The great drawback with this arrangement is the lack of any provision for keeping the food moist and fresh, thus rendering a change necessary at very frequent intervals; but this may be obviated by using damp sand as a foundation for the little twig of food plant. With this improvement, if you cover the top of the bottle with apiece of glass, a saucer, or any impermeable substance, you may keep the twig fresh for several days, generally until the disappearance of the last leaf calls for a fresh supply; but it is very doubtful whether the damp atmosphere resulting from this inclosure is not injurious to the larvae. It certainly does not seem to have much influence on some, but the unhealthy conditions that result must be detrimental to the inmates. It must also be remembered that many species require a _dry_ soil in which to burrow when about to change. When the time comes for the change of food, great care must be taken not to injure young and small larvae. In many cases they need never be touched, for if a fresh twig be placed beside the stale one, they will readily find their way to it; and to facilitate this, and also to afford a convenient foothold to those larvae that accidentally fall from the twig, the layer of sand at the bottom of the bottle should be covered with moss or cocoa-nut fibre. [Illustration: FIG. 57.--A LARVA GLASS.] [Illustration: FIG. 58.--A LARVA GLASS.] If you find it necessary to move the larvae yourself from the stale food, never touch them with your fingers, but lift them gently by means of a small camel-hair brush. Larger larvae need never be moved at all. They will always search out fresh food for themselves, and the stale may be removed after they have quitted it. For rearing larger species ordinary bottles are hardly satisfactory, and we must either use large jars or construct cages of some kind. An ordinary bell jar such as is used for covering ferns or for aquaria makes a very useful 'larva glass.' Place a small bottle of water at the bottom, and then introduce sufficient dry clean sand or sifted soil to reach up to its neck. On the top of this place a layer of moss or cocoa-nut fibre. Next introduce the food plant, fixing it firmly in the bottle of water, and plugging up the space between the stem and the rim with cotton wool. This precaution is to prevent the larvae from falling into the water as they attempt to pass up or down the stem, and the wool also helps to keep the twig in a vertical position. The glass is now ready for the caterpillars, but it is advisable to keep a covering of muslin or gauze over the top in all cases even though the larvae contained are unable to creep up the surface of glass, for the great enemies of caterpillars--the ichneumon flies--are always on the alert, and will often take advantage of an open window to 'sting' the larva rearer's pets.