DIPTEEA OR TRUE FLIES. 197 



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

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



Diptera are divided into two large sections, the Orthorrhapha 

 and the Gydorrhaplia. The first have their larvte with a distinct 

 chitinous head, and their pupae escape from the larval skin by 

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

 pupa or imago escapes from the larval skin by a circular 

 opening. 



There are two divisions in the Orthon-hapha — the Nema- 

 tocera and the Brachycera — the former with thread-like many- 

 jointed antennse, the latter with antennse composed of two or 

 three large basal joints with a multiarticulate bristle (fig. 

 115, a). 



There are many fl.ies extremely harmful 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 {Ceddomyia de- 

 sti-udm-), the Wheat Midge (Diplosis tritici), and the Pear 

 Midge (Diplosis pyrivora). The Cecids have very few veins in 

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

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

 composed of fourteen segmenta 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 

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

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

 hardens and forms a case or puparium. 



The Hessian Fly (Cecidomyia destruotoe). 



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

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



