COMBINATION OF MANY METHODS OF DEFENCE 277 



firmly adherent in the latter. They are thus tightly 

 fixed to the larva by both ends. The caterpillar is 

 nearly always allowed to become full grown and spin 

 a cocoon before the maggots have become large. In 

 this way the latter secure a safe retreat and more 

 abundant food. When they have grown large and 

 their prey is shrivelled and almost dead, they lose the 

 attachment to the egg-shell and devour from all points, 

 until nothing but a dry and empty 'skin is left. They 

 then spin their own cocoons within that of the cater- 

 pillar. The latter is also attacked by other parasites 

 and probably often by vertebrate insect-eaters. 



A well-protected larva is often especially liable 

 to attack 



Thus, in spite of the fact that the caterpillar pos- 

 sesses so many defensive appliances, it is especially 

 liable to attack, far more so than many other larvae 

 which are less' protected. Mr. G. C. Bignell enu- 

 merated seven species of parasites which attack it. At 

 first sight this seems to be a difficulty, but we must re- 

 member that we are probably dealing with an animal 

 which has been especially attacked for a long period 

 of time, and which has been saved from extermination 

 by the repeated acquisition of new defensive measures. 

 But any improvement in the means of defence has 

 been met by the greater ingenuity or boldness of foes ; 

 and so it has come about that many of the best pro- 



