PARTS OF IXSECTS. 35 



articulations in this part of tlie leg, as we obsen-e in Cermnhyx, Gri/l- 

 lus, and others: in Libdluh, Forficula, S,c. three: in the anterior feet 

 of Nepa only one. The figure of the tarsus is more varialjle than any 

 other portion of the leg, and is in a most singular manner adapted 

 to the insect's mode of life. The articulations in such as walk on the 

 surface of the earth are slender; those which burrow have them more 

 robust. Many of those which inhabit waters have them riat and cili- 

 ated at the edges, as in the Hijdrous. Others are furnished with 

 bristly tufts or vascular fleshy tubercles, which enable them to move 

 with security on smooth and slippery bodies in any direction : an ad- 

 mirable example presents itself in the common house-fly, which " treads 

 the ceiling, an inverted floor," with the same facility that other insects 

 walk on the surface of the ground. An occasional difl'erence in the 

 number and form of thejomts of the tarsus is sometimes observed in 

 the two sexes of the same species. The motion of each joint of the 

 tarsus is performed in a single plane, and is directed by two muscles in 

 each joint, one of which is small and placed on the dorsal surface, the 

 other larger and situated beneath. 



Unguis, or Clazc, the termination of the tarsus. In the greater 

 number of insects there are two claws attached to each tarsus : some 

 have only one; and in others furnished with two there is an interme- 

 diate process, forming by this means three. An appearance similar to 

 this is seen in the legs of the Liicanns; but this on minute examina- 

 tion is foimd to be a distinct joint also, armed with a pair of claws pre- 

 cisely resembling'those which more obviously, from their size, appear 

 to terminate the tarsi. It is considerably smaller, but is perfectly well 

 defined. 



Al.t, or Wings : the organs appropriated to flight. These are 

 either two or four, and are attached to the lateral part of the breast 

 close to the lower margin of the thorax. They are placed to an equal 

 amount and in a corresponding situation on both sides of the insect, 

 whether the niunber be two or foiu\ Those insects which are fur- 

 nished with only one pair of wings have in these organs both an uni- 

 form appeai'ance and size. Such as have two pair most frequently 

 differ, the first being larger than those behind : there is also a difference 

 in shape, and very commonly a considerable \ ariation in the spots, 

 markings, and other particulars, notwithstanding the prevailing hues 

 in all the wings may be the same. In general the posterior pair is 

 paler, and the marks obscure. 



A skeleton of nervures, (which are considered in the light of bones 

 by Dr. Leach, who has named them Pterigostia or Wing-^ones, 

 and are parts more or less numerous and differing exceedingly 

 iu disposition,) placed between two thin and closely united mem- 

 branes, constituies the true wing in insects. This conformation is very 



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