the antennæ of a tortoise-shell butterfly (
Vanessa
), showing the slender, knobbed form which butterfly antennæ assume; and Figs.
13
and 14 are specimens of moths’ antennæ, showing how they always terminate in a point. Fig.
13
is the beautiful feathery antenna of the ermine moth (
Spilosóma
); and Fig.
14
is the toothed one of the tiger moth (
Arctia caja
). In all these feathered and toothed antennæ of moths, the male insects have them much more developed than the female, probably for the purpose of enabling them to detect the presence of their mates, a property which some possess in wonderful perfection. The male oak-egger moth, for example, can be obtained in any number by putting a female into a box with a perforated lid, placing the box in a room, and opening the window. In the course of the evening seven or eight males are seen to make their appearance, and they are so anxious to get at their intended mate that they will suffer themselves to be taken by hand.
Fig.
9
is an antenna of the male gnat, a most beautiful object, remarkable for the delicate transparency of the joints, and the exquisitely fine feathering with which they are adorned.
We now arrive at the eyes of the insects, all of which are very beautiful, and many singularly full of interest.
In the centre of Plate
VI
. may be seen the front view of the head of a bee, showing both kinds of eyes, three simple eyes arranged triangularly in the centre, and two large masses, compound eyes, at the sides.
The simple eyes, termed “ocelli,” are from one to three in number, and usually arranged in a triangular form between the two compound eyes. Externally they look merely like shining rounded projections, and can be seen to great advantage in the dragon-flies. The compound eyes may be considered as aggregations of simple eyes, set closely together, and each assuming a more or less perfect six-sided form. Their number varies very greatly; in some insects, such as the common fly, there are about four thousand of these simple eyes in one compound one, in the ant only fifty, in the dragon-fly about twelve thousand, and in one of the beetles more than twenty-five thousand.
Fig.
18
shows a portion of the compound eye of the Atalanta butterfly, and Fig.
20
the same organ of the death’s-head moth. A number of the protecting hairs may be seen still adhering to the eye of the butterfly. Fig.
22
is a remarkably good specimen of the eye of a fly (
Helióphilus
), showing the facets, nearly square, the tubes to which they are attached, and portions of the optic nerves. Fig.
23
is part of the compound eye of a lobster, showing the facets quite square. All these drawings were taken by the camera lucida from my own preparations, so that I can answer for their authenticity.
On Plate VIII. Figs. 6 and 12, the reader will find two more examples of eyes, these being taken from the spiders. Fig.
6
is an example of the eight eyes of the well-known zebra spider, so common on our garden walls and similar situations, hunting incessantly after flies and other prey, and capturing them by a sudden pounce. The eyes are like the ocelli of insects, and are simple in their construction. The number, arrangement, and situation of the eyes is extremely varied in spiders, and serves as one of the readiest modes of distinguishing the species. Fig.
12
, Plate VIII., represents one of the curious eyes of the common harvest spider, perched on a prominence or “watch-tower” (as it has been aptly named), for the purpose of enabling the creature to take a more comprehensive view of surrounding objects.
Returning to Plate VI., in Fig.
21
we see a curiously branched appearance, something like the hollow root of a tree, and covered with delicate spiral markings. This is part of the breathing apparatus of the silkworm, extracted and prepared by myself for the purpose of showing the manner in which the tubes branch off from the “spiracle” or external breathing-hole, a row of which may be seen along the sides of insects, together with the beautiful spiral filament which is wound round each tube for the purpose of strengthening it. One of these spiracles may be seen in the neck of the gnat (Fig.
27
). Another spiracle, more enlarged, may be seen on Plate VII. Fig.
34
, taken from the wireworm,
i.e.
the larva of the skipjack beetle (
Eláter
), to show the apparatus for excluding dust and admitting air. The object of the spiral coil is very evident, for as these breathing-tubes extend throughout the whole body and limbs, they would fail to perform their office when the limbs were bent, unless for some especial provision. This is achieved by the winding of a very strong but slender filament between the membranes of which the tube is composed, so that it always remains open for the passage of air throughout all the bends to which it may be subjected. Flexible tubes for gas and similar purposes are made after the same fashion, spiral metal wire being coiled within the india-rubber pipe. A little piece of this thread is seen unwound at the end of a small branch towards the top, and this thread is so strong that it retains its elasticity when pulled away from the tube, and springs back into its spiral form. I have succeeded in unwinding a considerable length of this filament from the breathing-tube of a humble bee.
Fig.
28
represents the two curious tubercles upon the hinder quarters of the common green-blight, or Aphis, so very common on our garden plants, as well as on many trees and other vegetables. From the tips of these tubercles exudes a sweet colourless fluid, which, after it has fallen upon the leaves, is popularly known by the name of honey-dew. Ants are very fond of this substance, and are in the habit of haunting the trees upon which the aphides live, for the purpose of sucking the honey-dew as it exudes from their bodies. A drop of this liquid may be seen on the extremity of the lower tubercle.
The head of the same insect may be seen in Fig.
24
, where the reader may observe the bright scarlet eye, and the long beak with which the aphis punctures the leaves and sucks the sap. Fig.
29
is the head of the sheep-tick, exhibiting the organ by which it pierces the skin of the creature on which it lives. Fig.
25
is the head of another curious parasite found upon the tortoise, and remarkable for the powerful hooked apparatus which projects in front of the head.
Turning to Plate VII. Fig.
4
, we find the head of a ground beetle (
Cárabus
), valuable as exhibiting the whole of the organs of the head and mouth.
Immediately above the compound eyes are seen the roots of the antennæ, those organs themselves being cut away. Above there are two pairs of similarly constructed organs termed the “maxillary palpi,” because they belong to the lesser jaws or maxillæ, seen just within the pair of great curved jaws called the mandibles, which are extended in so threatening a manner. The “labial palpi,” so called because they belong to the “labium,” or under lip, are seen just within the others; the tongue is seen between the maxillæ, and the chin or “mentum” forms a defence for the base of the maxillæ and the palpi. A careful examination of a beetle’s mouth with the aid of a pocket lens is very instructive as well as interesting.
Fig.
1
on the same Plate shows the jaws of the hive bee, where the same organs are seen modified into many curious shapes. In the centre may be seen the tongue, elongated into a flexible and hair-covered instrument, used for licking the honey from the interior of flowers. At each side of the tongue are the labial palpi, having their outermost joints very small, and the others extremely large, the latter acting as a kind of sheath for the tongue. Outside the labial palpi are the maxillæ, separated in the specimen, but capable of being laid closely upon each other, and outside all are the mandibles.
VI.
VI.
The curiously elongated head of the scorpion-fly (
Panorpa
), seen at Fig.
7
, affords another example of the remarkable manner in which these organs are developed in different insects. Another elongated head, belonging to the daddy long-legs, is seen in Plate VI. Fig.
27
, and well shows the compound eyes, the antennæ, and the palpi. Fig.
2
represents the coiled tongue of the Atalanta butterfly; it is composed of the maxillæ, very greatly developed, and appearing as if each had originally been flat, and then rolled up so as to make about three-fourths of a tube. A number of projections are seen towards the tip, and one of these little bodies is shown on a larger scale at Fig.
3
. These curious organs have probably some connection with the sense of taste. Along the edges of the semi-tubes are arranged a number of very tiny hooks, by means of which the insect can unite the edges at will.
Fig.
11
, in the centre of the Plate, shows one of the most curious examples of insect structure, the proboscis or trunk of the common bluebottle-fly. The maxillary palpi covered with bristles are seen projecting at each side, and upon the centre are three lancet-like appendages, two small and one large, which are used for perforating various substances on which the insect feeds. The great double disc at the end is composed of the lower lip greatly developed, and is filled with a most complex arrangement of sucking-tubes, in order to enable it to fulfil its proper functions. The numerous tubes which radiate towards the circumference are strengthened by a vast number of partial rings of strong filamentary substance, like that which we have already seen in the breathing-tube of the silkworm. Some of these partial rings are seen on Fig.
12
, a little above. The mode in which the horny matter composing the rings is arranged upon the tubes is most wonderful, and requires a tolerably high power to show it. The fine hairs upon the proboscis itself afford most admirable practice for the young microscopist. They should, when properly lighted and focused, be quite black and sharp. Any errors of manipulation will cause them to be “fuzzy.”
Fig.
5
shows the tongue of the common cricket, a most elegantly formed organ, having a number of radiating bands covered with zigzag lines, due to the triangular plates of strengthening substance with which they are furnished, instead of the rings. A portion more highly magnified is shown at Fig.
6
, exhibiting the manner in which the branches are arranged.
The legs of insects now claim our attention.
Fig.
9
, Plate VII., shows the “pro-leg” of a caterpillar. The pro-legs are situated on the hinder parts of the caterpillar, and, being set in pairs, take a wonderfully firm hold of a branch or twig by pressure toward each other. Around the pro-legs are arranged a series of sharp hooks, set with their points inwards, for greater power in holding. Fig.
10
represents one of the hooks more magnified.
Fig.
15
is the lower portion of the many-jointed legs of the long-legged spider (
Phalángium
), the whole structure looking very like the antenna of the cricket. Fig.
17
is the leg of the glow-worm, showing the single claw with which it is armed. Fig.
26
shows the foot of the flea, furnished with two simple claws. Fig.
16
is the foot of the Trombídium, a genus of parasitic creatures, to which the well-known harvest-bug belongs. Fig.
26
, Plate VI., shows the leg of the green Aphis of the geranium, exhibiting the double claw, and the pad or cushion, which probably serves the same purpose as the pads found upon the feet of many other insects. Fig.
8
is the lower portion of the leg of the ant, showing the two claws and the curious pad in the centre, by means of which the insect is able to walk upon slippery surfaces. The Típula has a foot also furnished with a single pad (see Plate VI. Fig.
30
). This organ is seen under a very high power to be covered with long hair-like appendages, each having a little disc at the end, and probably secreting some glutinous fluid which will enable the creature to hold on to perpendicular and smooth surfaces. Many of my readers will doubtless have noticed the common fly, towards the end of autumn, walking stiffly upon the walls, and evidently detaching each foot with great difficulty, age and infirmity having made the insect unable to lift its feet with the requisite force.
Fig.
21
is the foot of one of the ichneumon-flies (
Ophíon
), the hairy fringe being apparently for the purpose of enabling it to hold firmly to the caterpillar in which it is depositing its eggs, and which wriggles so violently under the infliction that it would soon throw its tormentor had not some special means been provided for the purpose of enabling the latter to keep its hold. Fig.
20
is a beautiful example of a padded foot, taken from the little red parasitic creature so plentifully found upon the dor or dung beetle (
Geotrúpes
), and of which the afflicted insect is said to rid itself by lying on its back near an ant’s nest, and waiting until the ants carry off its tormentors.
Fig.
18
is the foot of the common yellow dung-fly (plentiful in pasture lands), having two claws and two pads; and Fig.
19
shows the three pads and two claws found in the foot of the hornet-fly (
Ásilus
).
Few microscopic objects call forth such general and deserved admiration as the fore-foot of the male water-beetle (
Dytiscus
), when properly prepared and mounted, for which see Fig.
13
.
On examining this preparation under the microscope, it is seen that three of the joints are greatly expanded, and that the whole of their under surface is covered profusely with certain wonderful projections, which are known to act as suckers. One of them is exceedingly large, and occupies a very considerable space, its hairs radiating like the rays of the heraldic sun. Another is also large, but scarcely half the diameter of the former, and the remainder are small, and mounted on the extremities of delicate foot-stalks, looking something like wide-mouthed trumpets. In the specimen from which the drawing was taken the smaller suckers are well shown, as they protrude from the margin of the foot.
One of the larger suckers is seen more magnified on Fig.
14
.
Plate VIII. Fig.
1
, exemplifies the manner in which the muscles of insects do their work, being well attached in the limbs to the central tendon, and pulling “with a will” in one direction, thus giving very great strength. This leg is taken from the water boatman (
Notonecta
), and has been mounted in Canada balsam.
On Plate VII. Fig.
29
, may be seen a curiously formed creature. This is the larva of the tortoise beetle (
Cássida
), the skin having been flattened and mounted in Canada balsam. The spiracles are visible along the sides, and at the end is seen a dark fork-like structure. This is one of the peculiarities of this creature, and is employed for the purpose of carrying the refuse of its food, which is always piled upon its back, and retained in its place by the forked spines, aided probably by the numerous smaller spines that project from the side.
Fig.
33
shows part of the stomach and gastric teeth of the grasshopper. This structure may be seen to perfection in the “gizzard,” as it is called, of the great green locust of England (
Ácrida viridíssima
). The organ looks like a sudden swelling of the œsophagus, and when slit longitudinally under water, the teeth may be seen in rows set side by side, and evidently having a great grinding power. The common house cricket has a similar organ of remarkable beauty. Just above (Fig.
27
) is the corresponding structure in the hive bee, three of the teeth being shown separately at Fig.
28
.
We now cast a rapid glance at the wings of insects.
They have no analogy, except in their use, with the wings of birds, as they are not modifications of existing limbs, but entirely separate organs. They consist of two membranes united at their edges, and traversed and supported by sundry hollow branches or “nervures,” which admit air, and serve as useful guides to entomologists for separating the insects into their genera. Indeed, the general character of the wings has long been employed as the means of dividing the insect race into their different orders, as may be seen in any work on entomology. The typical number of wings is four, but it often happens that two are almost wholly absent, or that the uppermost pair are thickened into a shelly kind of substance which renders them useless for flight; while in many insects, such as the ground beetles and others, the upper wings become hardened into firm coverings for the body, and the lower pair are shrivelled and useless.
Fig.
22
shows two of the wings of a humble bee, together with their nervures, and the peculiar system by which the upper and lower pair are united together at the will of the insect. At the upper edge of the lower wing, and nearly at its extremity, may be seen a row of very tiny hooks, shown on a larger scale at Fig.
25
. These hooklets hitch into the strengthened membrane of the upper wing, which is seen immediately above them, and so conjoin the two together. The curious wing-hooks of the Aphis may be seen on Fig.
24
, very highly magnified.
Fig.
31
is the wing of the midge (
Psychóda
), that odd little insect which is seen hopping and popping about on the windows of outhouses and similar localities, and is so hard to catch. The whole wing is plentifully covered with elongated scales, and is a most lovely object under any power of the microscope. These scales run along the nervures and edges of the wings, and part of a nervure is shown more highly magnified at Fig.
32
.
At Fig.
23
is shown the wing of one of the hemipterous insects, common along the banks of ditches and in shady lanes, and known by the name of Cíxius. It is remarkable for the numerous spots which stud the nervures, one being always found at each forking, and the others being very irregularly disposed.
Fig.
30
is one of the balancers or “haltéres” of the house-fly. These organs are found in all the two-winged insects, and are evidently modifications of the second pair of wings. They are covered with little vesicles, and protected at their base by scales. Some writers suppose that the sense of smell resides in these organs. Whatever other purpose they may serve, they clearly aid in the flight, as, if the insect be deprived of one or both of the balancers, it has the greatest difficulty in steering itself through the air.
The wings of insects are mostly covered with hairs or scales, several examples of which are given in Plate VIII. Fig.
4
shows one of the scales of the Adippe or fritillary butterfly, exhibiting the double membrane—part of which has been torn away—and the beautiful lines of dots with which it is marked. The structure of the scales is further shown by a torn specimen of tiger moth scale seen on Fig.
16
. On many scales these dots assume a “watered” aspect when the focus or illumination changes, an example of which may be seen in Fig.
15
, a scale of the peacock butterfly.
Fig.
11
is one of the ordinary scales of the azure blue butterfly, and Fig.
10
shows one of the curious “battledore” scales of the same insect, with its rows of distinct dottings. Fig.
14
is one of the prettily tufted scales of the orange-tip butterfly, and Fig.
8
is the splendid branched scale of the death’s-head moth. Fig.
19
shows a scale of the sugar-runner (
Lepisma saccharína
), a little silvery creature with glistening skin, and long bristles at the head and tail, that is found running about cupboards, window-sills, and similar places. It is not easy to catch with the fingers, as it slips through them like oil; but by holding a cover-glass in a pair of forceps, and pressing it upon one of the little creatures, a number of the scales may be caused to adhere to it, and these should be mounted dry for examination. The gnats also possess very pretty scales, with the ribs projecting beyond the membrane.
VII.
VII.
Fig.
21
is a scale from the common spring-tail (
Podúra plúmbea
), a little creature which is found plentifully in cellars and other damp places, skipping about with great activity. Some flour scattered on a piece of paper is a sure trap for these little beings. Fig.
3
is one of the scales taken from the back of the celebrated diamond beetle, showing the cause of the magnificent gem-like aspect of that insect. We have in England many beetles of the same family—the weevils—which, although much smaller, are quite as splendid when exhibited under a microscope by reflected light. The wing-case or “elytron” of a little green weevil, very common in the hedges, may be seen on Plate XII. Fig.
10
.
The reader will observe that all these scales are furnished with little root-like appendages, by means of which they are affixed to the insect. Fig.
13
shows a portion of the wing of the azure blue butterfly, from which nearly all the scales have been removed, for the purpose of exhibiting the pits or depressions in which they had formerly been fastened, and one or two of the scales are left still adherent to their places. The scales are arranged in equal rows like the slates of a housetop, as may be seen on Fig.
18
, which represents part of the same wing, to show the scales overlapping each other, and the elegant form which they take near the edges of the wing, so as to form a delicate fringe. The long hair-like down which covers the legs and bodies of the moths and butterflies (which are called Lepidóptera, or scale-winged insects, in consequence of this peculiarity), is seen under the microscope to be composed of scales very much elongated, as is shown in Fig.
17
, a portion taken from the leg of a tiger moth.
The eggs of insects are all very beautiful, and three of the most curious forms are given on Plate VIII.
Fig.
2
is the empty egg of the gad-fly, as it appears when fastened to a hair of the horse. Fig.
5
represents the pretty ribbed egg of the common tortoise-shell butterfly; and Fig.
7
is the very beautiful egg of the very horrid bed-bug, worthy of notice on account of the curious lid with which its extremity is closed, by means of which the young larva creeps out as soon as it is hatched.
The feathers of birds, and the fur of animals, will furnish many examples of the eggs of parasites, some of which are of extreme beauty. The feather or hair may be mounted in a cell without disturbing the eggs, which should, however, be heated sufficiently to kill the embryo if present.
Fig.
9
shows the penetrating portions of the sting of the wasp. The two barbed stings, which seem to be the minute prototypes of the many-barbed spears of the South Sea islanders, are seen lying one at each side of their sheath, and a single barb is drawn a little to the left on a very much larger scale. It is by reason of these barbs that the sting is always left adhering to the wound, and is generally drawn wholly out of the insect, causing its death in a short while.
The sting is only found in female insects, and is supposed to be analogous to the “ovipositor” of other insects,
i.e.
the instrument by which t