328 INTRODUCTION TO ZOOLOGY CHAP. 



solution of the whole body of the larva into a structureless, 

 creamy, or jelly-like mass, from which there gradually 

 develops the form of the pupa and then that of the perfect 

 fly. This group affords the most striking instances of meta- 

 morphosis amongst the Insecta. 



The abdomen bears no appendages; the number of segments 

 visible on it varies from four or five to nine, there being often 

 more visible in the male than in the female, owing to some 

 segments in the latter being indrawn at the end of the body. 

 There may be seven pairs of abdominal spiracles. 



The subdivision of the Diptera into natural 

 Cla t s i ^ ca " families is difficult, and until the group has been 

 more studied it may be well to continue to use 

 the old subdivisions of the order, the Nemocem (nema, a 

 thread ; keras, a horn), those with jointed delicate antennae 

 not ending in a bristle, and with a naked, mummy-like 

 pupa ; and the Brachycera (brachus, short ; keras, a horn), those 

 with short antennae of usually three joints, the last bearing 

 a bristle either basally or terminally. These latter forms 

 usually also have their pupae enclosed within the last larval 

 skin, which becomes more or less chitinous ; within the cases 

 so formed, the body breaks down into a creamy mass, and 

 only after some time does the recognisable pupa develop. 

 The order is a huge one, including already over 40,000 

 known species, and so each sub-order is again divided into 

 many families, of which only a few can be touched upon here. 



Sub-order 1 : Brachycera (the Short-horned Flies). 



Family 1 : MUSCIDAE (HOUSE-FLIES AND BLUE-BOTTLES, OR 

 BLOW-FLIES) 



The families of the Diptera are very difficult to define 

 concisely, for there are very few characteristics peculiar to 

 any one family, it being rather by a group of characteristics 

 occurring together that they are known. The Muscidae, 

 however, nearly always have antennae which are sunk into 

 a concavity on the face, and which have a feathered bristle 

 springing from the base of the terminal joint ; " squamae " 

 are present and hide the " halteres." 



