234 Arthropoda, 



proboscis of tlie butterfly. Maxillary palps are present^ but 

 feeble. The unpaired portion of the labium is not well developed, 

 but the palps are large setigerous lobes, enclosing the proboscis, 

 which is spirally coiled when not in use. In the Rhynchota the 

 suctorial tube is formed by the mandibles, which are represented 

 by two compressed blades without palps ; two grooves run down the 

 inner surfaces of the blades, which are so fitted together that they 

 form two tubes, an upper and a lower. The proximal opening of the 

 lower tube lies close to the opening of the duct of the salivary gland, 

 and saliva passes down it, to be mixed with the food before it is 

 sucked up into the mouth through the upper, wider canal. At the 

 sides of the mandibles lie two other dagger-like organs, the modified 

 first maxillae, pointed like them, and thus adapted to act as 

 stabbing weapons ; their palps are wanting, both pairs of appendages 

 are inserted in deep pits, and can be protruded or withdrawn. The 

 labium is characterised by the fusion of the palps, so that the 

 whole lower lip is an unpaired structure of three or four joints, 

 hollowed out like a gutter, and forming a sheath surrounding the 

 mandibles and first maxillae. This sheath is open above, but the 

 opening is simply a slit along most of its len_gth, widening out only 

 at the base, where it is covered by the triangular labrum. Formerly 

 the labium of the Rhynchota was regarded as the functional tube, but 

 it is now known to be simply a case for the sucking tube proper, which 

 is formed from the mandibles. In the D i p t e r a the relations are as 

 follows: In the most perfectly developed mouth parts {e.g., in 

 the Gnat and the Gad-fly \_Tabanus] ) there is a labium of consider- 

 able length, much hollowed out on its under surface ; beneath this lie 

 the flattened, sword-like mandibles, which together with the labrum 

 form the proboscis. Below the mandibles lies an unpaired, narrow, 

 flattened piece, the hypopharynx, which arises posteriorly from 

 the labium ; it is traversed by the salivary tube which opens at its 

 tip. Next below lie the first maxillse, which, like the mandibles, 

 are long, narrow, blade-like stabbing or cutting organs ; a large palp 

 arises from the base of each. All these parts are enclosed by a long- 

 furrowed labium, which has no palp, and like that of the Rhynchota, 

 only forms a case for the rest of the mouth parts ; the maxillary palps 

 alone are not surrounded by the labium, but project freely at its base. 

 In other Diptera {e.g., House-flies), the mandibles and first maxillae 

 (with the exception of the basal portions of the latter and their palps) 

 are wanting ; in this case, the place of the mandibles is supplied by the 

 hypopharynx, which closes the labial groove below. In the 

 Hymenoptera most of which possess simple biting mouth parts, 

 these may, as in the Bees, be both biting and sucking : the mandibles 

 performing the former, the first maxillae and labium together 

 subserving the latter, function. 



