FLIES. 



405 



sequence, firmer and more slender; in otherS, and by far the greater number, they are 

 adapted only for sucking up juices or such substances as may be dissolved by means 

 of their saliva. Grains of pollen have been observed in the digestive organs of the 

 Syrphidse, and other flower flies, but, as a rule, fluids alone 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 either projects in front or backwards under the abdomen. While it is 

 usually short, it may be as long or longer than the body. Finally, a few species have 

 the mouth parts rudimentary, and take no noui-ishment in the adult stage. 



The different parts consist of the labium, the maxillae, maxillary palpi, mandibles, 

 hypopharynx and labrum-epipharynx, a term used by Dimmock, to whom our clearest 

 knowledge of the mouth parts of Diptera is due. 

 The labial palpi are thought to be wholly want- 

 ing. The labium is always present, more or 

 less fleshy and provided with muscles, and is 

 grooved or channelled upon the upper side to 

 receive the other parts in a sheath completed 

 by the labrum. At its tip there is a pair of 

 joints called the labellae. In the mosquito these 

 are small, where they serve simply to guide the'"^ 



piercing portion between them, the labium itself mssss*— — — — - ^^ 



being bent backward beneath the thorax in its ^'''^^Z^^i^r::uZ:^.:&^',^tkP^^ 

 middle. Very often they are large and more gf^f^'l' ;S;aS?a?&T»: Sypiils"' ''^ 

 fleshy, and on the inner sides have a roughened 



surface composed of the pseudo-tracheae which, as in the housefly, serve as means for 

 attrition. The maxillae and mandibles are frequently absent, the latter most often ; 

 when present they are slender and bristle-like. The maxillary palpi are always pres- 

 ent and consist of from one to flve joints, in the latter case often long and whip-like ; 

 they are more or less hairy, and are attached near the base of the proboscis on the outer 

 side where the maxillae coalesce with the labium. In addition to the two pairs of max- 

 illae and mandibles there is a third, unpaired, slender organ which 

 is free, the hyjjopharynx. It is usually present and tube-like for 

 the passage of saliva, the outlet being near the tip on the upper 

 side ; its tip may be smooth, lance-like, or hairy. Its upper 

 side is continuous with the under surface of the pharynx, and 

 the whole, or in part may coalesce with the labium below. 

 Finally, the largest, except the labium, and uppermost, as well 

 as most important organ, is the labrum-epipharynx, which is 

 deeply channelled on the under surface and converted into a 

 canal by the apposition of the hypopharynx below. It is 

 through this channel that all the substances used as food 

 must pass. The two parts of which this organ is composed, 

 the labrum above and the epipharynx below, are sometimes 

 separable by the means of caustic potash, but are never so in 

 life. It may terminate in a single point or in several minute ones as in the mosquito. 

 It forms, as before stated, a covering to the channel in the labium, and may be 

 separable at the will of the insect as is readily seen in the mosquito when biting, or 

 it may always remain tightly closed, as in the housefly. 



Fig. 610. — Mouth-parts of 

 house-fly; (, labellae; 

 znaxillse; mp, maxillary 

 palpi. 



