Fig. 68.—Steel Bone-scrapers.
Scraping a Ligamentary Skeleton.—The removal of the flesh and other animal matter from a small skeleton is accomplished by scraping the bones with various chisel-edged scrapers specially designed for this work, and by clipping and trimming on the joints with either curve-pointed or straight scissors. The principles to be learned in skeleton-scraping are comparatively few and simple. In the first place, a sufficient quantity of the connecting ligament at each joint must be left to hold the two bones together in proper shape when the specimen dries. This must not be left in a thick, unsightly mass, but requires to be scraped and trimmed down so that it is reduced to as small a quantity as will serve the purpose. In scraping the flesh off the main stem of a bone, such as the humerus, for example, always begin at the end and scrape toward the middle. The skeletons of turtles, lizards, and the like are an exception to this rule by reason of their structure, and should be scraped from the middle toward each end. Ifyou scrape from the middle of a mammalian or avian bone toward either end, before you are aware of it, you have loosened the attachment of the ligament, and have nothing left to hold the joint together. By beginning on the ligament itself, and working away from it, you can scrape it down so thin at the point of attachment that its identity is quite lost, and the point where it ends is hardly visible. This principle applies to the scraping of all ligamentary skeletons, except a few reptiles.
In cleaning bird skeletons beware of injuring the little tack like points which project downward from each of the cervical vertebræ. Have a care also for the soft bones of the coccyx, and the uncinate process which projects backward from the posterior edge of each rib. In fishes the greatest difficulty lies in leaving the ribs attached to the remainder of the skeleton, for if the operator is at all as the writer used to be in the days of his youth, he will be prone to scrape some of the ribs loose, and be obliged to glue them in place in the dry skeleton, with glue and cotton batting that has been clipped up finely with a sharp pair of scissors.
While a small skeleton is undergoing the scraping process it must not be allowed to get dry until it is finally set up in position. When the skeleton is not being worked upon, it must be kept soaking in clean water; but remember that this cannot go on very long, or maceration will set in, the ligaments will give way, and the bones will all come apart. A little borax in the water serves to arrest decomposition, and will allow a skeleton to remain soaking for several days longer than could otherwise be allowed. After a skeleton has been well scraped, in order to get it as white as possible and free from grease, it must be treated with
Javelle Water.1/2 pound chloride of lime.1 pound common washing soda.1 gallon of boiling water.
Keep this on hand in a glass-stoppered jar, in the dark. In using it, draw off a small quantity in a broad, shallow, earthen dish. Lay every small skeleton in it, and with a soft tooth-brush of the right size, brush all the bones thoroughly for about five minutes. At the end of that process wash the skeleton thoroughly with clear water, and perhaps it is then ready to mount.
Often the bones of a small skeleton contain an inordinate amount of grease. The easiest and simplest way to remove it is to soak the greasy bones for several days or weeks, as may be necessary, in a jar of pure naphtha.
Mounting a Small Skeleton.—The skeleton of every bird, mammal, and reptile requires to have the spinal cord replaced by a stout zinc wire, to give both strength and rigidity to the structure. Zinc wire is necessary because iron wire will rust, and brass wire is too expensive to use when something cheaper and better is obtainable. If you cannot procure zinc wire, use good galvanized iron wire. For very large specimens you may use iron wire, but it must be covered with two coats of asphaltum, applied with a brush, like black paint. After inserting the wire the full length of the cavity of the spinal cord, leave enough of the end protruding beyond the first vertebra of the neck to afford a means for the attachment of the skull. The extra length to be allowed should always be nearly equal to the lateral depth of the brain cavity.
Attitude.—It is often somewhat difficult to decide upon the attitude the skeleton is to have when finished. The possibilities in this line are extensive, and the result depends entirely upon the character of the subject, and the knowledge and good taste of the operator. In the first place, the position of the skeleton must be a correct representation of some characteristic attitude of the species. For example, a sloth skeleton should hang underneath a branch; a monkey should be climbing, or walking on a stout bough; a hyena should sneak and crouch; a passerine bird should always perch, while the penguins and the auks must stand erect on flat pedestals. If the young osteologist can do so, it will pay him well to travel several hundred miles, if need be, to see the beautiful, and even elegant, collection of skeletons and other preparations in Mr. F.A. Lucas's Department of Comparative Anatomy in the National Museum, all of the specimens in which have been prepared, mounted and displayed by Mr. Lucas and his assistant, Mr. Joseph W. Schollick. I know of no other osteological collection which in the beauty and scientific accuracy of mounting, and exhibition arrangement of its specimens, can be considered equal to this.The museum-builder may well consider it a model of its kind. Every skeleton, from that of a tiny humming-bird to a whale forty-eight feet long, is as nearly perfect as human skill can make it, and the variety of the characteristic attitudes represented in the smaller species makes this collection a particularly attractive one.
PLATE XX.Ligamentary Skeleton (Domestic Cat).—Mounted and Drying.
Process with Mammals.—We will assume that the skeleton has been carefully scraped, and is now ready for mounting. The successive steps in this work from start to finish are about as follows:
1. In case the skeleton has been dried after scraping, as is often done, it must be soaked in clear water until the ligaments are relaxed.
2. Cut a zinc or galvanized iron wire of the right length and size to replace the spinal marrow, and long enough that the upper end of it will project beyond the axis into the brain cavity of the skull. Sharpen one end of this wire so that you can force it well down into the sacrum, and insert it in its place in the spinal column.
3. Bend the vertebral column to its permanent shape. In doing this, draw the sternum well forward so that the ribs will spread out, and show a chest cavity of the right size for inflated lungs. If you are not careful in this regard, the chest cavity will be too narrow.
4. Hang the body in a frame made of light strips of wood, as shown in the accompanying plate. Let the body hang at just the right height from the pedestal to receive the legs (Plate XX.).
5. Space the ribs carefully by starting a thread from the neck, and taking a turn around each rib from the first to the last, finally making fast the remaining end of the thread to one of the lumbar vertebræ.
6. Put on each hind leg by drilling a small hole straight through the head of the femur and the socket of the pelvis (innominate bone), through which a small brass wire is to be passed and clinched down closely at each end, to hold the head of the femur firmly in place.
7. Place each leg in the attitude chosen for it, plant the footaccording to its osteological character, and pin each toe in its proper place, as shown in the accompanying plate. The leg must be held in place by attaching threads to it, and making them fast to the various parts of the gallows.
8. In putting on the foreleg, the position of the scapula must be defined with accuracy, in order to avoid placing it too low or too high, and thus making an incorrect representation of the height of the animal. Bear in mind that the scapula never lies prone upon the ribs, but is separated from them by a cushion of muscle. It is therefore necessary to leave a certain space between ribs and scapula.
9. Next cut two stiff brass wires of the proper length for the two standards that must support the skeleton (seeA AandB Bin Plate XX.). To make the U-shaped fork at the upper end of each standard, to clasp the vertebral column, heat one end of the rod red-hot, and plunge it into cold water, which softens the metal. Now put it in a vise, and with a hack-saw split the rod down the middle as far from the end as necessary. Finish neatly by rounding off the ends with a fine file, and bending them in shape with the pliers. The lower end must have a thread cut on it an inch or so in length, a neat brass "rosette" screwed upon it (R) to do duty on the top of the pedestal, and a small brass nut made to screw on underneath the pedestal, to hold the standard firmly upright. These standards need not be put in place under the skeleton until it is mounted finally on its handsomely polished, permanent exhibition pedestal.
10. Mr. Lucas has two methods for attaching a small skull to the skeleton. One is to cut a piece of cork to fit snugly in the occipital hole of the skull (foramen magnum), then pierce a hole through its centre, and fit it tightly on the projecting end of the vertebral wire, close up to the first cervical vertebra (the axis). The cork thus becomes stationary, and the skull may be put in place and removed at will.
The other method is to place the skull exactly in position on the skeleton, fitting it closely to the axis. Then drill a small hole through each side of the axis in such a manner that in its passage from top to bottom the drill will also pass through the occipital condyle of the skull. By fitting a wire through each of these holes the skull will be held fast in positionso long asthe skeleton remains in its place, right side up. If the skeleton is to be packed for shipment, the skull (unless it be very small and light) must be taken off, wrapped, and packed separately for safety in transit.
Fig. 69.—Skeleton of a Bat, as exhibited by Mr. Lucas.
11. If any bones have been broken, they must now be repaired, either by gluing them together, or by joining with a short wire fitted into the axis of each piece, and the missing particles of bone may be restored by a filling of best sinew glue mixed with plaster Paris into a paste, and applied hot, so that it will adhere. As it cools it can be shaped properly, and when thoroughly dry and hard, its surface must be dressed down with a fine file and sand-paper until the form of the bone is once more perfect. This is work which very often calls for considerable skill in the operator, but the process itself is a very simple one.
If ligaments are missing and a small bone is completely detached, it should be put on as follows: Procure some fine cottonbatting, cut it up very finely with the scissors, then apply some hot glue to the joint, lay a bit of clipped cotton upon it, and work it into the glue so that when dry it will form a false ligament and hold the bone firmly in its place without attracting any attention to the fact that the ligament has been made for the occasion.
12. Finally, transfer each skeleton to its permanent pedestal, which we will assume has been prepared while the specimen has been drying. Mr. Lucas puts all his small skeletons on handsome ebonized pedestals, which are the thingpar excellence. The limbs for his climbing animals, and the thin, black boards for his bat skeletons are also ebonized. The illustration on page 291 (Fig. 69) shows one of his bat skeletons complete, as it stands in its case, bearing a label of black letters on an olive-gray card, with no ornamentation. In the final mounting the standards are put in place, and the upper end of each fitted fast to the backbone. Each toe is fixed firmly in its place, and held down by the bent-over end of a headless pin, or by having a pin put through it, and cut off close down to the bone.
Fig. 70.—Skeleton of a Bird Mounted and Drying.
Cautions and Exceptions.—It is only the tiny skeletons, suchas mice, shrews, small squirrels, and the like, that can safely be mounted without standards. To be sure, a large cat skeletoncanbe mounted on its own legs, without any standards, and so can a man drink a pint of bad whiskey; but in each case the falling from grace will be in about the same degree, if not the same in kind also. In long-continued moist weather, ligaments are apt to soften and let large unsupported skeletons come down, without neatness, but plenty of despatch.
Birds.—The foregoing principles, which have been described in detail for small mammals, apply so fully and with such complete general similarity to birds, that it is only necessary to add the two accompanying illustrations.
Fig. 71.—Wiring a Skeleton Wing.
Reptiles.—Serpents.—The skeletons of serpents should always be scraped and mounted as ligamentary specimens, andnot macerated. The skeleton should be supported on from three to five low brass standards clasping the vertebral column at proper intervals, the body curved naturally, and the ribs spread out and spaced evenly as in life, according to the curves of the body. The skeleton looks best when placed low down on the pedestal. The ribs must be spaced with threads where the ligaments are soft, but when dry require no wires. The skeleton may be mounted in any life-like attitude, either coiled or in motion.
Lizards.—Small species are to be treated the same as small mammals.
Crocodiles and Alligators.—It is best that all saurian skeletons, even the largest, should be scraped and mounted without maceration, on account of the elaborate cartilaginous sternum and false ribs. The head requires a special standard, and the tail requires a pair, while the tip of the latter is to be pinned down with a wire. Of course the feet must rest down on the pedestal as in life. One thing which would greatly enhance the scientific value of every crocodile and alligator skeleton wouldbe the preparation and display, in its proper place, of one side of the skin of the back with its wonderful shield of bony plates nicely articulated together. This remarkable covering of the vital organs seems to have been specially designed to ward off glancing bullets, and it has saved the lives of thousands of crocodilians. (Of course this shield is not proof against a bullet fired squarely against it.) So far, all collectors and osteologists have ignored this remarkable feature of the saurians, but it should have the attention it deserves.
Fig. 72.—Skeleton of Turtle, as Exhibited.
Turtles and Tortoises.—The skeleton of a tortoise, if mounted on its feet in a life-like attitude, has the best part of its anatomy concealed by its shell. This difficulty Mr. Lucas meets occasionally by sawing out and laying back one-half the carapace, to expose the interior. The commonest method, however, is that shown in the accompanying illustration (Fig. 72), which is self-explanatory. The plastron is hinged at one side, furnished with a latch, and opens like a door. The skeleton is mounted on a single standard, which is split at the upper end like a Y, the arms bent to fit the curvature of the shell, and riveted to the carapace. Each leg is held in place by a small wire attached to the shell at its edge.
Fishes.—There is nothing in the mounting of fish skeletons that has not been fully described in the foregoing pages. Ofcourse fish skeletons are never macerated, but must be scraped and mounted with their natural ligaments in place. Each skeleton requires two brass standards, one clasping the vertebral column close to the tail, the other near the head. A very long fish, or one with a large skull, requires three standards, one for the skull and one for the middle of the body. Where only two are used for a large fish, the head requires to be supported by a wire running from the centre of the backbone.
It will be well for anyone who intends to mount a large skeleton,if he has not already a fair knowledge of osteology, to take some book which contains a description of the skeleton, for example, of the domestic cow, and familiarize himself with the names of the various bones and the different anatomical terms used in describing them. In fact it is next to impossible to describe the process of mounting a skeleton without making use of quite an array of technical terms.
In order to make our description of this intricate process as clear as possible, we will choose as our typical subject the skeleton of an American bison, and go through with it in detail, aided by an abundant supply of illustrations. We of course assume that the macerating, cleaning, and bleaching has been done.
In mounting a disarticulated skeleton, begin with the vertebral column as the key to the situation. It is, in point of fact, the keel upon which the whole structure is to be built. The vertebræ should be arranged, each in its place, and, then they should be numbered with pen and ink on the anterior articulating surface of the body of each one, beginning with the first vertebra in front of the sacrum. This vertebra (the last lumbar) should be marked No. 1, the next in front No. 2, and so on to the axis.
The next step consists in boring two holes through the sacrum from its under surface (Fig. 73,a, a) to its anterior articulating surface (b, b), and these holes should be continued on through the body of each of the succeeding vertebrae to the axis. They should come out underneath that vertebra (the axis), where the wires which pass through all these holes are afterward to betwisted together. The holes should be somewhat larger than the brass wires which are to pass through them.
It is necessary to mark the place for starting the drill into the posterior surface of each vertebra by fitting two articulating surfaces together, and passing the drill through the holes already made. The points at which the drill should come out on the anterior surface of a vertebra should be marked with a lead pencil. The beginner will find some difficulty in making the drill come through at precisely the right spot. The greatest difficulty will be experienced in getting these holes through the cervical vertebrae.
When the axis is reached, bore the holes so that they will come out underneath, about half way between each extremity of the vertebra, and about three-fourths of an inch apart.
It is just as well to now bore the holes through which the wires which fasten the axis and atlas together are to pass, though these need not be actually united until the remainder of the spinal column has been, articulated. The wires uniting the atlas and axis are smaller than those passing through the spinal column. The holes for these wires are made by boring two of them through each of the two surfaces by which the axis articulates with the atlas. These holes should come out underneath the axis. Then, placing the axis and atlas together, mark on the atlas the places through which the holes are to pass by running the drill through each of the holes already made.
The next thing to be done is to cut pieces of artificial cartilage, called "buffle," to fit the posterior articulating surface of the body of each vertebra, and each piece should be fastened to the vertebra to which it belongs by a small wire nail through its centre. The holes in each vertebra should be continued straight on through the false cartilage. Now cut a brass wire three times the length of the spinal column, double it, pull it straight, pass the two ends through the sacrum, and so on forward through all the vertebræ.
When the vertebræ have all been strung on the two wires and tightened up, it will be seen that the spinal column assumes a curve approximating very nearly to the natural one. Mark this curve with chalk on a table or a board.
Unstring the vertebræ from the wire. Then take a square rod of iron, a foot or so longer than the spinal column, and over which each of the vertebra will fit easily. Have the blacksmith flatten out one end into a sort of spear, so that it will fit snugly in the spinal canal of the sacrum (Fig. 73,c).
Fig. 73.—The Sacrum and Spinal Rod.
Drill a hole through the under surface of the sacrum, and on through the iron rod: Into this a brass pin is to be fitted atd. Bend the iron rod to correspond exactly with the curve previously marked with chalk on the board. Paint the rod black, and when it has dried place it again in the sacrum, drive in the brass pin, leaving enough of the end exposed to be seized with a pair of pliers and pulled out if desired. Now string the vertebræ over the rod and wires. If all fit properly they can then be unstrung preparatory to attaching the ribs to them.
Fig. 74.—The Attachment of the Ribs to a Vertebra.
Each rib should have a hole bored through its lower end at the middle, to come out on the inner surface (Fig. 74,b, b). Through these holes wires are to pass, as seen in the accompanying figure, and to these wires the sternum is presently to be attached.
Having arranged the ribs so that you know the place of each, take the first pair, and the first dorsal vertebra to which this pair attaches. Bore a hole with the drilling machine through the rib, beginning at the centre of the articular surface of the tubercle of the rib, directing the drill so that it will come out on the under side (Fig. 74,c, c); then drill a hole through the head of the rib (d, d). Now fit the rib to the vertebra, and with a small awl,a sharp-pointed wire, or drill, mark, through the holes already made, the points on the articular surface of the vertebra through which the holes should be drilled (a, a). Bore similar holes through the rib of the opposite side, then through the vertebra at the points marked, and the wire will pass through as in the figure.
PLATE XXI.Skeleton of an American Bison.
Continue this same process for the remaining ribs. It will be found, however, that the process of carrying a single wire through the heads of both ribs and the anterior portion of the body of the vertebra cannot be continued with all. In the last of the dorsal vertebrae the wires will have to be put through the head of the rib and the pedicles of the vertebrae into the neural canal.
Make a loop on the end of each wire, as ata, a, Fig. 74, and put the ribs on each vertebra as they belong, having only one end of the wire—the one on which is made a double loop—pulled up snugly. The other end of the wire should be left a few inches in length, but bent slightly close to the rib, to hold the latter in place.
The innominate bones should be attached to the sacrum either by two brass bolts, one passing through each side at about the middle of the articular surfaces between the sacrum and each ilium, or by passing a heavy double wire through each of these places. Before tightening permanently, apply "plaster-glue" (the mixture of glue and plaster Paris already described) to the articular surfaces between the sacrum and ilia, thus when dry making the pelvis firm.
Now that the ribs are attached to the vertebræ, and the innominate bones to the sacrum, proceed to string the vertebræ again on the wires and rod. The atlas can now be attached to the axis by passing wires through the holes previously made, after which the wires are to be twisted firmly together.
When all the vertebræ with their ribs attached have been put in place, hang the backbone to a framework similar to that used for suspending the alligator (Plate XIV.), or, what is much better, to the ceiling, by two small ropes attached at the neck and pelvis.
With the pliers now twist tightly together the wires underthe axis, then take a screw-driver and work between each pair of vertebrae from underneath, beginning with the last lumbar, and prying back toward the sacrum. By the time you have reached the axis a considerable space will have been gained. Shorten the wires by twisting them, and continue this process until the vertebrae all fit snugly together, and are tight one against the other.
Fig. 76.—Middle Joint of the Hind Leg.
The next step is to put on the sternum, which has been soaked in water containing a little washing soda, and thus made flexible. Of course it has been previously cleaned by the scraping process. A hole should be bored through the end of each sternal rib, coming out on the inner surface. The sternum is suspended temporarily by strings attached to the vertebral column, and the single wires that have previously been placed through the end of each rib are now run, one by one, through the end of the sternal rib it is to support.
Now space the ribs temporarily with a string that will hold each one of them exactly in its place. Having done this, two brass wires can now be used to hold the ribs permanently in place, running them on each side from the inferior process of the last cervical vertebræ to the transverse process of some one of the lumbar vertebra, or to the pelvis. What is much better for a large skeleton, because it is both firmer and more elegant, is a long, narrow strip of polished brass on the inside, bent carefully to fit the curve of the ribs, and fastened by a brass pin through each rib, the posterior end of the brass strip being attached to a transverse process of one of the lumbar vertebræ (see Plate XXI.). After this has been done, each rib can then be permanently fastened at top and bottom by making the loop and cutting off the long end of each wire.
Fig. 76.—Middle Joint of Hind Leg: Rear View.
The next step is to put on the tail. A hole should have been bored into the middle of the articular surface of the posterior end of the sacrum, and on each side a little hole coming out below (see Fig. 73). The large middle wire (e, e) should be ofstiff brass, and extend through the entire length of the tail, the tapering end being filed small so that the small vertebræ can fit over it. The small side wires of soft brass (f, f) should only extend through a few of the larger tail vertebræ, and are for making things firm.
Fig. 77.—Bones of the Foot: Side View.
To articulate the bones of the hind leg, first arrange them so as to know the precise place of each. Take first the tarsal and metatarsal bones. In articulating these it is necessary for one to use his judgment largely, and put wires through so as to make the joint firm. Bore holes through the astragalus and os calcis (Fig. 75,a, a) so as to put a double wire through these and hold them together firmly. Next send two strong double wires through these and through the other tarsal bones, and bring them out on the posterior surface of the metatarsal or canon bone (candd).
Fig. 78.—Bones of the Foot: Rear View.
Next articulate the bones of the feet. This is very simply done by passing a single heavy wire through the lower end of each half of the canon bone to each set of phalanges, making a loop at each end of the wire (see Figs. 77 and 78). In large skeletons it will frequently be found necessary to further strengthen the articulations of the phalanges by means of brass pins, as shown in the figure ata. The sesamoids are fastened on by two stiff brass pins through each atb.
The femur and tibia can be fastened together by a double brass wire passing through each condyle of the femur, and through each side of the head of the tibia, or,what is better, a strip of brass set into the middle of the joint, and fastened firmly by two stout brass pins driven transversely through from side to side, as indicated in Fig. 79.
Fig. 79.—The Knee-Joint.
The patella is fastened on by passing a wire through it and twisting it, or erecting it on a small strip of brass set into the tubercle of the tibia. The joint is further strengthened by putting a brass pin through the patella into the end of the femur.
The tibia is articulated to the lower portion of the leg, or, more properly, the pes, by putting stiff wire pins into it. The femur is articulated to the pelvis by a brass bolt. The front foot is articulated on the same principles as the hind foot.
Fig. 80.—Front View of Knee-Joint.
In articulating the knee-joint, as it is called, send two heavy wires through, letting them come out on the posterior surface of the radius and metacarpal bone, and insert two wire pins diagonally through the joint, as shown in Fig. 81. The scapula is fastened to the humerus by brass pins. The humerus, radius, and ulna are also fastened by brass pins, three in number.
Fig. 81.—Elbow Joint: Front View.
The scapula is attached to the body by two brass bolts attached to the ribs. Sections of spiral spring wire or pieces of brass tube are placed on the bolts between the scapula and the ribs, to hold the former off the latter the same distance as when the flesh surrounding the scapula was all present. As to the position of the legs, the operator must use his own judgment. It is of course to be understood that the attitude of the legs has been decided upon before their articulation began, and that the work of wiring together has been carried out in accordance with this plan. It is hard to do more with a large skeletonthan to place the legs in an easy walking attitude, of which the buffalo skeleton already figured may fairly be taken as a model.
The rod extending through the spinal canal is cut off so that the head will hang on it properly. Two brass pins are passed through the atlas, one into each occipital condyle. Two iron rods, with lacquered brass shoulders, are used to support the skeleton, as shown in the figure. These rods should be painted black.
The lower jaw is fastened to the skull by means of brass spiral spring wire, which permits it to be moved freely up and down by any one who is studying the animal's dentition.
The method of mounting any large disarticulated skeleton of a quadruped is essentially the same as that described for the buffalo, the principle variation occurring in the feet. In articulating the feet of a wolf, for example, the method of wiring the tarsal bones, carpal bones, metatarsals, metacarpals, and the phalanges, is very similar to that described for the buffalo, but the workman must here also depend largely on his own ingenuity. A single wire passes through the phalanges of each digit, and two sesamoids are fastened on by a single wire.
Where the digits are more than two in number, a wire is passed transversely through the lower ends of the metacarpals and metatarsals, and on this are placed short pieces of fine coiled brass spring, to hold the digits at proper distances from one another.
The tools used in mounting large skeletons are by no means so numerous or costly that any one need be deterred from trying his hand at practical osteology on the score of facilities or the lack of them. Of course the complete outfit of a professional osteologist includes an extensive array of tools, some of which are rather costly. The most important item is a good drilling machine, chuck and lathe, to work by foot-power. This can be procured of Goodnow & Wightman, of Boston, and in ordering it will be necessary to have a 1/4-inch hole drilled through the centre of the axle, to receive the long, steel drills of various sizes that are to drill the many holes required in the various bones.
The amateur who can not afford an expensive plant and a first-class drilling machine, can get along very well with a Millers' Falls hand-drill and a good assortment of first-class steel drills to fit it. I once saw an old German anatomist mount a cow skeleton for a Western college with hardly more tools than I could hold in one hand—but, of course, that skeleton was not mountedà la Lucas, by a considerable difference.
Itis estimated that four-fifths of the species comprised withinthe animal kingdom belong to the class of the Insecta. Fully one hundred and seventy-five thousand species of insects have already been named and described. Nevertheless vast territories teeming with insect life have been as yet only very imperfectly explored. The life-history and habits of only a few thousands of species have as yet been accurately investigated. There remains, therefore, a broad field for discovery and research in this portion of the animal creation.
Many insects are polygoneutic, that is, the species is represented by two or more annual broods, or generations, and thousands of individuals may, by careful treatment, be reared from the eggs of a single female. In the case of the higher animals the development and multiplication of individuals takes place but slowly, and it is obvious, therefore, that there is in the domain of insect life a far more convenient field for the investigation of the great problems of variation in animal forms, than among the vertebrate animals. Aberrant forms are not uncommon, especially among butterflies and moths, and are worthy of careful study. The various broods often present great and striking differences. The phenomena of seasonal and sexualdimorphism are nowhere more clearly developed than among the lepidoptera. Hybridization also often takes place between allied species of insects, especially in the case of the bombycid moths, and it is possible for the skilful entomologist to conduct investigations in this interesting department of inquiry with almost as much freedom and success as have attended the labors of the botanist in the domain of plant life.
The economic importance of the study of entomology can scarcely be overestimated. Some of the best friends of the agriculturist, as well as multitudes of his worst enemies, are found among the insects. The silkworm, the cochineal insect, and the bee have aided in the accumulation of many fortunes, and their culture has provided employment for millions of human beings. On the other hand, property worth millions of dollars is annually destroyed by insect ravages. It has been asserted by competent authorities that the depredations of the Codling moth (Carpocapsa Pomonella) have resulted, in a single year, within the limits of the State of Pennsylvania alone, in the destruction of fruit worth over a million of dollars, and the terrible Phylloxera at one time threatened the total overthrow of viticulture in Southern Europe.
Various schemes have been proposed for the classification of insects, and there is as yet only partial agreement among students upon this subject.
Insects belong to that great group of animals designated by zoologists as theArthropoda. As a means of assisting to a better understanding of the practical hints and suggestions which follow, a sketch of the classification of the Arthropoda is here given.
Animals possessing an external skeleton composed of chitinous rings, or somites, and provided with articulated limbs.
Ceratophora.
Heterometabola. For the most part undergoing only a partial metamorphosis in the development from the egg to the imago.
Metabola. Undergoing for the most part a complete metamorphosis from egg through larva and pupa to imago.
Acerata.
The Egg.—The Arthropoda are developed from eggs. The eggs of these animals are often exceedingly curious in form and remarkable in color. The eggs of insects are generally deposited upon those substances upon which the animal feeds during its larval or rudimentary stage of existence. They are most frequently found attached to the leaves and twigs of plants and trees. Some insects are carnivorous as larvæ, and deposit their eggs upon dead animal matter, or even, as the ichneumon-flies and other parasitic forms, upon the tissues of living animals. Some lay their eggs upon decaying wood, or upon the ordure of animals. Some deposit their eggs in water. The female of some of the myriapoda deposits her eggs in a mass under the bark of decaying trees, and, coiling up about them, apparently guards them with maternal instinct until they are hatched. The spawn of many of the crustacea is carried about by the female, attached in masses to the lower surface of the body. The eggs of some insects, as the cockroach and the mantis, are deposited in masses concealed within cases, and so united as to appear to form composite or multiple eggs. These are conspicuous objects. A similar arrangement is found in the case of the ova of Hydrophilus and allied aquatic Coleoptera. The eggs of the mosquito are deposited upon the surface of the water in small, boat-shaped masses, composed of from fifty to one hundred ova. The eggs of the Lepidoptera, which are generally deposited upon the leaves and blossoms of trees and plants, are not difficult to find, and have been more carefully observed and described than those of other orders. By confining impregnated females of many species of butterflies and moths in nets of gauze drawn over the branches of thefood-plant, it is often possible to obtain their eggs in considerable numbers. The insects thus confined should be supplied with food and drink. This may be done by sprinkling upon the leaves water sweetened with sugar, or preferably honey. The females of many of the bombycid moths and hawk-moths will lay freely, if enclosed in a dark box, without the presence of the food-plant. When eggs are found and their parentage is unknown, a few should be preserved as hereafter described, and the remainder should be retained and kept until they have been hatched and the perfect insect has been reared therefrom. Insect eggs may often be obtained by dissecting the gravid female, but it is always preferable to obtain them, if possible, after oviposition has taken place, since in many cases the color of the egg in the oviduct is somewhat different from what it is after having been laid.
The eggs of insects may be deprived of their vitality by immersion in alcohol or by exposure to heat. The albumen of ova coagulates at 160° F., and the temperature of the egg should not be raised above 175°. They are best killed by being placed in the stove used for drying the skins of larvæ, which is described on page 315. It is better to kill by means of a gentle heat than by immersion in alcohol, as by the latter process a change in color is sometimes produced. After they have been deprived of their vitality they may be preserved in small phials in dilute glycerine, or, if this cannot be had, in a solution of common salt. The phials should be kept tightly corked, and should be numbered by a label, written in lead pencil and placed within the bottle, to correspond with the note made in the collector's note-book giving an account of the place of discovery, the food-plant, the date when found, and the name of the insect which deposited them, if known. In the latter case it is best to put the name of the insect in the phial with the number. Unless insect eggs are preserved in a fluid they are apt in many cases to shrivel with the lapse of time and become distorted, through the drying up of their contents, which, on account of their small size, it is impossible to void. The shell of some eggs is often very neatly voided by the escape of the larva, but there is generally a large orifice left, the color is frequently materially altered, and great vigilance in securingthe shell must be exercised, as the young larvæ of many species have the curious habit of whetting their appetites for future meals by turning about, as soon as they have been hatched, and eating the shell which they have just left.
The eggs of insects are best mounted in the form of microscopic slides in glycerine jelly contained in cells of appropriate depth and diameter. It is well to mount several upon the same slide, exhibiting the lateral as well as the terminal aspect of the eggs. At the upper end of all insect eggs there are one or more curious structures, known as micropyles (little doors), through which the spermatozoa of the male find ingress and they are fertilized. The peculiar, and often very beautiful, features of this part of the egg are, in a well-mounted specimen, exposed to view. In some cases it is advisable to slice off the end of the egg with the micropyle and mount it microscopically. The best display of this curious structure is thus often obtained.
The slides should be kept in a cabinet arranged in shallow trays. They should be accurately named, and have references to a book into which, from time to time, should be carefully transcribed from the field-book the observations of the collector, or his assistants and correspondents. Such a collection of insect ova is not only valuable but intensely interesting.
The Larva.—By reference to the table of the classification of the Arthropoda, given in Chapter XL., it will be observed that the Insecta are broadly divisible into two groups, the Heterometabola and the Metabola. The animals classified in the first group do not undergo metamorphosis in the development from the egg to the perfect insect to the same extent and in the same manner as the Metabola. In this respect the Peripatidea, the Myriapoda, and the various classes included under the Acerata agree with them. The young myriapod and the young spider are found immediately after they have emerged from the egg to present most of the features of the mature insect, and so also the immature grasshopper and squash-bug resemble the perfect insect in nearly everything but size and the absence of fully developed wings. In preparing a suite of specimens of these insects, designed to illustrate their life-history, the directions which are given for the preparation of the imago applyequally well to the larva. It is simply necessary, for instance, in preparing a series of specimens of the Rocky Mountain Locust, to make sure that a specimen representing the creature after each successive moult has been secured, and these are mounted upon pins, and treated exactly as specimens of the adult insect are treated. Be careful not to pin, however, too soon after the moult.
In the case of many of the Coleoptera, and of all the Metabola the work of the collector is rendered far more laborious, for these pass from the egg into vermiform larvæ, which undergo in some cases many moults, are then transformed into pupæ, which are either naked or contained in a protecting envelope known as the cocoon, and then finally, after a longer or shorter period in the pupal state, are transformed into the perfect insect.
The student and collector, if intending to benefit science by their efforts, dare not neglect these rudimentary forms.
The larvæ of most insects which undergo a complete metamorphosis are very small when first emerging from the egg, and before they make the first moult are, for the most part, best preserved as microscopic objects in cells filled with glycerine. In the case of the larvæ of the great bombycid moths, which at the time of hatching are dark in color, it is possible to make a fairly good specimen by piercing the anal extremity of the caterpillar, and spitting it upon the extremity of a thick, black bristle, or a fine copper wire wrapped with black silk. Specimens so mounted will not shrivel greatly, and may be attached to pins and placed in the cabinet after the slide containing the egg, as the first in the series of slowly maturing forms. After each successive moult the larvæ increase rapidly in size. The method of preparing the larger forms which is now preferred by good collectors is that of inflation.
In inflating larvæ the first step is to carefully remove the contents of the larval skin. This is best effected by making an incision with a stout pin or needle at the anus, and then, between the folds of a soft towel, gently pressing out the contents of the abdominal cavity. The pressure should be first applied near the point where the pellicle has been punctured, and then be carried forward until the region of the head is reached. Greatcare must be exercised not to apply such a degree of pressure as will expel those tissues lying nearest to the epidermis, in which the pigments are located, and in the case of hairy larvæ not to rob them of their hair. Practice can alone make perfect in this regard. The contents of the larva having been removed, the next step is to inflate and dry the empty skin. Some persons, as preliminary to this step, recommend that the empty skin be soaked for a period of a few hours in pure alcohol. By this process undoubtedly a certain portion of the watery matter contained in the pellicle is removed, and the process of desiccation is facilitated, but it is objectionable in the case of all larvæ having light colors, because these are more or less effaced by the action of the alcohol.
The simplest method of inflating the skins of larvæ after the contents have been withdrawn is to insert a straw or grass stem of appropriate thickness into the opening through which the contents have been removed, and then by the breath to inflate, while holding over the chimney of an Argand lamp, the flame of which must be regulated so as not to scorch or singe the specimen. Care must be taken in the act of inflating not to unduly extend the larval skin, thus producing a distortion, and also to dry it thoroughly. Unless the latter precaution is observed a subsequent shrinking and disfigurement will take place. The process of inflating in the manner just described is somewhat laborious, and while some of the finest specimens, which the writer has ever seen, were prepared in this primitive manner, various expedients for lessening the labor involved have been devised, some of which are to be highly commended.