TWO-WINGED FLIES AND FLEAS 95 



into the mouth, probably consists of the labrum and 

 tongue in conjunction, the needles which represent 

 the mandibles and maxillae being chiefly used to 

 enlarge the wound. In the Central Hall of the 

 British Museum (Natural History) some enlarged 

 models of a gnat's head and mouth-parts are exhibited. 

 These show clearly the positions of the several organs, 

 and the manner in which they work. Students who 

 are able to visit the Museum should not fail to examine 

 these models carefully. 



Among those Diptera that are able only to suck, 

 all the mouth-parts except the labium have become 

 obsolete ; but the labium is a very wonderful organ, 

 which ends in a bilobed, fleshy pad. When not in 

 use it lies in a cavity beneath the head ; but, being 

 jointed, it can be extended when the insect is feeding. 

 The pad is traversed by numerous ducts, that act as 

 tributaries to two main tubes which communicate 

 with the mouth. It acts like a kind of sponge, 

 readily absorbing liquids, or even small solid bodies, 

 such as pollen grains. Moreover, the fly is able to 

 flood the ducts of the pad with saliva in order to 

 soften or dissolve hard substances that would other- 

 wise be unsuitable for food. It is in this way that 

 the house-fly is able to eat lump sugar. 



Diptera may be divided into two sub-orders in 

 accordance with the character of the larva, and the 

 manner in which the pupal skin splits to allow for the 

 escape of the imago. All Dipterous larvse are 

 destitute of thoracic legs; but in the first sub- 

 order ! there is a small but distinct head, while 

 the pupal skin splits longitudinally down the back. 

 The flies of this sub-order are arranged in numerous 

 1 Orthorrapha. 



