STRUCTURE. 



11 



the margin of the wing. Besides these veins there are -cross- veins, 

 sometimes few in number, in other cases very numerous so that (as 

 in the Dragon-flies) the wing has the appearance of network. To 

 avoid confusion these cross-veins are called nervures. 



In the classification of insects the way in which the veins are 

 arranged is of great importance. The principal veins have received 

 various names, but as it is, or was, impossible to ascertain the 

 corresponding veins in the different Orders, each author has used 

 the names that best suited his purpose. The 

 names most commonly in use are given in 

 the anatomical case at the end of the gallery. 

 In the diagrams in the table-cases the veins 

 are only numbered and coloured, those that 

 are believed to be homologous being similarly 

 coloured throughout. 



The eyes are of two kinds; simple and 

 compound. The simple eyes, called ocelli, are 

 placed on the front or upper part of the head ; 

 three is the most usual number, bub some in- 

 sects have only two, and a few only one. They 

 have the appearance of glass beads imbedded 

 in the surface of the head. The compound 

 eyes are placed at the sides of the head. 

 They are termed compound because they 

 consist of a number of lenses, varying from 

 seven to twenty-seven thousands. In some 

 insects these lenses are placed close together 

 but retain their round form ; in others they 

 have the appearance of having been pressed 

 together, so that each lens is six-sided (hexa- 

 gonal) and the whole eye presents the appear- 

 ance of a honeycomb. 



The legs (1 020) are composed of five principal 

 parts : 1, the coxa, which fits into a sucket 

 in the body ; 2, the trochanter, which in 

 some cases is divided into two ; 3, the femur ; 

 4, the tibia ; 5, the tarsus, which normally consists of five joints, but 

 the number is sometimes four or three, and in exceptional cases two 

 or even one. The last joint is provided with a pair of claws, and 

 between these there is often a small piece, which has received various 

 names, such as pulvillus, arolium, onychium, according to its form. 



Leg of a beetle (Ghiaso- 

 gnathus). (1020.) 



a, coxa ; b, trochanter ; 

 c, femur ; d, tibia ; e, 

 tarsus ; /', claw-; g, 

 onychium (enlarged). 



