310 Mr. J. A. Dell on the Structure 



the tracheal extension is thinner than anywhere else, and 

 lacks the annular thickenings. This appears to be an 

 arrangement for snapping the tracheal extension just at 

 the required spot. Whether the pupal tracheae persist as 

 those of the adult, or whether all those of the thorax are 

 withdrawn through the adult metathoracic spiracle, I am 

 unable to say, but in all pupal skins hitherto examined no 

 trace of trachea? could be discovered, while the end of the 

 tracheal extension always appeared to have been snapped 

 across just where it originally entered the imaginal thorax. 

 The metathoracic spiracle is a small, circular opening 

 immediately below the base of the haltere. 



The Ahdomcn of the Fly and its Appendages (Fig. 13). 



The abdomen consists of eight segments. The first of 

 these is only distinguishable dorsally, being ventrally 

 obscured by the bases of the metathoracic legs. The 

 seventh segment shows some indication of being divided 

 into two parts. The last (eighth) segment in the male is 

 flattened from above downwards, and slightly bifurcated at 

 its hinder end, from which arises a long forceps, reaching 

 forwards when at rest as far as the middle of the seventh 

 segment. It is composed of two joints, a large basal and 

 a small terminal one. From the fore end of the eighth 

 segment arises a second forceps, also two-jointed. This runs 

 upwards and backwards, and is enclosed by the other. 

 There are also two processes arising from the hinder end 

 of the seventh segment in the middle line ; one is dorsal 

 to the other. They are possibly analogous in function to 

 the penis and titillator of a cockroach. 



In conclusion, I desire to express my indebtedness to 

 Professor Miall, at whose suggestion the work was first 

 taken up, and without whose assistance and encouragement 

 it could never have been carried out, and also to Mr. T. H. 

 Taylor for much helpful advice and criticism. 



Bibliography of Psyehoda. 



Balbiani, E. G. (1881.) Sur la structure du noyau des 

 cellules salivaires chez les larves de Chironomus. 

 Zool. Anz. pp. 637-G41 and 662-G66, with figs, in 

 text. 



(1890.) Etudes anatomiques et histologiques sur 

 le tube digestif des Grypbops. Arch. Zool. Exper., 

 2 e ser., VIII, pp. 1-82, PI. I-VI. 



