THE ANATOMY OF THE PUPA 69 



segment, the only locomotor organ of the pupa, the wings 

 and legs lying immovable, and even adhering to each other, 

 though they are easily separated in specimens preserved in 

 alcohol. 



Nine segments are easily recognised in the abdomen, 

 and the last one, though it is probably composed' of no 

 less than three condensed and highly modified segments, is 

 the smallest. Each segment has a chitinous terguni and 

 sternum, and setae are sparingly distributed over them, 

 those present being mostly on the hinder part of the terga. 

 Of these a pair placed on the hinder part of the first seg- 

 ment alone require mention. Each consists of a triangular 

 basal plate, articulated to the tergum by a soft membrane, 

 and distally divided into a number of bars, which by re- 

 peated sub-division give rise to about a hundred setae, all 

 lying in one plane parallel with that of the median, of the 

 body. When at rest the pupa floats with the tips of these 

 setae and those of the respiratory syphons at the surface of 

 the water, and the setae probably assist in maintaining 

 equilibrium, as well as serving as sensory organs for the 

 perception of disturbances of the water. 



The eighth segment bears the fins, a large pair of thin 

 oval plates about 1"2 mm. in length, each of which is 

 strengthened by a midrib, which projects as a spine beyond 

 its hinder border. Beneath and behind them is the ninth 

 segment, a small though probably composite segment, 

 which contains the anus, and is provided on either side 

 and in front of it with a pair of blunt processes, larger in 

 the male than in the female. 



The digestive canal differs but little in any stage of the 

 insect, the main change being the casting off of the thick 

 large-celled lining of the stomach and the substitution of 

 the more delicate mucosa of the adult. The cast-off larval 

 mucous membrane appears to be disposed of by digestion. 

 During this period also is developed the peculiar chitinous 

 dilatation of the anterior part of the thorax, already described 

 in the adult. Mr. Hurst describes it as triangular in section 

 with incurved sides, to the concavities of whose sides are 

 attached muscular fibres originating from the sides of the 



