26 



AN ECONOMIC ENTOMOLOGY. 



portion belongs to the leg and is called a " coxa," and the socket 

 is the " coxal cavity." Attached to the coxa is the "femur," 

 or thigh, usually the stoutest part of the leg, and corresponding 

 to the same portion of the leg in the higher animals, most of the 

 muscular system being found in it. It is often strengthened at 

 the base by a small supplementary piece called the "trochan- 

 ter," which sometimes forms an intermediate segment between 

 coxa and femur, and may be even two-jointed, as in the parasitic 

 Hymenoptera. At the end of the femur is attached the "tibia," 

 or shank, which varies greatly in structure according to the 

 habits of the insect, being modified for digging, tearing, swim- 

 ming, clasping, or other special functions. It is armed in many 

 ways, and becomes an index to the life history of the insect, 

 furnishing, also, many good characters of use in classification. 

 Economically, it is useful in that it is a guide from which we can 

 often get an idea of the kind of existence that the insect leads, — 

 whether it is a digger, a swimmer, or a carnivorous type. The 

 terminal part of the leg is the "tarsus," or foot, and this is also 

 jointed, —five joints being usual, though variations frequently 

 occur. The last joint is armed with a pair of claws, and between 

 them we may have "pulvilli," or small pad-like structures, by 

 means of which flies and other insects cling to smooth surfaces 

 and are able to walk back downward on ceilings. The differ- 

 ences in the leg structure are easily seen with a little experience, 

 and in some orders, notably the Coleoptera, they serve to divide 

 the insects into large groups or series. 



The abdomen normally contains nine segments, but these are 

 rarely all distinct. As a rule, one or more are modified to form 

 sexual organs or appendages, or shields, or other accessory 

 structures. It sometimes happens that the segments on the 

 dorsal, or upper, and ventral, or under, side do not correspond, 

 one part of the ring being lost or modified. In the adult the ab- 

 domen never bears organs of locomotion, and, as a rule, no appen- 

 dages of any kind, except an ovipositor in the female of certain 

 groups, or a pair of claspers in the male. In some of the lower 

 orders it bears jointed filaments or appendages resembling feelers, 

 and these are called " cerci." Only in the lowest of all types do 

 we find traces of rudimentary legs on the abdominal segments, 

 indicating a relationship between the insects and myriapods. 



