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 shellac is placed upon their tips, and they are then inserted into the opening at the anal extremity of the larval skin. Upon the release of pressure they spread apart, and after the shellac 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 larvæ of Coleoptera and Diptera as well as to those of the Lepidoptera.
[a]Fig. 85.]—Wire Bent into Shape for Mounting Larva. (After Riley.)
An account of the manner of preserving larvæ would not be complete without an account of the manner of rearing them. In rearing the larvæ of Coleoptera, Diptera, and Hymenoptera, the student must be left in a large degree to his own devices. A few large glass 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 larvæ 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 larvæ of the Coleoptera. The larvæ of many beetles which are carnivorous may be reared in glass 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 larvæ 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 larvæ of other species which might be introduced. The cages should have a sufficient supply of moisture, and, so far as possible, the circumstances should be made to approximate those under which the larvæ were found. The larvæ 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 larvæ of leaf-eating beetles may be bred as the larvæ of lepidoptera. The larvæ 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 larvæ, and the larvæ of ants. The larvæ 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.
[a]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 glass sides and hinged door; c, removable cap of box covered with wire gauze.
The breeding of the larvæ 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 glass. 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 butterflies, 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 larvæ 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 larvæ 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.
[a]Fig. 87.]—Breeding Cage. B, Jar with food-plant; E, box with soil; G, gauze lid.
In the breeding of larvæ experience must be the great instructor, and practice can alone make perfect. No department of entomological study is, however, quite so fascinating as this, even though its prosecution may be somewhat laborious.