STRUCTURE AND CLASSIFICATION. 25 



insects with this figure, and by identifying the different pieces 

 observe for themelves what modifications have taken place. The 

 upper side is always called the dorsum, or dorsal surface, while 

 the under side, between the legs, is the sternum, or breast. 

 Laterally, between the sternum and the dorsum we find the 

 pleurum, which is a region rarely referred to in general descrip- 

 tion. The "dorsum," " dorsal surface," and "sternum," on the 

 other hand, are terms of frequent occurrence in the literature of 

 economic entomology, and it is well to know exactly what is 

 meant by them. 



As has been previously said, all the organs of locomotion in 

 the adult insect are attached to the thoracic segments ; never 

 more than three pairs of legs, one pair to each segment, and 

 usually two pairs of wings, borne on the meso- and metathorax. 

 The prothorax never bears wings, and, when only a single pair 

 is present, as occurs normally in the flies, or Diptera, this is on 

 the meso- or middle thoracic segment. 



Of the wings, or organs of flight, all that need be said here 

 is, that they are more or less membranous, flattened, and stiff- 

 ened by " veins" or chitinous rods. They vary greatly in char- 

 acter, and must be especially described for each order, their 

 general structure being all that it is necessary to give here. In 

 their origin wings are merely collapsed sacs, — that is, each is a 

 bladder-like outgrowth from the body wall, that assumes definite 

 form, but remains filled with liquid until ready to assume its final 

 shape. Then it is gradually distended to full size, the liquid is 

 absorbed, and the walls of the bladder unite and become so 

 closely welded together that it is impossible afterward to separate 

 them. The wing seems like a single thin membrane, or horny 

 shell, as the case may be, and the veins appear like hollow rods 

 or cylinders, though originally they were mere thickened grooves 

 on opposite sides of the distended bladder. The arrangement of 

 the veins, or " venation," is of very great importance in classifi- 

 cation, but needs no particular exposition here. 



The legs, always six in number in the adult, are jointed, or 

 made up of a definite number of parts or pieces. They are fixed 

 into the thorax at the sides of the sternum, or between the breast 

 and the sides, and are fitted into a body cavity by raeans of a 

 ball-and-socket joint, giving a great range of motion. The ball 



