USE OF THE MICROSCOPE IN ZOOLOGY. 65 



If you have ever been bitten by a horse-fly, you will 

 have a lively appreciation of the effects of these 

 lancets piercing your skin. At c you will notice the 

 maxillary palpi. But by far the most beautiful piece 

 of mechanism in the mouth of the fly is the end of 

 the labium, which consists of two lobes forming the 

 ligula (a). It is a wide muscular membrane, which con- 

 tains a number of delicate semi-spiral tubes, through 

 which the little insect sucks up fluids. These tubes 

 remind one strongly of the trachea, those exquisite 

 little spiral vessels by means of which insects breathe, 

 only there is this difference in their construction in . 

 the tracheae the rings are a continuous spire, in the 

 ligula they are distinct, and do not, as in the tracheae, 

 surround the whole tube, but perform about two-thirds 

 of a circle. 



" In the Diptera, or two-winged flies, generally, the 

 labrum, maxillae, mandibles, and the internal tongue 

 (where it exists), are converted into delicate lancet- 

 shaped organs, termed setce, which, when closed 

 together, are received into a hollow on the upper side 

 of the labium, but which are capable of being used 

 to make punctures in the skin of animals or the 

 epidermis of plants, whence the juices may be drawn 

 forth by the proboscis. Frequently, however, two or 

 more of these organs may be wanting, so that their 

 number is reduced from six to four, three, or two. In 

 die Hymenoptera (bee and wasp tribe), however, the 

 labrum and the mandibles much resemble those of 

 mandibulate insects, and are. used for corresponding 

 purposes. The maxillae, c, (see page 66), are greatly 

 elongated, and form, when closed, a tubular sheath for 

 the ligula (e), or ' tongue/ through which the honey is 

 drawn up ; the labial palpi (), which are greatly deve- 

 loped and fold together like the maxillae, so as to form 

 an inner sheath for the ' tongue,' while the ligula itselt 



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