THE CHARACTERISTICS OF INSECTS. 



237 



among useful secretions are silk and wax. Silk is furnished more 

 or less abundantly by all the caterpillars and many other larvae. 

 The silkworms produce 

 it in large quantities.* 

 Wax is produced by a 

 number of Hymenop- 

 tera, which construct 

 cells of it to hold hon- 

 ey. Aphides and cochi- 

 neals secrete fatty mat- 

 ters, the white tufts of 

 which form a kind of 

 down on their bodies 

 and on the plants they 

 frequent. In other in- 

 sects the secretions be- 

 come a defensive armor. 

 Thus the Hymenoptera 

 drop poison in the 

 wound made in animal 

 tissues by their sting, 

 which it causes to swell, 

 the excrescences called galls, with which the leaves of trees are 



Fig. 19. Appearance of a Wasp in Flight. 



An analogous stinging gives rise to 



r^ 



Fig. 20. Scaphinus repelling the Attack of a Carabts. 



often covered. Many of the Cdleoptera emit penetrating odors. 

 The Cicindela smells of the rose, the Aromia moschata of musk ; 

 the anal glands of the carabicus produce butyric acid ; while 



* See Leo Vignon, La Soie au point dc vue scientifiqtie et industriel. Paris, 1890 (Bibli- 

 otheque des Connaissanees utiles). 



