314 



INSECTA. 



in other cases, as in the common Flesh-fly, soft and muscular, and 

 folds up when at rest in such a manner as to form two angles, repre- 

 senting the letter Z. At the base of this sheath or proboscis there is a 

 small upper lip, between which and the sheath are lodged the setae, 

 knives, or lancets, which form such terrible instruments for cutting or 

 piercing the skin of their victims. These cutting parts vary in number 

 from one to five : when they are all present, the upper pair (e.ultelli, or 

 knives) represent the mandibles of a perfect mouth, the two lower ones 

 (scalpella, the lancets) are the maxillaB, the fifth or middle piece (glossa- 

 rium) is the tongue ; and between them all is the oral opening. The 



Fig. 157*. 



Head of a Flea. 



strength of the above piercing instruments varies greatly : in the Gnat 

 they are finer than a hair, very sharp, and barbed occasionally on one 

 side ; while in the Horse-fly they are flat, like the blades of a lancet or 



* Head of the Flea, as represented by the Solar microscope in Canada balsam ; 

 dedicated by permission to the President and Members of the Entomological Society, 

 by W. Lens Aldous. 



