THE INSTINCTS OF ANIMALS 157 



animal, for the caterpillar is longer and altogether of greater volume 

 than the pupa. The same thing is seen in the stag-beetle (Lucanus 

 cervus), the largest of our indigenous beetles, which gets its name 

 from the powerful antler-like jaws which distinguish the male. It 

 also undergoes its pupal metamorphosis in the earth, and makes 

 a large hard ball of clay, hollow inside, and as smooth as if polished, 

 and its cavity is exactly the size of the future pupa, or to speak more 

 precisely, of the fully-formed beetle. For, as Rosel von Rosenhof in 

 his day ' observed with amazement,' the balls in which the males lie 

 have a much longer cavity than those built by the females, and for 

 this reason, that when the fully-formed beetle emerges from the 

 pupa it must, if it is a male, have room to stretch out its horns, 

 which have till then lain upon the breast. ' For the beetles do not 

 leave their dwelling-place until all their parts are sufficiently strong 

 and properly hardened, and till the season has arrived in which they 

 are wont to fly about.' The male larva thus makes a much longer 

 pupa-house than the female larva, in anticipation, so to speak, of the 

 enormous size of the jaws which will grow out later ! 



Here the instinct has two modes of expression, according as the 

 bodily parts are male or female. Here we have to do with an action 

 which is performed once in a lifetime, and thus the possibility of 

 any other explanation of the origin of this instinct than through 

 natural selection is excluded. 



Not less significant is the case of the silk-cocoons. The cocoons 

 spun by the silkworm are egg-shaped, and consist of a single thread 

 many thousand yards in length, which is wound round the spinning 

 caterpillar so that not a space is left uncovered. The web is firm, 

 tough, and very difficult to tear ; therefore we must grant that the 

 pupa resting within will enjoy a very considerable degree of security 

 against injury. But the moth must be able to get out, and that this 

 may be possible the caterpillar is impelled by instinct to make its 

 spinning movements such that the cocoon is eventually looser at the 

 anterior end, so that the insect, when it is ready to emerge, can tear 

 it asunder with its feet and make a way out for itself. For this very 

 reason, because the silk must be torn and spoilt by the emerging- 

 insect, silk-breeders kill the pupating insect before it begins to make 

 its way out. 



But there are species whose cocoons are provided from the very 

 start with an outlet, for the caterpillar spins the silk round itself in 

 such a way that a round opening is left. But this opening would 

 be not only a convenient door for the butterfly to emerge by, but an 

 equally convenient entrance for all its enemies. It is, therefore, closed 



