26 ON THE ORIGIN AND [CHAP. 



faculty of secreting round itself a quantity of frothy 

 fluid which serves to protect it from its enemies. 

 But the best known insects of this group are the 

 Aphides or Plant-lice ; while the most useful belong 

 to the Coccidae, or scale insects, from one species of 

 which we obtain the substance called lac, so exten- 

 sively used in the manufacture of sealing-wax and 

 varnish. Several species also have been used in 

 dyeing, especially the Cochineal insect of Mexico, a 

 species which lives on the cactus. The male Coccus 

 is a minute, active insect, with four large wings ; 

 while the female, on the contrary, never acquires 

 wings, but is very sluggish, broad, more or less 

 flattened, and in fact, when full grown, looks like a 

 small brown, red, or white scale. 



The larva of the order Lepidoptera are familiar to 

 us all, under the name of caterpillars. The insects 

 of this order in their larval condition are almost all 

 phytophagous, and are very uniform both in structure 

 and in habits. The body is long and cylindrical, con- 

 sisting of thirteen segments ; the head is armed with 

 powerful jaws ; the three following segments, the 

 future prothorax, mesothorax, and metathorax, each 

 bears a pair of simple articulated legs. Of the pos- 

 terior segments, five also bear false or pro-legs, which 

 are short, unjointed, and provided with a number of 

 hooklets. A caterpillar leads a dull and uneventful 

 life ; it eats ravenously, and grows rapidly, casting its 

 skin several times during the process, which generally 

 lasts only a few weeks ; though in some cases, as for 

 instance that of the goat-moth, it extends over a 

 period of two or three years, after which the larva 

 changes into a quiescent pupa or chrysalis. 



