DIPTEEA OE TRUE FLIES. 197 



the tnie pupa being found in the brown case, formed of the 

 maggot's old skin much hardened (fig. 108, d). 



Diptera are divided into two large sections, the Ortliorrhapha 

 and the CydorrhapJia. The first have their larvoe with a distinct 

 ohitinous head, and their pupte escape from the larval skin liy 

 a T-shaped rent ; the second liave no chitinous head, and the 

 pupa or imago escapes from the larval skin by a circular 

 opening. 



There are two divisions in the Ortliorrhapha — the Nenia- 

 iocera and the Brarhi/rera — the former with thread-like many- 

 jointed antennas, the latter with antennas composed of two or 

 three large basal joints with a multiarticulate bristle (fig. 

 115, a). 



There are many flies extremely haTmful to the agriculturist 

 and gardener. The following are some of the most injurious : — 



Cecidomyidae, or Gall -Gnats. 



This family contains only small flies, some of which are 

 distinctly harmful, such as the Hessian Fly (Cecidomyia de- 

 structor), tlie Wheat Midge {Diplosis tritici), and the Pear 

 Midge (Diplods pyrivnra). The Cecids have very few veins in 

 their wings, thread-like hairy antennas, and often hairy wings. 

 The larvse are sometimes white, at others red in hue, and 

 composed of fourteen segments. They always have on the 

 under surface, close to the anterior extremity, a curious process 

 called the Breast Bone or Anchor Process (fig. 94). Many 

 larvse form galls, in which they live, and may or may not 

 pupate there. The larva; may change in their old skin, which 

 hardens and forms a case or puparium. 



The Hessiax Fly (Cecidomyia destructor). 



In 1886 there was a great scare concerning this pest, which 

 was thought to have been introduced from America. It was not, 



