Part 33 (1/2)
Some insects are carnivorous as larvae, 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 ma.s.s under the bark of decaying trees, and, coiling up about them, apparently guards them with maternal instinct until they are hatched. The sp.a.w.n of many of the crustacea is carried about by the female, attached in ma.s.ses to the lower surface of the body. The eggs of some insects, as the c.o.c.kroach and the mantis, are deposited in ma.s.ses 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 ma.s.ses, 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 b.u.t.terflies and moths in nets of gauze drawn over the branches of the food-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 alb.u.men 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 larvae, 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 sh.e.l.l 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 securing the sh.e.l.l must be exercised, as the young larvae of many species have the curious habit of whetting their appet.i.tes for future meals by turning about, as soon as they have been hatched, and eating the sh.e.l.l 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 a.s.sistants and correspondents. Such a collection of insect ova is not only valuable but intensely interesting.
THE LARVA.--By reference to the table of the cla.s.sification 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 cla.s.sified 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 cla.s.ses 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 gra.s.shopper 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 ill.u.s.trate their life-history, the directions which are given for the preparation of the imago apply equally 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 pa.s.s from the egg into vermiform larvae, which undergo in some cases many moults, are then transformed into pupae, which are either naked or contained in a protecting envelope known as the coc.o.o.n, 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 larvae 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 larvae 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 a.n.a.l 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 larvae increase rapidly in size. The method of preparing the larger forms which is now preferred by good collectors is that of inflation.
In inflating larvae 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 a.n.u.s, 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. Great care 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 larvae 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 larvae having light colors, because these are more or less effaced by the action of the alcohol.
The simplest method of inflating the skins of larvae after the contents have been withdrawn is to insert a straw or gra.s.s 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.
[Ill.u.s.tration: FIG. 82.--Apparatus for Inflating Larvae. B, Foot-bellows; K, rubber tube; C, flask; D, anhydrous sulphuric acid; E, overflow flask; F, rubber tube from flask; G, standard with c.o.c.k to regulate flow of air; H, gla.s.s tube with larva upon it; I, copper drying-plate; J, spirit-lamp.]
A comparatively inexpensive arrangement for inflating larvae is a modification of that described in the ”Entomologische Nachrichten,” 1879, vol. v., p. 7, devised by Mr. Fritz A. Wachtel. It consists of a foot-bellows such as is used by chemists in the laboratory, or, better still, of a small cylinder such as is used for holding gas in operating the oxy-hydrogen lamp of a sciopticon. In the latter case the compressed air should not have a pressure exceeding fifty pounds to the square inch, and the c.o.c.k regulating the flow from the cylinder should be capable of very fine adjustment. By means of a rubber tube the air is conveyed from the cylinder to a couple of flasks, one of which contains concentrated sulphuric acid and the other is intended for the reception of any overflow of the hydrated sulphuric acid which may occur. The object of pa.s.sing the air through sulphuric acid is to rob it, so far as possible, of its moisture. It is then conveyed into a flask, which is heated upon a sand-bath, and thence by a piece of flexible tubing to a tip mounted on a joint allowing vertical and horizontal motion and secured by a standard to the working-table. The flow of air through the tip is regulated by a c.o.c.k.
Upon the tip is fastened a small rubber tube, into the free extremity of which is inserted a fine-pointed gla.s.s tube. This is provided with an armature consisting of two steel springs fastened upon opposite sides, and their ends bent at right angles in such a way as to hold the larval skin firmly to the extremity of the tube. The skin having been adjusted upon the fine point of the tube, the bellows is put into operation and the skin is inflated. A drying apparatus is provided in several ways. A copper plate mounted upon four legs, and heated by an alcohol lamp placed below, has been advocated by some. A better arrangement, used by the writer, consists of a small oven heated by the flame of an alcohol lamp, or by jets of natural gas, and provided with circular openings of various sizes, into which the larval skin is introduced. (See Fig. 83.) A modification of the oven is given in Fig. 84.
[Ill.u.s.tration: FIG. 83.--Drying Oven. A, Lamp; B, pin to hold door open; C, door open; D, gla.s.s cover.]
[Ill.u.s.tration: FIG. 84.--Oven for Drying Larva-skin, made of tin joined without solder and with top made of gla.s.s. (After Riley.)]
A less commendable method of preserving larvae is to place them in alcohol.
The larvae should be tied up in sacks of light gauze netting, and a label of tough paper with the date and locality of capture, and the name, if known, written with a lead pencil, should be attached to each such little sack.
Do not use ink on labels to be immersed, but a hard lead pencil. Alcoholic specimens are liable to become shrivelled and discolored, and are not nearly as valuable as well-inflated and dried skins.
When the skins have been inflated they may be mounted readily by being placed upon wires wrapped with green silk, or upon annealed aluminium wire.
The wires are bent and twisted together for a short distance and then made to diverge as in Fig. 85. The diverging ends are pressed together, a little sh.e.l.lac is placed upon their tips, and they are then inserted into the opening at the a.n.a.l extremity of the larval skin. Upon the release of pressure they spread apart, and after the sh.e.l.lac has dried the skin is firmly held by them. They may then be attached to pins by simply twisting the free end of the wire about the pin, or they may be placed upon artificial imitations of the leaves and twigs of their appropriate food-plants. This method of preparation is applicable to the larvae of Coleoptera and Diptera as well as to those of the Lepidoptera.
[Ill.u.s.tration: FIG. 85.--Wire Bent into Shape for Mounting Larva. (After Riley.)]
An account of the manner of preserving larvae would not be complete without an account of the manner of rearing them. In rearing the larvae of Coleoptera, Diptera, and Hymenoptera, the student must be left in a large degree to his own devices. A few large gla.s.s jars capable of being closed with a gauze top are necessary, though in the case of the Hymenoptera reliance must be mainly placed upon finding the larvae in their nests. Bees and wasps construct various larval edifices, and these must be explored as found in nature for a knowledge of the immature insect. Breeding them in captivity is attended by difficulties which are rarely overcome by the most expert, except in a few isolated cases. This is also true, but to a less extent of the larvae of the Coleoptera. The larvae of many beetles which are carnivorous may be reared in gla.s.s jars, or boxes, covered with fine wire gauze, at the bottom of which earth or sand has been placed, and in which a supply of appropriate food can be put, such as the soft larvae of beetles, maggots, and bits of meat. It is best to previously scald the earth and sand placed at the bottom of the breeding cages in order to destroy the eggs and small larvae of other species which might be introduced. The cages should have a sufficient supply of moisture, and, so far as possible, the circ.u.mstances should be made to approximate those under which the larvae were found. The larvae of wood-boring beetles may be bred in portions of the wood which they frequent. A tight barrel with a cover made of wire gauze fitting closely over the top is a good device. In the fall of the year it may be filled with fallen twigs and pieces of branches from the forest, on which beetles have oviposited, and in the spring there will be generally found a large number of beautiful specimens of species, some of which are otherwise very difficult to secure. The barrels should be placed in a covered spot in the open air, and the twigs and wood occasionally lightly moistened with water. The larvae of leaf-eating beetles may be bred as the larvae of lepidoptera. The larvae of neuropterous insects, such as Myrmeleon, may be easily reared in boxes at the bottom of which sand to the depth of six inches has been placed. They may be fed with house-flies which have been deprived of their wings, and soft bodies of coleopterous larvae, and the larvae of ants. The larvae of the Odonata and aquatic beetles must be reared in aquaria in which there is a muddy bottom provided, and in which there are a few pieces of rotting wood, with loose bark upon it, so that they protrude some inches above the surface of the water, and in which aquatic plants are kept growing. Many aquatic insects pupate under the bark of trees growing at the edge of the water.
[Ill.u.s.tration: FIG. 86.--Breeding Cage. (After Riley.) _a_, Bottom board; _g g_, battens to prevent warping; _f f_, zinc pan four inches deep; _d_, zinc tube soldered to bottom of pan and intended to hold jar of water for food-plants; _e_, earth in pan; _b_, box with gla.s.s sides and hinged door; _c_, removable cap of box covered with wire gauze.]
[Ill.u.s.tration: FIG. 87.--Breeding Cage. B, Jar with food-plant; E, box with soil; G, gauze lid.]
The breeding of the larvae of lepidopterous insects has received far greater attention than that of other insects, and many modifications of devices for this purpose have been suggested. The simplest devices are often the best, and in the early stages of the smaller forms the best plan is to pot a specimen of the appropriate food-plant, when it is low and herbaceous and capable of being thus treated, and then put it under a cover of tarletan or under a bell gla.s.s. When the larva undergoes its transformations in the ground a bed of earth several inches in depth, upon which some dead leaves and litter are placed, should be provided. A convenient form of a breeding-cage is represented in Fig. 86. Mr. W.H. Edwards, who has done more than any other person to elucidate the life-history of North American b.u.t.terflies, often uses a breeding-cage made of a nail keg, the top of which has been knocked out, and over which gauze netting is tied. The writer has successfully employed, for breeding moths upon a large scale, common store boxes, with about eight inches in depth of good soil at the bottom, covered with a close-fitting frame lid over which mosquito-netting is tacked. Branches of the food-plant are set into the box in jars of water, in which they remain fresh for several days (see Fig. 87). If possible, and if operations are to be prosecuted upon a large scale, it is well to appropriate to breeding purposes a small room from which all the furniture and carpets have been removed. The windows should be closed with gauze netting tacked over them, and the doors should also be made tight so as to prevent the escape of the insects. When the caterpillars descend from the food-plants which are placed in the apartment in jars of water, or in pots, and thus indicate their readiness to undergo transformation, they should be secured and placed in smaller boxes fitted up as before described, and, in case the insect pupates in the soil, provided with a sufficient depth of earth. In case it is desired to go to still greater expense, a small house, arranged after the manner of a greenhouse, and with suitable cages and compartments, may be provided. Such an insect-house exists at Cornell University, and is under the care of that admirable investigator, Professor Comstock, who no doubt would be glad to furnish students with a knowledge of the details of its construction. The larvae of many lepidopterous insects emerge from the egg in the fall of the year, and after feeding for a time and undergoing one or two moults, hibernate, and upon the return of the springtime begin feeding again, and finally pupate.
It is best in the case of such to leave the larvae in the fall in a cold place, as an icehouse, and to suffer them to remain there until an abundant supply of the proper food-plant can be obtained.