40 



In Culex the part which corresponds to the fulcrum of Miisca is a trifle less 

 easily recognizable than it is in Bombylius. By an examination of the longitudinal 

 median section of the head of Culex (pi. 1, fig. 11) it is easily discoverable that, 

 just behind the place where the mouth-parts join to form the mouth, a set of 

 pumping muscles exists similar to those found in Musca and Eristalis. These 

 muscles are, however, enclosed in a chitinous case or box (fig. 11, c) of their 

 own in front of the head. By studying cross-sections through this box (pi. 1, 

 fig. 9, in female; fig. 15, in male), the muscles {pm and pm''), which have their 

 origins in the upper and lateral portions of this chitinous case, are inserted on 

 the basal supports of the labrum-epipharynx, in the same general way as they 

 are attached to the pharyngeal walls in the fulcra already described; and that, 

 by their contraction, these muscles would serve, in the same way as has been already 

 described, to separate from each other the upper and lower walls of the pharynx. 

 The upper and lower walls of the pharynx themselves have a similar form, in 

 cross-section, to that which they have in Musca and Eristalis ; their lateral portions 

 turn upward and form a slight internal chitinous framework about the pharynx; 

 on the upper walls of the pharynx originate the pharyngeal muscles. Here we 

 have, then, as in all the diptera previously compared, a fulcrum, only that, in 

 Culex, its upper portion is more rudimentary than is usual, the necessary rigidity 

 for the origin of the pharyngeal muscles being supplied by the forward wall of 

 the head (fig. ll, c). The position of the fulcrum in Culex is not essentially 

 different, because located in a case apparently external to the head, from the 

 position which it occupies in the basal part of the proboscis of Musca, when the 

 proboscis is extended. 



The relative position or condition of the fulcrum in the genera of diptera 

 which I have most studied is this: in Culex it is permanently extended; in 

 Bombylius, permanently retracted ; and in Eristalis and Musca, capable of extension 

 or retraction at will. 



Before essaying to discover the homological significance of the fulcrum itself, 

 it is well to turn attention to the parts which surround it. In Bombylius the 

 chitinous wall (pi. 2, fig. 3, c) of the head, forming the covering to the upper, 

 or dorsal, open side of the fulcrum, bearing, on its inner surface, the pharyngeal 

 muscles, and continuous with the labrum in front, would, without doubt, be recog- 

 nized as the clypeus (epistom of some authors). The chitinous wall which 

 surrounds the fulcrum beneath, continuous at the sides with the clypeus above, 

 is the under surface of the head. The under surface of the head is continuous 

 anteriorly with the labium, but often, between the two, exhibiting folds, which 

 (pi. 2, fig. 3, I', I") may be reckoned as belonging to the head or to the labium. 



