26 - 



them in perspective, when Bombylius is feeding, with their teeth turned outward 

 (fig. 6, I), is a cross-section of the same). 



The functions and finer structure of these pseudotracheae, together with other 

 points in the anatomy of the mouth-parts of Bombylius, will be further discussed 

 in the part of this paper devoted to a comparative study of the mouth-parts of 

 diiferent diptera. 



ANATOMY OF THE MOUTH-PARTS OF ERISTAL1S. 



Little has been done in the study ofj the anatomy of the mouth-parts of 

 Syrphidae, of which I have chosen Eristalis horticola for anatomical investigation. 

 Gerstfeldt 3 (p. 28-29) gives a general description of the mouth-parts of the 

 Syrphidae, which can be condensed as follows : labrum blunt, with the tip separ- 

 ated into several points ; hypopharynx always present ; mandibles more or less 

 rudimentary and coalesced with the sheath of the proboscis; maxillae fairly well 

 developed and setiforrn; labium with well-developed terminal lobes. Meuzbier 8 

 (p. 57-60) describes the proboscis of Syrphw taeniatus. His results, as regards 

 the typical structure of the mouth-parts of Syrphus, may be summarized thus: 

 labrum and epipharynx united ; hypopharynx with a channel for salivary secretion; 

 mandibles present; only remnants of the maxillae are the two maxillary palpi; 

 labium with well developed terminal lobes. Menzbier, in describing the proboscis of 

 Syrphidae, divides it into three portions, a basal, a middle, and a terminal portion. 

 Leaving all reasons for my own views to be given in detail later, I will only say 

 here, that this division of the proboscis of the Syrphidae, Mmddjie, and other 

 families of diptera, into basal, middle, and terminal portions, has served to render 

 the study of the mouth-parts of these insects more complex and more difficult than 

 it would otherwise be, and that, while retaining the term basal portion, for reasons 

 to be given later, as a convenient designation for a part of the proboscis, I lay 

 little value on such a division of parts. The so-called middle and terminal portions 

 of the proboscis of the Syrphidae and Muscidae are really a single portion. 



The proboscis of Eristalis horticola (side view in PL 3, fig. 1) is hung upon 

 the under side of the head, and, unlike the proboscis of Cukx or of Bombylius, 

 is extensible and retractible. When extended the proboscis of E. horticola is as 

 long as the head, and points nearly directly perpendicular toward the surface on 

 which the insect is resting. Its retraction is accomplished by means of joints, 

 near the points indicated on the figures by d and g. If the retraction is begun, 

 by a partial revolution toward the head, of the portion of the proboscis between 

 d and g, around the extremity d as axis, then the end g will follow the course 



