THE EXO-SKELETON OF THE PROBOSCIS. 137 



a latero-ventral tube ijnx), which has the same relation to the 

 mesolabrum that the theca has to the prelabrum. In the 

 nymph, at an early stage of development, the separation of 

 the pre- and mesolabrum is not apparent, and the whole lies 

 in a groove, so that the mouth at this period extends back 

 to the epistome ; it then very closely resembles the labrum 

 and sheath of the Hemiptera. 



Subsequently the fulcrum is developed between the meso- 

 labrum and the grooved part of the rostrum, and the meso- 

 labrum becomes inseparably united with the ventral portion of 

 the rostrum by the fusion of the outer and inner plates of the 

 lateral walls of the fulcrum (Fig. 28, '2 Ip). 



I regard the latero-ventral portion of the rostrum as the basal 

 portion of a pair of united maxillai ; it bears the palpigerous 

 scales and the maxillary palpi. It is probable, however, that 

 part of the thin integument on its posterior or ventral aspect is 

 common to the maxillae and the rudimentary labium, so that it 

 may be regarded as an extension of the ventral integument 

 between the insertion of the labium and the maxillee. In the 

 more generalised Insecta (Fig. 22, ^) it is easy to see that 

 the mere obliteration of the deep grooves between the bases of 

 the labium {mx~) and the maxillae ijnx^) would result in the 

 formation of a rostrum similar to that of the Blow-fly. Indeed, 

 the fusion of the bases of the maxillae and labium is far more 

 complete even in the most generalised forms than has hitherto 

 been supposed. 



The Fulcrum, or cephalo-pharyngeal skeleton (Fig. 28, / and 2), 

 has been compared by Kraepelin [70] to a Spanish stirrup-iron 

 with a double foot-plate. It may be described as consisting 

 of a flattened tube, the pharyngeal tube, formed by two 

 pharyngeal plates, a dorsal or anterior, cpipharynx, and a 

 ventral or posterior plate, hypopharynx. The epipharyngeal 

 plate exhibits a strong median raphe, which projects into the 

 pharynx. A pair of horns, cornua {a, b), project at either end 

 of the pharyngeal tube, and give insertion to several muscles. 



On either side of the pharyngeal tube there is a somewhat 

 triangular lateral plate (//>), formed by the adjacent walls of the 



