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represented by the dotted segment of a circle fjn ; -and if, during the time that 

 the above mentioned revolution is progressing, a revolution of the portion of the 

 proboscis gk around the axis </ takes place, then the tip of the proboscis, A, will 

 follow the direction indicated by the paraboloid curve kd. When these two partial 

 revolutions, around the axes of the joints at d and g, are completed, as far as 

 possible, the points g and ?i, and k and d, will have been closely approximated, 

 and the proboscis will have been folded upon itself by a fold near its middle 

 point, and, folded thus, the whole proboscis will have been retracted into a groove 

 on the under side of the head. When fully retracted the dorsal * portion of the 

 basal half of the proboscis is closely pressed against the dorsal portion of its 

 distal half. Fig. 2 represents a longitudinal section of the head of Eristalis 

 horticola with the basal half of the proboscis wholly retracted, and its distal half 

 partly retracted. A section through the. distal half of the proboscis of Eristalis 

 (Fig. 1, /?') shows all its mouth-parts in their normal positions. They consist 

 of a labrum-epipharynx (lr-e), closed beneath by a hypopharynx (/*). To each 

 side of the above named parts lay the only paired mouth-parts, the maxillae (mx) 

 and their palpi (>np). Beneath these parts the labium forms a channel for all 

 the other mouth-parts, a channel into which they fit when the proboscis is retracted. 

 No mandibles are present. A section through the proximal half of the proboscis 

 (fig. 1, d'} shows none of the true mouth-parts. In the middle of this section 

 is the channel for the passage of food, the pharynx (;>), surrounded by the 

 chitinous distal end of the so-called fulcrum, and its muscles (pm). At each side 

 of the pharynx is a chitinous rod (./) which supports at its distal extremity the 

 maxilla. Around these parts is a thin elastic membrane (c I') which is continuous, 

 proxirnally, with the chitinous covering of the head ; and, distally, on the upper side, 

 with the labrum, and on the under side with the lower walls of the labium. 

 This elastic membrane, surrounding the forward extremity of the fulcrum, folds 

 itself together, on the under side, between n and g (fig. 2), when the proboscis 

 is retracted ; on the upper side, when the proboscis is retracted, it receives in its 

 folds the dorsal surface of the labrum. 



The labrum-epipharynx is, in section (fig. 1, p' and /', lr-c), nearly the shape 

 of a horse-shoe, convex above, and is composed of the labrum, the continuation 

 of the flexible walls of the upper side of the basal portion of the proboscis, and 

 of the epipharyux, the continuation of the upper walls of the pharynx; but, in 



' I shall use dorsal and ventral, upper and under, distal and proximal, and anterior 

 and posterior, of the parts of the proboscis, in the same way and with the same meaning 

 as if the proboscis were extended out from the front of the head. 



