64 ZOOLOGY 



all parts of the body- -the muscles ami all other internal 

 organs. These air-tubes are called trachea. They can be 

 seen best in some aquatic larvae with the aid of a strong 

 magnifying-glass. Air is driven in and out of these tubes 

 by means of expansions and contractions of the body wall. 



Although so very numerous, flies would be still greater 

 pests if it were not for the fact that they are preyed upon 

 by various parasitic animals and plants. The larva of the 

 house-fly is sometimes infested by minute hymenopterous 

 parasites. A still more important foe is a minute plant - 

 a fungus called Empusa 1 muscce 2 - -which infests house- 

 flies in the autumn. The dead flies may often be seen on 

 window panes with fine white threads sticking out of the 

 body, and surrounded by a halo made up of the spores 

 discharged from the fungus upon the glass. 



The order Diptera is a large one and a difficult one to 

 study, for it contains many species and many of the 

 species are composed of small individuals which are 

 comparatively unknown. The members of the group can 

 usually be distinguished by having only two wings, the 

 posterior pair being rudimentary and transformed into 

 knobbed " balancers." An account of the principal fami- 

 lies follows. 



The short-horned Diptera (Brachycera 3 ), to which family 

 the house-fly belongs, includes flies in which the third seg- 

 ment of the antenme is unsegmented, called true Brachy- 

 cera, and flies in which the third segment of the antennae 

 is segmented, called anomalous Brachycera. The true 

 Brachycera include, besides the common flies, several 

 other common or especially interesting flies. 



a hobgoblin. 2 Of a fly. 



i'S, short ; /ce'pas, horn or antenna. 



