56 C. HERBERT HURST. 



The three pairs of ventral appendages of the thorax, or legs, are 

 long cylindrical bodies folded upon themselves, and lying beneath 

 the thorax and between the wings. The same segmentation into 

 femur, tibia, etc., is recognisable as in the adult gnat, but the 

 segments are more nearly equal in the pupa, and the joints of the 

 developing and shrinking legs of the future imago soon lose their 

 correspondence with those of the pupal cuticle enclosing them. 

 They arise in the larva, like other appendages, as folds of epidermis 

 enclosing mesoblastic tissues. 



The abdomen is dorso-ventrally compressed and exceedingly flexible 

 dorso-ventrally, though not from side to side. It is the only part 

 of the pupa in which the segmentation of the body is readily recog- 

 nisable, and as I shall very frequently have to refer to the various 

 segments by number, I shall use the terms "first segment," etc., to signify 

 "first segment of the abdomen,'" etc. 



Nine segments are readily recognised in the abdomen, and the 

 last one, though it is probably composed of no less than three 

 condensed and highly modified segments, I shall call simply "ninth 

 segment." 



Each abdominal segment has a chitinous tergum and sternum, and 

 setae are distributed sparingly over them, being almost confined to 

 the hinder parts of the terga. The terga and sterna of successive 

 segments are united by soft arthrodial membranes. 



Of the setce, only one pair need special mention. These are 

 placed on the hinder part of the first segment, the base of each 

 being a triangular plate attached by one angle to a soft membrane, and 

 the distal side of the plate is divided into a number of bars which, 

 by repeated division or branching, give rise to about one hundred 

 setae all lying in one plane parallel to the median plane of the body. 

 Each seta bears a few fine hairs. When at rest, the pupa floats with 

 the tips of these setae, and the tips of the respiratory siphons, at the 

 surface of the water, and these setae probably assist in maintaining the 

 equilibrium of the animal in this position, as well as serving as sensory 

 organs by means of which any disturbance of the surface is felt. 



The eighth segment bears a pair of large fins, thin oval plates about 

 1-2 mm. in length, attached by the narrow end beneath the tergum 

 behind. Each is stiffened by a midrib which projects beyond the 

 hinder border of the fin as a spine. (Fig. 2.) 



