Common Objects of the Microscope
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A very remarkable, and not uncommon, fresh-water diatom is the Bacillária paradóxa. It looks, when at rest, like a broad brown ribbon of varying length. The diatoms lie across the ribbon, on edge, and slide upon each other exactly like the ladders of a fire-escape, so that the broad ribbon is converted into a fine long thread, which speedily closes up again into the original ribbon, and so da capo. The reason for this movement, and how it is effected, is absolutely unknown; indeed, nothing certain is known as to the way in which diatoms move, nor has ever a probable guess yet been made.
The last of the diatoms which we shall be able to mention in this work is that represented on Fig. 31. The members of this genus have the name of Navícula, on account of their boat-like shape and their habit of gliding through the water in a canoe-like fashion. There are many species of this genus, all of which are notable for the graceful and varied courses formed by their outlines, and the extreme delicacy of their markings. In many species the markings are so extremely minute that they can only be made out with the highest powers of the microscope and the most careful illumination, so that they serve as test objects whereby the performance of a microscope can be judged by a practical man.
The large spherical figure in the centre of Plate IV. represents an example of a family belonging to the confervoid algæ, and known by the name of Volvox globator. There seems to be but one species known.
This singular plant has been greatly bandied about between the vegetable and animal kingdoms, but seems now to be satisfactorily settled among the vegetables. In the summer it may be found in pools of water, sufficiently large to be visible to the naked eye, like a little green speck proceeding slowly through the water. When a moderate power is used, it appears as shown in the figure, and always contains within its body a number of smaller individuals, which after a while burst through the envelope of the parent and start upon an independent existence. On a closer examination, a further generation may be discovered even within the bodies of the children. The whole surface is profusely covered with little green bodies, each being furnished with a pair of movable cilia, by means of which the whole organism is moved through the water. These bodies are analogous to the zoospores already mentioned, and are connected with each other by a network of filaments. Reproduction also takes place by conjugation as in other algæ. A more magnified representation of one of the green bodies is shown immediately above the larger figure. The volvox is apt to die soon when confined in a bottle.
Fig. 25 is the common yeast-plant, consisting simply of a chain of cells, which increase by budding, and only form spores when they have exhausted the nutriment in the fluid in which they live. Fig. 26 is a curious object, whose scientific name is Sárcina ventrículi. It is found in the human stomach. Similar forms are often to be found in the air; for instance, a piece of cocoa-nut will exhibit this, and many other kinds of Bacteria and moulds, after a few days’ exposure to the air, preferably in a dark cupboard.
We now come upon a few of the blights and mildews. A very interesting series of forms is first to be alluded to. Upon the bramble-leaf may often be found spots, at first red, then orange, then reddish black. These are known as Œcidium berberidis. Fig. 32 shows the “red-rust” of wheat, the Urédo; and Fig. 33 is the mildew of corn, known as Puccinia. The interest lies in the fact that these three forms are successive stages in the life-history of the same plant. Another species of Urédo, together with a Phragmídium, once thought to be another kind of fungus, is seen on a rose-leaf on Plate V. Fig. 1. On Fig. 10, however, of the same Plate, the Phragmídium may be seen proceeding from Urédo, thus proving them to be but two states of the same plant. There is room for any amount of observation and work in connection with the life-histories of many of these fungi.
Another species of Puccinia, found on the thistle, is shown on Plate V. Fig. 7. Fig. 34 is the mould found upon decaying grapes, and called therefrom, or from the clustered spores, Botrýtis. Some of the detached spores are seen by its side. Fig. 35 is another species of the same genus, termed Botrýtis parasítica, and is the cause of the well-known “potato-disease.”
The mosses and ferns afford an endless variety of interesting objects to the microscopist; but as their numbers are so vast, and the details of their structure so elaborate, they can only be casually noticed in the present work. Fig. 38 represents a spore-case of the Polypodium, one of the ferns, as it appears while in the act of bursting and scattering the contents around. One of the spores is seen more magnified below. The spore-cases of many ferns may be seen bursting under the microscope, and have a very curious appearance, writhing and twisting like worms, and then suddenly filling the field with a cloud of spores. Fig. 9, Plate V., is a piece of the brown, chaff-like, scaly structure found at the base of the stalk of male fern cells, showing the manner in which a flat membrane is formed. Fig. 39 is a capsule of the Hypnum, one of the mosses, showing the beautiful double fringe with which its edge is crowned. Fig. 2, Plate V., is the capsule of another moss, Polytríchum, to show the toothed rim; on the right hand is one of the teeth much more magnified.
Fig. 3, Plate V., is the capsule of the Jungermannia, one of the liverworts, showing the “elaters” bursting out on every side, and scattering the spores. Fig. 4 is a single elater much magnified, showing it to be a spirally coiled filament, that, by sudden expansion, shoots out the spores just as a child’s toy-gun discharges the arrow. Fig. 5 is a part of the leaf of the Sphagnum moss, common in fresh water, showing the curious spiral arrangement of secondary fibre which is found in the cells, as well as the circular pores which are found in each cell at a certain stage of growth. Just below, and to the left hand, is a single cell greatly magnified, in order to show these peculiarities more strongly. Fig. 8 is part of a leaf of Jungermannia, showing the dotted cells.
Fig. 6, Plate V., is a part of a rootlet of moss, showing how it is formed of cells elongated and joined end to end.
On the common mare’s-tail, or Equisétum, may be seen a very remarkable arrangement for scattering the spores. On the last joint of the stem is a process called a fruit-spike, being a pointed head around which are set a number of little bodies just like garden-tables, with their tops outward. One of these bodies is seen in Fig. 40. From the top of the table depend a number of tiny pouches, which are called sporangia; these lie closely against each other, and contain the spores. At the proper moment these pouches burst from the inside, and fling out the spores, which then look like round balls with irregular surfaces, as shown in Fig. 40, c. This irregularity is caused by four elastic filaments, knobbed at the end, which are originally coiled tightly round the body of the spore, but by rapidly untwisting themselves cause the spore to leap about, and so aid in the distribution. A spore with uncoiled filaments is seen at Fig. 40, b. By breathing on them they may be made to repeat this process at will.
Fig. 36 is a common little sea-weed, called Ectocarpus siliculósus, that is found parasitically adhering to large plants, and is figured in order to show the manner in which the extremities of the branches are developed into sporangia. Fig. 37 is a piece of the common green laver, Ulva latíssima, showing the green masses that are ultimately converted into zoospores, and by their extraordinary fertility cause the plant to grow with such rapid luxuriance wherever the conditions are favourable. Every possessor of a marine aquarium knows how rapidly the glass sides become covered with growing masses of this plant. The smaller figure above is a section of the same plant, showing that it is composed of a double plate of cellular tissue.
Fig. 41 is a piece of purple laver or “sloke,” Porphýra laciniáta, to show the manner in which the cells are arranged in groups of four, technically named “tetraspores.” This plant has only one layer of cells.
On Plate V. may be seen a number of curious details of the higher algæ.
Fig. 11 is the Sphacelária, so called from the curious capsule cells found at the end of the branches, and termed sphacelæ. This portion of the plant is shown more magnified in Fig. 12. Another sea-weed is represented in Fig. 13, in order to show the manner in which the fruit is arranged; and a portion of the same plant is given on a larger scale at Fig. 14.
A very pretty little sea-weed called Cerámium is shown at Fig. 15; and a portion showing the fruit much more magnified is drawn at Fig. 22. Fig. 23 is a little alga called Myrionéma, growing parasitically on the preceding plant.
Fig. 16 is a section of a capsule belonging to the Hálydris siliquósa, showing the manner in which the fruit is arranged; and Fig. 17 shows one of the spores more magnified.
Fig. 18 shows the Polysiphónia parasítica, a rather common species of a very extensive genus of sea-weeds, containing nearly three hundred species. Fig. 19 is a portion of the stem of the same plant, cut across in order to show the curious mode in which it is built up of a number of longitudinal cells, surrounding a central cell of large dimensions, so that a section of this plant has the aspect of a rosette when placed under the microscope. A capsule or “ceramídium” of the same plant is shown at Fig. 20, for the purpose of exhibiting the pear-shaped spores, and the mode of their escape from the parent-cell previous to their own development into fresh plants. The same plant has another form of reproduction, shown in Fig. 21, where the “tetraspores” are seen imbedded in the substance of the branches. There is yet a third mode of reproduction by means of “antheridia,” or elongated white tufts at the extremities of the branches. The cells produced by these tufts fertilise the rudimentary capsules, and so fulfil the function of the pollen in flowering plants.
Fig. 25 is the Cladóphora, a green alga, figured to illustrate its mode of growth; and Fig. 26 represents one of the red sea-weeds, Ptilóta élegans, beautifully feathered, and with a small portion shown also on a larger scale, in order to show its structure more fully. A good contrast to this species is seen on Fig. 27, and the mode in which the long, slender, filamentary fronds are built up of many-sided cells is seen just to the left hand of the upper frond. Fig. 24 is a portion of the lovely Delesséria sanguínea, given in order to show the formation of the cells, as also the arrangement by which the indistinct nervures are formed.
V.
V.
The figure on the bottom left-hand corner of Plate V. is a portion of the pretty Nitophyllum lacerátum, a plant belonging to the same family as the preceding one. The specimen here represented has a gathering of spores upon the frond, in which state the frond is said to be “in fruit.”
Fig. 27 represents a portion of the common sea-grass (Enteromorpha), so common on rocks and stones between the range of high and low water. On the left hand of the figure, and near the top, is a small piece of the same plant much more magnified, in order to show the form of its cells.
CHAPTER 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.