346 ANIMAL DEFENCES 



superfluous rabbit-flesh would have been used up as food by more 

 powerful animals. 



Insects again, as we have had occasion to learn, furnish food 

 to a great variety of other animals, notwithstanding the large 

 number of protective devices exhibited within the limits of their 

 class. And it is only by means of immense powers of increase 

 that the ravages of their enemies are made good; indeed so great 

 are these powers, that there are more insects in the world than 

 any other land animals. A single illustration will perhaps suffice 

 (taken from Wallace's Darwinisvt). Speaking of the rapid in- 

 crease of organisms this author says: — " In the lower orders this 

 increase is particularly rapid, a single flesh-fly {Ahisca carnarid) 

 producing 20,000 larvae, and these growing so quickly that they 

 reach their full size in five days ; hence the great Swedish 

 naturalist, Linnaeus, asserted that a dead horse would be devoured 

 by three of these flies as quickly as by a lion. Each of these 

 larvae remains in the pupa state about five or six days, so that each 

 parent fly may be increased ten thousand-fold in a fortnight. 

 Supposing they went on increasing at this rate during only three 

 months of summer, there would result one hundred millions of 

 millions of millions for each fly at the commencement of the 

 summer — a number greater, probably, than exists at any one 

 time in the whole world. And this is only one species, while 

 there are thousands of other species increasing also at an enor- 

 mous rate ; so that, if they were unchecked, the whole atmosphere 

 would be dense with flies, and all animal food and much of animal 

 life would be destroyed by them." The following example will 

 serve to show how the numbers of a particular species may be 

 kept down by even a single kind of enemy. 



" During a very cold winter in the district of Hanau several 

 thousand old oaks were cut down, in the hollow trunks of which 

 many tens of thousands of bats sheltered during the rigorous part 

 of the year. As the trees were felled and sawn into pieces most 

 of these useful animals perished, either of cold or maltreatment at 

 the hands of boys. In the following year much larger numbers 

 than usual were seen of the Oak Procession-Moth {Cnethocampa 

 processioned), and for a number of years after the caterpillars of 

 this moth became a most destructive pest in the Hanau district, 

 over an area of miles in circumference. Not only were oaks 

 stripped of leaves, but also a large number of other forest trees 



