ANIMAL SELF-DEFENSE. 603 



mimic poisonous species. And this mimicry is found even among 

 birds. 



Active defense implies such organs and methods of defense as are 

 under control of the animal's will, or matters of conscious action. We 

 shall here find much greater variety. 



The homes of animals nests, houses, burrows, etc. are protection 

 from the storm and for the young as well as from foes. This is a 

 most interesting and extended field, and requires separate treatment. 



But many small creatures build individual shells or cases wholly for 

 defense against enemies. These are frequently carried about with the 

 creature, as armor, wherever it goes. A familiar example, found in 

 any brooklet, is seen in the case of the young caddis-fly. To hide and 

 protect itself from the ever-hungry fishes, the larva of this insect in- 

 closes its body in a tube formed by gluing together bits of wood, 

 shells, sand, and all sorts of matter that 

 may be found at the bottom of a stream. 

 This case has a silken lining, and out of 

 the end the larva protrudes its head and 

 legs for locomotion, or wholly withdraws 

 out of sight and danger. Other water larvae 

 reside within a bit of hollow straw or plant- Fig. 8. caddis-Worm, with its 

 stem. A similar habit characterizes a group 



of sea-worms, to which belongs the Serpula. Thin tubes may be formed 

 of a limy secretion, or built by cementing sand, shells, etc. One of 

 the tentacles of the Serpula is terminated by an expansion which, when 

 the worm withdraws into its case, serves as a stopper (operculum) to 

 securely close the opening (see Monthly, February, 1882, page 452, 

 Fig. 4). The silk pupa-cases of the moths are very wonderful, even 

 if very common, examples of artificial covering. 



The singular hermit-crabs are obliged, on account of their lack of 

 a hard epidermis, to inhabit some empty mollusk-shell. And they are 

 exactly fitted for that sort of life : the tail-fin is changed into hooks 

 for holding the shell ; some of the legs are strong levers for dragging 

 its heavy house ; and one of its claws is disproportionately large, in 

 order to close the opening of the shell. 



Keen senses combined with swiftness of locomotion are the chief 

 reliance of birds and mammals. The rabbit is a fair example. Inno- 

 cent and timid, entirely without weapons, it is always on the alert. 

 With its large eyes, ears sensitive to the slightest sound, and a delicate 

 sense of smell, it is as difficult to surprise as it is to " catch a wea- 

 sel asleep." Every deer-stalker knows he must approach his game on 

 the side opposite the wind. Most mammals, especially the herbivor- 

 ous, scent danger, and flee away. Many of them use their natural 

 weapons only when brought to bay, and in despair. This is true, in- 

 deed, of many carnivorous beasts when they are not bold with hunger. 

 Birds rely, for warning of foes, more exclusively upon their eyes. 



