26 INTRODUCTION. 



two main tracheal trunks in the base of the abdomen to form 

 air-sacs, the presence of the sucking-stomach as in the Lepidoptera, 

 the constant number (four, rarely five) of the Malpighian tubes, 

 and the absence of a bursa copulatrix in the females."* 



The alimentary canal presents behind the oesophagus, an ex- 

 pansion known as the diverticulum or sucking-stomach,t the 

 ventriculum or true stomach lying behind it, with, usually, two 

 caeca. The heart is of the usual type, but in the more specialised 

 families has two chambers only. " In the larva of Corethra the 

 heart is a simple elongated tube without chambers." The two 

 main tracheal trunks expand at the base of the abdomen into 

 conspicuous air-sacs. The two pairs of spiracles of the thorax are 

 provided with " vocal cords " and it is these that cause the 

 humming when the fly is on the wing. 



The nervous system in the XEMATOCERA generally comprises 

 five or six abdominal ganglia and three distinct thoracic ganglia. 

 Intermediate forms are numerous between this type and those of 

 the MUSCIDJE, in which the abdominal and thoracic ganglia are 

 united into a large mass in the thorax. A minute structure called 

 Johnston's organ, placed in the 2nd antennal joint, is supposed to 

 contain the auditory nerves. 



The internal genital organs consist in the male of two oval testes 

 with short vasa deferentia, a well developed penis with accessory 

 copulatory appendages, which are of the most diverse structure 

 even in allied species of the same genus. In the female the ovi- 

 positor is remarkably uniform ; there are a large number of egg- 

 tubes, three spertuatheca3, paired accessory organs, and no true 

 bursa copulatrix. 



3. THE EABLY STAGES OF DIPIERA. 

 The Larva. 



The larvae of all Diptera are destitute of jointed legs. The 

 larvae of other groups most resembling them are those of the 

 CuECULiONiDjK (Coleoptera), but whereas these latter have little 

 or no power of locomotion, the larvae of Diptera can generally 

 move about freely by means of projections on the body called 

 pseudopods ; or by the aid of short bristles arranged so as to 

 favour progression, such being present even in completely maggot- 

 like forms. 



The spiracles are also of great aid in enabling us to decide 

 whether a larva is Dipterous or not. 



Schiner counted thirteen segments in the larva, first the head, 

 then three representing the thorax, the remainder forming the 

 abdomen. " There is, however, no morphological criterion yet 



* This is from Willirton, after Kellogg. 



t This function of this organ has been questioned. 



