58 ON THE ANATOMY OF THE FLY. 



of the alimentary canal. They are hollow, conical, glandular or- 

 gans, about l-30th of an inch in length, enclosed in the dilated 

 portion of the rectum, and having their bases only external to its 

 cavity. Their function is probably the excretion of a urinary 

 fluid. 



Each papilla consists of three parts ( Plate IV, Figs. 9 and 12 ) : an 

 internal central cavity, surrounded by a transparent structureless mem- 

 brane ; around this a hollow cone of gland cells, the secreting portion 

 of the organ, is disposed; and external to this again, a tough transparent 

 cone of membrane, Avhich 3 shall call the calyx of the papilla, perforated 

 by numerous minute pores, 'surrounds the whole of that portion of the 

 organ -which is internal to the rectum. By a little dexterous manipula- 

 tion these parts may be separated completely from each other, or, by 

 hardening the papillae in chromic acid, sections may be made showing 

 the relations of the parts in situ. Fig. 9 represents such a section. 



The calyx is perforated by about 300 minute pores, each pore being 

 surrounded by a nipple-like projection, which is surmounted by from 

 three to eight minute setse, ( Plate IV, Fig. 11 ). The whole membrane 

 of the calyx becomes thickened towards its apex, where it has a faint, 

 yellow tint ; it is near the apex that the nipple-like projections and their 

 setas are best seen. Indeed a casual observer might overlook their 

 presence entirely at the upper portion of the calyx. 



The calyx itself is marked by faint reticulations, and its margin is 

 deeply crenated. These are the only indications of structure which it 

 presents, and these seem to point to its being a fibrous membrane, 

 especially as two sets of muscular fibres arise from the crenations of its 

 margin. 



These muscular fibres are, first, a set from the muscular coat of the 

 rectum, and, secondly, a layer of converging fibres which cover the whole 

 base of the papilla to within a very short distance of its centre, and 

 which apparently end in the edge of the membrane forming the 

 boundary of the central cavity, although from the extreme transparency 

 of this membrane it is nearly impossible to be certain of their insertion. 



Each papilla is supplied with air by a large tracheal vessel from the 

 fifth abdominal spiracle, which divides into several generally five or six 

 large trunks before entering the papilla. The tracheae of the papilla 

 may be divided into two sets as soon as they enter the base of the organ. 

 First, from twenty to thirty radiating lateral branches run to the 

 edge of the base, and then pass over the outer surface of the glandular 

 structure to the apex of the cone, giving off numerous branches, which 

 anastomose freely, and form a fine reticulation around the gland cells } 

 the larger branches running directly towards the central cavity, and 

 forming loops by anastomosing with other similar vessels. 



