USE OF THE MICROSCOPE IN ZOOLOGY. 63 



biting jaws of a beetle, the piercing proboscis of a 

 bug, the long elegantly-coiled sucker of a butterfly, 

 the licking tongue of a bee, the cutting lancets of a 

 horse-fly, and the stinging tube of a gnat, that each of 

 these organs was composed on a plan of its own, and 

 that no common structure could exist in instruments 

 so diverse."* But such is the case ; underlying the 

 great varieties of form in the details there is a com- 

 mon type. In order, therefore, to get some good 

 definite idea of the typical insect mouth, you should 

 examine the parts in that of a beetle, which possesses 

 them in their most distinct form. You will notice, 

 then, in a beetle (i) an upper lip, or labrum; (2^ a 

 lower lip, or labium; (3) a pair of jaws, or mandibles, 

 which frequently are provided with strong teeth, and 

 open laterally on either side of the mouth ; (4) a pair 

 of secondary jaws, maxillce, situated beneath the 

 mandibles ; these serve to hold the food, the man- 

 dibles or biting jaws working on it, and to convey it 

 to the mouth ; (5) one or two pairs of jointed appen- 

 dages attached to the maxillae, called maxillary palpi ; 

 (6) a pair of labial palpi. The. under lip, or labium, 

 is generally composed of several parts, the basal part 

 being called the chin, or mentum a wide horny piece 

 the upper part being often much prolonged, forming 

 what has been termed the ligula. Now, it is this 

 tongue-shaped organ that we see so highly developed 

 in the common fly, blow-fly, and other relatives. The 

 plate, p. 64, represents a magnified view of the under side 

 of the fly's tongue. The broad dark part at the bottom 

 of the figure is the mentum \ b is the portion formed 

 by the metamorphosis of the maxillae, being modified 

 into a kind of sheath for the mandibles, which, in the 

 fly, assumes the form of a pair of sharp cutting lancets. 



* Gosse's " Evenings at the Microscope," page 168. 



