[CHAPTER XLI.]
EGGS AND LARVÆ: BREEDING AND REARING.
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 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 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 securing the 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 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 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.