PASSIVE DEFENCE 337 



requires great force if it is once allowed to fix itself firmly 

 although, if taken unawares, a smart tap is generally sufficient. 



Jointed-limbed Invertebrates (Arthropods] are invested in a 

 firm, horny external skeleton, sometimes thick enough to con- 

 stitute veritable plate-armour, and not infrequently garnished with 

 protective spines. Insects and Crustacea afford the best ex- 

 amples. 



Insect Armour. Beetles are often distinguished by the thick- 

 ness of their integuments, and, as previously mentioned (see p. 



Fig. 497. Caddis- Worm Tubes of various kinds (enlarged) 



315), some of the weevils are so well oft; in this respect as to be 

 avoided by insectivorous birds. Certain tropical beetles (of the 

 genus Hispd] present a chevaux-de-frise of long, sharp-pointed 

 spines. 



The larvae of Caddis- Flies, commonly known as " caddis- 

 worms ", make protective tubes by cementing together all sorts 

 of available substances (fig. 497). 



Crustacean Armour. To illustrate the smooth variety of 

 defensive armour-plating among Crustaceans, a better example 

 could hardly be chosen than the common Edible Crab (Cancer 

 pagurus]. Here the greater part of the body is enclosed in 



VOL. II. 



