THE GLASS AND ORDERS OF INSECTS 185 



families with free and mandibulate pupae, and also some 

 with the pupa non-mandibulate and partly obtect. 



Sub-Order ii. Heteroneura 



In this section, including the great majority of the 

 order, the hindwing has only one radial nervure or two 

 (radial trunk and radial sector), while the forewing has 

 the normal five. The pupa, more or less obtect, is always 

 non-man dibulat e . 



Order 20. Diptera 



The Diptera (two-winged flies) have the labium highly 

 modified as a sucking organ ; mandibles may be absent or 

 present as piercing-organs. Parts of the maxillae may also 

 be modified as piercers ; maxillary palps are present, labial 

 palps absent. The prothorax is of small extent and closely 

 united with the large mesothorax ; the membranous forewings 

 are well developed for flight, while the hindwings are reduced 

 to short, clubbed equilibrating organs. The abdomen carries 

 cerci, which with the genital armature and hinder segments 

 generally are often concealed. 



The larvae of Diptera are eruciform or vermiform, all without 

 true thoracic legs, but prolegs may be present on some of the 

 segments. The pupa is free or obtect, the pupal wing has the 

 tracheal system reduced. 



Two sub-orders of Diptera have been recognized and are 

 distinguished by the nature of the early stages. 



Sub-Order i. Orthorrhapha 



In this section (including the various families of midges 

 and gnats, crane-flies, black-flies, breeze-flies and others) 

 the larva has a distinct head with mandibles. The cuticle 

 of the pupa splits lengthwise, as is usual in insects, to 

 allow the emergence of the imago. 



Sub-Order ii. Cyclorrhapha 



In this section (including the hover-flies, the house-fly 

 and blue-bottle group of families, the bot-flies, etc.) the 



