The age to which the larva generally attains before this change takes place is exceedingly variable. To some it is permitted to reach the venerable age of four or five years; others, three; and many, not nearly so much. The dung and fungus-feeding insects, as if hastened forward by the warmth of their position, are at the end of their larva life in a week; others live a fortnight in this state; and others, several months. We are told of a curious anecdote of a beetle which had existed in a desk, set up in an office in London, for upwards of twenty years, half of which period, according to Messrs. Kirby and Spence, it must have lived as a larva. But this is quite an exception to the general rule. For some days before, the larva loses its long distinguishing characteristic--its appetite--and now forsakes the plants upon which it formerly fed; or if it does not forsake them, no longer consumes any part of them. It becomes, apparently, in some degree sensible that it is approaching the most dangerous period of the history of the insect--the state of sleep or torpor in which it is to exist as a pupa--of which we shall speak more particularly in our next chapter. Before, although a weak and comparatively a defenceless creature, it was still able to exercise a vigilant watch against its enemies, and thus to avoid them, or to prepare for their reception; but now, its eyes are to be closed in a death-like slumber; its body to hang a tempting bait to any wandering foe, or hungry bird, and it can in no way resist the attacks of the feeblest adversary. As if fully aware of this, the wonderful instinct of the larva meets the difficulty by teaching it to select the best place for its concealment from a dangerous notice. With this view, it sets out in search of a hiding-place, and rests not until it has obtained a city of refuge which may prove to it a defence against the enemy. It may be some little recess, covered by an irregular stone, and sheltered round the sides by moss and the upspringing grass; or, perhaps, a little hole in the earth has received the poor fugitive from future harm, and in order to conceal itself more completely, it has drawn over the mouth of the cave a covering of leaves. Here, safe from observation, the larva bids a farewell--to speak as the poets speak--to the busy staring world for awhile, and loses all recollection of its past history, in the night of torpidity upon which it has now entered. The larvæ of the lady-bird, and other insects, glue themselves fast to the under side of a twig or leaf, and here pass the days of their sleep in safety. Many of the larvæ of insects whose life, up to this period, has been passed beneath the surface of some glassy pool, now quit the pure element, and seek the shore, on which they are to pass the remaining portion of their existence.