THE PUPAL STAGE OF CULEX. 67 



not conspicuous in the pupa, though it is just recognisable. During 

 pupal life its parts undergo considerable change, and these will be 

 best understood if I describe the adult structure first. 



In the imago the antenna? differ markedly in the two sexes. In 

 the female the shaft is longer than in the male, and the hairs with 

 which it is beset are less numerous and very much smaller. In both 

 sexes the basal joint is enlarged, and forms a nearly hemispherical 

 cup, with small cavity and very thick walls, covered and lined 

 with chitin. The shaft of the antenna arises from the centre of 

 the cup, and the chitinous floor of the cup is strengthened by a 

 series of radial thickenings. In the female the edge of the cup is 

 turned in, so that the aperture of the cup is narrower than the cavity 

 immediately below. The structure in the male is really an exaggera- 

 tion of this ; the edge is folded in so completely that it unites with 

 the floor, and the walls of the cavity of the cup of the female thus 

 come to be represented by a concave double disc, the two laminae of 

 which are closely united, and the space between them, the equivalent 

 of the cavity of the cup in the female, is here obsolete. The attach- 

 ment of the shaft to the floor of the cup appears to be rigid, and the 

 organ would appear to be adapted for the perception of sound-waves 

 coming in the direction of the axis of the shaft alone. 



A section taken along the axis of the organ shows the following 

 structures : A layer of flattened epidermal cells, next to the cuticle of 

 the outer wall; then a layer of cells I shall call "ganglionic," thickest 

 at the base of the cup, and continuous with the antennary lobe of 

 the " brain." Between this layer and the inner wall of the cup 

 is a double (perhaps treble) layer of long narrow rod-like cells, at 

 right angles to the surface, that is, radiating from the centre of the 

 cup. 



These structures form a thick ring round the cup, perforated at 

 the base by the nerve supplying the shaft of the antenna. 



The basal joint is supplied by an enormously large nerve arising 

 from the ventral portion of the suprauesophageal ganglion at the side 

 of the oesophagus. This nerve is broader than the abdominal double 

 nerve cord, and is independent of the nerve supplying the shaft of 

 the antenna, which lies ventral to it. The nerve, after entering the 

 organ, divides, one layer penetrating the "ganglionic" layer; another 

 runs between the ganglionic layer and the layer of rods, and a third 



