Many caterpillars of _Pieris rapæ_ have, during this autumn, fed below my windows. On searching for suitable positions for passing into chrysalides, some eight or ten individuals, in their direct march upwards, encountered the plate-glass panes of my windows; on these they appeared to be unable to stand. Accordingly in every case they made silken ladders, some of them five feet long, each ladder being formed of a single continuous thread, woven in elegant loops from side to side. . . . The reasoning, however, seems to be but narrow, for one ladder was constructed parallel to the window-frame for nearly three feet, on which secure footing could be had by simply diverting the track two inches.[100] In this case it appears clear that we have to do with instinct, and not with reason. No doubt it is the congenital habit of these caterpillars to overcome impediments in this way; but the instinct is one of sufficient interest to be here stated. The following is quoted from Kirby and Spence:-- A caterpillar described by Bonnet, which, from being confined in a box, was unable to obtain a supply of the bark with which its ordinary instinct directs it to make its cocoon, substituted pieces of paper that were given to it, tied them together with silk, and constructed a very passable cocoon with them. In another instance the same naturalist having opened several cocoons of a moth (_Noctura verbasci_), which are composed of a mixture of grains of earth and silk, just after being finished, the larvæ did not repair the injury _in the same manner_. Some employed both earth and silk; others contented themselves with spinning a silken veil before the opening.[101] The same authorities state, as result of their own observation, that the-- Common cabbage caterpillar, which, when building web under stone or wooden surfaces, previously covers a space with a web to form a base for supporting its dependent pupa, when building a web beneath a muslin surface dispenses with this base altogether: it perceives that the woven texture of the muslin forms facilities for attaching the threads of the cocoon securely enough to support the weight of the cocoon without the necessity of making the usual square inch or so of basal support.[102] The instincts of the larva of the _Tinea_ moth are thus described by Réaumur:--