NORTH AMERICAN DIPTERA. 



MOUTH-PARTS. 



The mouth-parts of diptera are wholly suctorial. The}' 

 differ not a little in different flies, as might be supposed 

 from their diverse habits. In some they are adapted for 

 piercing animal or vegetable substances, and are, in con- 

 sequence, firmer and more slender; in others, and by far 

 the greater number, they are short and soft, with a thick- 

 ened extremity used for the attrition of small particles of 

 solid substances. Grains of pollen have been observed in 

 the digestive organs of the Syrphidae and other flower 

 flies, but, as a rule, fluids only serve as food. Many have 

 the proboscis wholly retractile into the oral cavity, and 

 furnished with one or even two hinges, by which when at 

 rest it may be folded up. In others the proboscis is not 

 retractile, and projects either in front, or downward or 

 backward, beneath the body. While it is usually short, 

 it may be much longer than the body. Finally, a few 

 species have the mouth-parts vestigial and take no nour- 

 ishment in the adult state. 



The more commonly accepted homologies of the mouth- 

 parts are as follows: labium, maxillae, maxillary palpi, 

 mandibles, hypopharynx, and labrum or labrum-epiphar- 

 ynx. The labial palpi are thought to be wholly wanting, 1 * 

 or represented by the labella. The labium is always 

 present, more or less fleshy and provided with muscles. 

 It is grooved or channeled on the upper side to receive 

 the other parts, with the exception of the maxillary palpi, 

 which are free. This sheath is often nearly complete, 

 the thin margins touching each other above. At its tip 

 are the pair of joints of variable size called the 'lips' 

 or labella. The maxilla and mandibles are sometimes 



*This has very recently been contested by Wesche, who asserts that 

 either the labial or maxillary palpi may be present and functional; but 

 no cases are known of both pairs being functionally present. I am 

 inclined to be skeptical. 



