INTRODUCTION. XXVII 



to form air sacs, the presence of a sucking stomach as in the 

 Lepidoptera, the constant number (four, rarely five) of the 

 Malphigian tubes, and the absence of a bttrsa copulatrix in the 

 females. 



The alimentary canal presents behind the oesophagus an 

 expansion which is a crop or sucking stomach. The ventri- 

 culus, or true stomach, lying behind it, has usually two cceca ; 

 and the long, slender, Malpighian vessels are, in almost all 

 species, four in number, a surprising constancy compared with 

 the condition in other groups of specialized insects. The ves- 

 sels open singly into the alimentary canal in some flies and in 

 others they unite in pairs before reaching the canal and open 

 into it by two ducts. 



The heart is of the usual type, with two chambers in the 

 more specialized families, owing to the concentration of the 

 body. In the larva of Corethra the heart is a simple, elongate 

 tube without chambers. 



The two main tracheal trunks expand at the base of the 

 abdomen into conspicuous air sacs similar to those found 

 among Hymenoptera, Lepidoptera, the lamellicorn beetles and 

 some other insects. The two pairs of spiracles of the thorax 

 are provided with " vocal cords ", and a considerable part of 

 the humming sound is produced by these structures. The 

 abdominal spiracles of some flies are as primitive as are to be 

 found among insects, being simply unlipped openings. 



The condition of the nervous system varies greatly within 

 the order. In the elongate, more generalized Nematocerous 

 forms there are five or six abdominal ganglia, and three dis- 

 tinct thoracic ganglia. From this condition to that shown by 

 the Muscidse, where all the thoracic and abdominal ganglia 

 are united into one large ganglion in the thorax, a most in- 

 structive series of gradatory forms is presented. In the 

 Empididse, which stand intermediately as regards the concen- 

 tration of the ventral cord, the two anterior thoracic ganglia 

 are fused into one ; this condition is radically different from 



