EXTERNAL ANATOMY. 7 



devoid of strong bristles on the body (the MYCETOPHILID^E 

 possess conspicuous and characteristic bristles on the legs), it 

 seems unnecessary to dilate here on the scheme of chsetotaxy, or 

 the systematic study of these appendages ; but a brief outline of 

 the system is given in the Glossary under Chcetotaary. 



The Abdomen. 



Few special terms are in use regarding the abdomen except that 

 the external male genital organs are spoken of collectively as the 

 "hypopygium,* whilst the female organs are known as the ovipositor. 

 The eight segments of the abdomen are numbered from the base 

 onwards, the usual terms used in the TIPULIDJE, where the 

 covering of this part of the body is represented by a distinct 

 dorsal and ventral plate, being tergum and sternum respectively .t 



The Legs. 



These also call for but little description, most of the terms being 

 in common use. The acetabulum is a minute joint attaching the 

 coxa to the body ; the coxa is the short first obvious joint which is 

 united to the femur, the first long conspicuous part of the legs, by 

 another minute, ring-like piece, known as the troclianter. The tibia, 

 succeeds the femur and is the second conspicuous division, followed 

 by the tarsus, or foot, which is invariably composed of five joints.* 

 These joints are numbered from the basal one, which is known either 

 as the metatarsus, or the first tarsal joint. Care must be taken to 

 note that the next joint following the metatarsus is the second. 



Attached to the extremity of the ultimate tarsal joint are the 

 ungues, or claws, two curved hooks, and below these are two pad- 

 like cushions or <pulvilli. These are often absent in the ORTHO- 

 BRHAPHA. Between the claws, and below them, attached, like the 

 pulvilli, to the last tarsal joint, is the empodium, which sometimes 

 takes the shape of another pad, in which case it is said to be 

 pulvilliform, and sometimes that of a thick bristle or spine. Osten 

 Sacken placed much reliance on it as a classificatory character 

 in TIPFLID^E, but I am inclined to doubt its value to such an 



* Bergroth has proposed propygium, but hypopygium is universally adopted 

 by dipterologists, whenever a special term is used. 



t Westhoff terms the upper and lower sides of the 8th segment the lamella 

 basalts supera and in/era, respectively, and those of the 9th segment or the one 

 actually developed into the genital organs, the lamella terminalis supera _ and 

 in/era, respectively, but I know of no one who has adopted these unwieldy 

 names. 



\ The exceptions are very few and only amongst abnormal forms. 



Too much importance should not be given to minor differences, such as the 

 comparative size, toothed nature, and so on, of the ungues, presence or absence 

 of empodia, etc. When Theobald's first volume on the CULICID* of the world 

 was published, great stress was laid on very microscopic differences in the shape 

 and size of the different pairs of claws, and in my Catalogue of Oriental 

 CULICID.*: the great importance of these characters was questioned. In sub- 

 sequent volumes of Theobald's work it was admitted that they did not possess 

 the value at first accorded to them. 



