218 THE COLOURATION OF INSECTS 



Thus a Roller which was examined at Jinja was found 

 to have devoured numbers of a bright green ball-rolling 

 dung beetle, a member of a family whose habits render 

 them nauseous to monkeys, and which emit a very foul 

 smelling fluid when handled. 



Bee-eaters (Merops) feed on bees and other stinging 

 HymenopteraP- cuckoos appear to feed largely on very 

 hairy caterpillars. Now it is well known that every living 

 thing produces very many more offspring than can possibly 

 survive, and Wallace pointed out, in 1858,^ that, of the 

 offspring of one pair, on the average all but one must die 

 or at least fail to produce young. So that any one who 

 refuses to accept the explanation that aposematic colours 

 and habits are produced by Natural Selection is quite 

 justified in saying, " If you claim that these species escape 

 being devoured by vertebrate enemies, the burden is laid 

 on you of proving by what means they are prevented 

 from overrunning the earth." 



In order to answer this, all the restraining factors must 

 be considered. 



Firstly, there are the vertebrate enemies, birds and 

 beasts, against which it is claimed aposematic insects 

 are protected. Secondly, predaceous insects, such as 

 dragon flies, carnivorous beetles and bugs, and Asilid 

 flies, which eat or suck juices of other insects. Spiders 

 are not here included, for they appear to be indiscriminate 

 feeders on protected and unprotected alike. For the 

 same reason fossorial Hymenoptera, which feed their 

 young on other insects, are not here included. Thirdly, 

 there are parasitic Hyinenoptera and Diptera, which lay 

 their eggs on other insects ; and fourthly, the micro- 

 organisms of disease. 



^ See Reports of Sleeping Sickness Commission, vol. xiv, 1913, pp. 15, 

 16, and table extracted therefrom in Chap, viii, p. 172. 



* " On the Tendency of Varieties to depart indefinitelj'from the Original 

 TjT)e." Essay reprinted in Natural Selection and Tropical Nature, 1895 

 edition, pp. 2-i-5. 



