44 A BOOK OF INSECTS 



maxillae elongated to form needle-like processes, but the 

 tongue and the labrum (or upper lip) are similarly modi^ 

 fied, and work in conjunction. All these six lancets, 

 when not in use, lie within a grooved sheath formed by 

 the labium. But when the tip of this sheath is applied 

 to the skin of a victim, and the rapid plunging and cut- 

 ting of the lancets begin, the sheath curves bow-like 

 beneath the insect's head, while the rigid lancets escape 

 from the groove and are thus free to be driven deeper 

 into the tissues. The bifid tip of the labium, however, 

 continues to surround the lancets, and slides along them 

 as the skin is penetrated. The actual sucking-tube 

 appears to be formed by the union of the modified upper 

 lip and the tongue, the mandibles and maxilla? being 

 chiefly used to enlarge the puncture and increase the flow 

 of blood. Each salivary gland of the gnat is three-lobed, 

 and the middle lobes secrete a poisonous fluid which runs 

 down the tube formed by the tongue and the labrum. 

 This poison probably retards coagulation of the blood and 

 stimulates its flow, although originally it may have acted 

 upon proteids in the juices of plants — the blood-sucking 

 habits of gnats and their kindred being comparatively 

 recent in origin. 



In the common house-fly, or the bluebottle, all the 

 mouth-parts except the labium are suppressed ; but the 

 latter is a complex organ, and expands at the tip into a 

 bilobed, fleshy pad. Each lobe contains about thirty 

 channels that act as tributaries to two central tubes, 

 which, in their turn, lead to the mouth, and are in com- 

 munication with the salivary ducts. The sixty channels 

 are really so many inverted gutters. Each one is open to 

 the air on the underside of the pad, while each is sup- 

 ported inwardly by a closely set series of incomplete rings, 

 or arches, of chitin. The salivary glands of the fly are 



