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 off in this respect as to be 

 avoided by insectivorous birds. Certain tropical beetles (of the 

 genus Hispa) present a chevaitx-de-frise of long, sharp-pointed 

 spines. 



The larvee 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 

 pagtirus). Here the greater part of the body is enclosed in 



VOL. II. 



54. 



