INSECTA. 257 



diblcs and maxillae strangely altered in their form and excessively 

 lengthened, so as not merely to become efficient piercing instru- 

 ments, but so disposed as to form by their union a suctorious tube, 

 through which animal or vegetable fluids may be imbibed. This kind 

 of mouth, when not employed, is usually laid under the thorax be- 

 tween the legs, in which position it is easily seen in most Hemi- 

 ptera : in some families, as, for example, in the plant-lice (Aphides), 

 it is of extraordinary length ; thus, in the aphis of the oak it is three 

 times as long as the whole body of the insect, projecting posteriorly 

 like a tail, and in the fir-aphis it is still longer. 



(297.) The second kind of mouth is that met with among the 

 Diptera, and from its construction in some tribes we may well under- 

 stand how they are enabled to become so seriously annoying. The 

 gnat and the mosquito furnish sufficiently well-known examples of 

 the formidable apparatus in question, which, in the horse-fly (Taba- 

 nus), seems to attain its maximum of developement. The oral 

 organs of the Diptera are composed of a sheath or proboscis, that 

 represents the lower lip of the mandibulate insects ; it is sometimes 

 coriaceous or horny in its texture, or in other cases, as in the com- 

 mon flesh-fly, soft and muscular, and folds up when at rest in such a 

 manner as to form two angles, representing 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 (cultelli^ or knives) 

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

 (scalpella, the lancets) are the maxillae, the fifth or middle piece 

 (glossarium) is the tongue, and between them all is the oral 

 opening. The strength of the above piercing instruments varies 

 greatly ; in the ghat 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 penknife : occasionally 

 they are so constructed as to form a tube by their union, through 

 which the liquid aliment is sucked up and conveyed into the 

 stomach. 



(298.) The mouth of the flea, although described by Kirby and 

 Spence as forming a distinct type of structure, differs very little 

 from that of the Diptera described above, as will be at once evi- 

 dent on inspecting the accompanying figure, reduced from a beau- 

 tiful drawing by Mr. W. Lins Aldous. 



