68 THE STUDY OF ANIMAL LIFE CHAP, iv 



moth larva " 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-protected larvae 

 are often those which die in the largest numbers from 

 the attacks of enemies. The exceptional standard of 

 defence has been only reached through the pressure of 

 an exceptional need." 



12. Surrender of Parts. Among the strange life-pre- 

 serving powers which animals exhibit, we must also 

 include that of surrendering parts of the body in the 

 panic of capture or in the struggle to escape. A rat 

 or a stoat will sometimes gnaw off a leg to free itself 

 from a trap. But the cases to which we now refer are 

 not deliberate amputations, but reflex surrenders. Many 

 lizards (such as our British ' slow-worm ") will readily 

 leave their tails in their captor's grasp ; crustaceans, 

 insects, and spiders part with their limbs and scramble 

 off maimed but safe ; starfishes, brittle-stars, and feather- 

 stars resign their arms, and the sea-cucumbers their viscera. 



Among Crustacea the habit is most perfectly developed 

 in the crabs, e.g. the common shore-crab (Carcinus 

 mcenas), and in the spiny lobster (Palinurus), but it is 

 also exhibited by the crayfish (Astacus), the common 

 lobster (Homarus), the shrimp (Crangon), and the prawn 

 (Palcemori). In the higher crustaceans there is a gradual 

 series leading up to the perfection of ' autotomy ' ' seen 

 in crabs. In some cases, e.g. Gammarus and hermit- 

 crab, the creature will nibble at an injured limb, eating 

 it down to the base ; in some cases, e.g. prawns, an 

 energetic stroke of the tail is required if the self-mutila- 

 tion is to be effected ; in some cases, e.g. crayfish, the 

 animal tugs at an injured limb. In edible-crabs (Cancer) 

 and shore-crabs (Ccwcinus), the end of the limb to be 

 thrown off must be forcibly gripped or pressed against 

 something ; in the swimming-crabs (Portunus) and sand- 

 crabs (Hyas) this is not necessary. In the more finished 

 cases there is a pre-determined plane of breakage at the 

 base of the limb, a special autotomist muscle, and a 



