Let us consider the Lycosa in particular.  In her, at the moment of the exodus, a sudden instinct arises, to disappear, as promptly and for ever, a few hours later.  This is the climbing-instinct, which is unknown to the adult and soon forgotten by the emancipated youngling, doomed to wander homeless, for many a long day, upon the ground.  Neither of them dreams of climbing to the top of a grass-stalk.  The full-grown Spider hunts trapper-fashion, ambushed in her tower; the young one hunts afoot through the scrubby grass.  In both cases there is no web and therefore no need for lofty contact-points.  They are not allowed to quit the ground and climb the heights.

Yet here we have the young Lycosa, wishing to leave the maternal abode and to travel far afield by the easiest and swiftest methods, suddenly becoming an enthusiastic climber.  Impetuously she scales the wire trellis of the cage where she was born; hurriedly she clambers to the top of the tall mast which I have prepared for her.  In the same way, she would make for the summit of the bushes in her waste-land.

We catch a glimpse of her object.  From on high, finding a wide space beneath her, she sends a thread floating.  It is caught by the wind and carries her hanging to it.  We have our aeroplanes; she too possesses her flying-machine.  Once the journey is accomplished, naught remains of this ingenious business.  The climbing-instinct conies suddenly, at the hour of need, and no less suddenly vanishes.

CHAPTER VII:  THE SPIDERS’ EXODUS

Seeds, when ripened in the fruit, are disseminated, that is to say, scattered on the surface of the ground, to sprout in spots as yet unoccupied and fill the expanses that realize favourable conditions.

Amid the wayside rubbish grows one of the gourd family, Ecbalium elaterium, commonly called the squirting cucumber, whose fruit—­a rough and extremely bitter little cucumber—­is the size of a date.  When ripe, the fleshy core resolves into a liquid in which float the seeds.  Compressed by the elastic rind of the fruit, this liquid bears upon the base of the footstalk, which is gradually forced out, yields like a stopper, breaks off and leaves an orifice through which a stream of seeds and fluid pulp is suddenly ejected.  If, with a novice hand, under a scorching sun, you shake the plant laden with yellow fruit, you are bound to be somewhat startled when you hear a noise among the leaves and receive the cucumber’s grapeshot in your face.

The fruit of the garden balsam, when ripe, splits, at the least touch, into five fleshy valves, which curl up and shoot their seeds to a distance.  The botanical name of Impatiens given to the balsam alludes to this sudden dehiscence of the capsules, which cannot endure contact without bursting.

In the damp and shady places of the woods there exists a plant of the same family which, for similar reasons, bears the even more expressive name of Impatiens noli-me-tangere, or touch-me-not.