22 INTRODUCTION. 



joints only, the terminal joint takes the most varied and at times 

 extraordinary shapes, and may be elongate and porrect as in Ceria 

 (SYRPHiDJi:), elongate and pendant as in the higher MUSCID^E, 

 short and rounded as in most of the ANTHOMYID^E and ACALYP- 

 TRATA. It may be furnished with an arista, a long, gently curved, 

 bristle-like hair, which may be plumose, subplumose, pubescent, 

 or bare, or even extraordinarily pectinate as in one or two abnor- 

 mal genera of Tachinids ; the arista itself may be dorsal, sub- 

 apical, or apical, or may be replaced by a more solid style-like 

 appendage. 



The Thorax. 



Little can be added to the notes given under " Terminology." 

 In most Diptera the mesothorax, usually known as the ruesonotum 

 when only its upper part is referred to, occupies tlie greater 

 portion of that part of the body ; both the prothorax (except in 

 some TIPULIIXE) and metathorax being very much aborted. 



The thorax is normally oval or subquadrate ; in many genera 

 exceedingly elongated ( Calobata, Micropeza) ; in others highly 

 arched (Simulium, Platypeza, Hybos, CYRTID^E, etc.). In rare 

 instances the sides of the thorax are furnished with a strong 

 spine (EpJiippium). The sides of the thorax or the pleura? have 

 been described above (p. 6). 



The scutellum is variable in relative size, but is most frequently 

 semicircular or subtriangular. It reaches its most extraordinary 

 development in Celyphus, in which it forms an enormous spherical 

 cover for the whole of the abdomen, and as the insects are 

 generally smooth and shining, and of a metallic blue, green, 

 or yellowish colour, they are easily mistaken by the beginner for 

 beetles. The scutellum may be distinctly spined on the posterior 

 margin (as in many STRATIOMYID^E), serrate, smooth, furnished 

 with bristles or pubescence, or quite bare. 



The metanotum is in most cases hidden by the scutellum above, 

 except in some families of NEMATOCERA in which it is often the 

 more conspicuous of the two. The vestiture of the thorax is as 

 variable as that of the abdomen. 



The Abdomen. 



The abdomen in the Diptera varies in the number of segments 

 from four (some ANTHOMYID^ and ACALYPTRATA) to as many as 

 nine (in some MYCETOPHILID^E) ; the normal number in the 

 NEMATOCERA being eight. The basal segment is often much short- 

 ened and almost invisible from above, the first two being some- 

 times more or less fused together. They are numbered from, the 

 base, on the upper side, the genital organs not being counted as 

 a separate segment. 



The variations in shape of the abdomen are very great. It is 

 very elongate, narrowed, and cylindrical in TIPTJLLD.S:, CHIRONO- 

 MID.S:, and CuLlCiDjE ; stout and shorter in some of the other 

 nematocerous families ; very short and exceedingly broad and 



