64 Wild Life in Wales 



argument, we suppose that the whole brood of one female 

 reached maturity, and that the sexes were equally divided, 

 we should have 150 moths, which would produce 45,000 

 eggs. Repeating the process, the army of leaf-devouring 

 caterpillars would have multiplied to 6,750,000 in the third 

 generation, and so on. But it seems to be an undoubted 

 fact that there are in Nature certain influences, occasionally 

 brought to bear, which greatly alter the proportion of the 

 sexes to one another. It matters not to our argument 

 whether the cause originates in an augmented food-supply, 

 or otherwise ; it is sufficient to admit that circumstances may 

 arise in which three-fourths or even more of a brood will 

 be females, and the result in increased caterpillars is at 

 once apparent. The argument may be carried still further. 

 Birds and small mammals undoubtedly play an important 

 part in keeping within bounds the increase of such prolific 

 creatures as all insects are, but, as is well known, the most 

 deadly foes with which lepidoptera have to contend are the 

 Ichneumon Flies that are parasitic upon them. Without 

 doubt the same laws that influence the excess of one sex 

 over the other, and the consequent periodic increase in the 

 numbers of the host, will equally apply to the parasite, and 

 it is only necessary to suppose a time when a year of 

 abundance of ichneumons shall coincide with a lean year in 

 lepidopterous larvae, to see the disastrous effect upon the 

 latter. The reverse side of the picture, of course, supplies 

 " a caterpillar plague," and we begin to appreciate the subtle 

 means often adoptod by Nature to swing the scales and yet 

 eventually to hold the balance even. 



A French proverb tells us that with time a Mulberry leaf 

 may become a silk dress, a slight variation of which might 

 be read into the present picture. These oaks, though now 

 stripped of their verdure, will, as soon as the caterpillars 

 have disappeared, put forth new leaves, and by autumn will 

 show but little outward sign of the misfortune that overtook 

 them in the spring ; but the drain of producing two crops 

 of foliage in one season must leave its mark in another 

 direction ; and when, years hence, the trees come to be 

 converted into timber, their trunks will show a narrow 



