1 42 INTRODUCTION TO ZOOLOGY. 



The eleventh does not consist of active female krvje alone, 

 but of males antl females. These acquire wings, rise into the 

 air, sometimes migi-ate in countless m}Tiad3, and produce eggs 

 which, glued to twigs and leaf-stalks, retain their vitality 

 through the winter. When the advance of spring again 

 clothes the plants with verdui-e, the eggs are hatched, " and 

 the larva, without having to wait for the acquisition of its 

 mature and winged form, as in other insects, forthwith begins 

 to produce a brood as hungiy and' insatiable, and as fertile as 

 itself." Supposing that oue Aphis produced 100 at each 

 brood, she would at the tenth brood be the progenitor of one 

 quintilUon of descendants!— 1,000,000,000,000,000,000!* 



There is another tribe known to gardeners as scale insects, 

 or mealy bugs, which ai-e very destructive, especially to our 

 hot-house plants. They constitute the family Coccidce. The 

 female, from her motionless aspect, bears a greater resemblance 

 to a gall or excrescence upon a leaf than to a living insect 

 with numerous yoimg. But if these singular and inert beings 

 are the cause of occasional injury to man, they repay the 

 damage a hundredfold, by furnishing him with the brilliant 

 scarlet dye known in commerce by the name of cochineal. 

 The insect from which this is procured is the Coccus Cacti, of 

 Mexico. It is found upon a plant termed " Cactus Cochinel- 

 lifer," and is collected in such quantities, that, according to 

 Humboldt, 80,000 poimds of cochineal are annually brought 

 to Europe, each pound containing about 70,000 insects; and 

 Dr. Bancroft estimated the weight of that annually consumed 

 in England at 150,000 pounds, worth £370,000.t Lac, a 

 substance much used for varnishes, sealing-wax, &c. is pro- 

 duced by another species of the same family. 



Every pond affords examples of other insects 

 whose structure exhibits, in a more obvious 

 manner, the characteristics of the order. There 

 we find the Boat-fly (Notonecta, Fig. 131), which 

 rows gracefully along upon its back; and the 

 Water-scorpion {Nepa, Fig. 132), in which the 

 dark external covering of our most common 

 native species contrasts beautifully with the 

 Fig. 131. scarlet body underneath; and others which gUde 



NOTONECTA. 



• Owen, page 235. 



■f Wcstwood, pages 448, 449. 



