INSECTS. 267 



fly to appear in the spring), and numbers of others, the 

 catalogue of the names of which would 

 prove dry reading. Only one needs 

 more mention. This is the White 

 Mountain butterfly, found only on the 

 tops of the White Mountains, on the 

 tops of the higher peaks of Colorado, 

 and in Labrador. It is supposed that 

 this form is a remnant of an Arctic 

 fauna which extended over the north- 

 ern United States when the country FIG. 85. White Mountain 



11,1 , i butterfly (CEneis semi- 



was covered by the great ice-sheet (see dea). 

 Geology), and on the retreat of the glacier these colonies 

 were left stranded upon these points as the only places cold 

 enough for them. 



ORDER IX. DIPTERA (Flies). 



This order contains the true flies, and these forms are 

 sharply marked off from other insects. The name means 

 two-wings, and the flies have but a single pair of these 

 organs, while on the metathorax is a pair of knobbed hairs, 

 the so-called balancers (p. 237). The mouth-parts are 

 fitted for sucking (fig. 86). The larvae, commonly known 

 as maggots (fig. 87), are worm-like, lack feet, and in some 

 species even lack a distinct head. In some the pupa is 

 motionless, but in others, as in the mosquito, it has great 

 powers of motion. The balancers are sensory organs, and 

 the)' also serve as a means of maintaining the equilibrium, 

 for if they be cut off from a fly, the animal can no longer 

 direct its motions. 



The group of flies is very large in number of species, 

 some being beneficial, while others are decided pests, 



