138 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



mesolabrum and the base of the maxillae. This plate consists 

 of two laminae towards the pharynx, but the two are inseparably 

 united throughout the greater part of its extent. The anterior 

 or dermal edges of the lateral plates of the fulcrum are con- 

 tinuous with the clypeus, so that the mesolabrum is bounded 

 by the clypeus, the lateral plates of the fulcrum, and the epi- 

 pharyngeal plate ; it contains the dilator muscle of the pharynx, 

 which, by withdrawing the epipharyngeal from the hypo- 

 pharyngeal plate, draws fluid through the tubular mouth into 

 the pharynx, so that the fulcrum is a powerful instrument of 

 suction. It also forms a movable support for the whole 

 haustellum. 



Morphology of the Fulcrum. Gerstfeldt [55] is, I believe, the 

 only author who has anticipated me in the statement that the 

 maxillae enter into the composition of the fulcrum, but he 

 merely observes, 'The anterior lancet (labrum) shows distinctly, 

 by the presence of a median raphe, that it is formed of two 

 halves, which must be the blades of the maxillae (Kieferladen). 

 They rest upon a piece extending backwards (the fulcrum), 

 which appears to be the united stipites, from which two slender, 

 nail- shaped parts diverge downwards and backwards. These 

 Straus termed glosso-pharyngeal apophyses in Melolontha, 

 they are analogous with the cardines ' [55, p. 25] . I so far 

 agree with Gerstfeldt as to regard the outer portion of the 

 lateral plate of the fulcrum as a portion of the stipes of the 

 maxilla. His other statement, that the nail-shaped processes 

 represent the cardines, is undoubtedly incorrect, and the fusion 

 of the blades of the maxillae with the labrum is by no means 

 proved. Although I formerly held this view myself, I now think 

 it far more probable that the lacinae are undeveloped in all 

 those flies in which only two median lancets exist. The lacinae 

 of the maxillae are present, however, when paired lancets 

 enter into the composition of the mouth armature, as in the 

 Tabanidae. 



Menzbier [50] states that the fulcrum is developed in the 

 chitinous lining of the stomodaeum, and regards it as an internal 

 organ. This view is inconsistent with its developmental history 



