146 



ELEMENTARY STUDIES IN INSECT LIFE 



and parasitic insects may be taken from the fact that 

 the army worm rarely appears in considerable numbers 

 two seasons in succession, showing that these enemies 

 severely check its increase. There are a number of 

 flies somewhat similar in appearance to our common 



FIG. 116. Different stages in transformation of parasitic fly (Sarcophaga sp.) : 

 J, 2, 3, 4, white maggot in active stage ; 5, maggot beginning to pupate dirty white 

 in color, pupa-case light brown ; C, 7, fully developed pupa-case of two species. X 5. 



house-fly. These flies are parasitic upon grasshoppers, 

 and aid in keeping the grasshoppers in check. Some 

 of these flies are illustrated in Figures 117 and 171. 

 In regard to pollination of crops, reference has already 

 been made to the fact that the honey-bee materially in- 

 creases the yield of the alfalfa plant. (See page 107.) 

 It is known that the bumblebee is very influential in 

 bringing about the proper fertilization of the common 

 red-clover blossom. However, injurious insects, such as 

 the grasshopper, may not be looked upon as wholly in- 

 jurious, since they frequently furnish food for the 

 farmer's poultry. High-class farm culture tends to re- 

 duce the numbers of injurious forms, and at the same 



