THE INSECT WORLD. 



327 



sheeting, so that the vapor may become effective before it escapes. 

 It has been found by experiment that grain can be exposed to 

 this vapor for a considerable time without losing in germinating 

 qualities, but seed wheat should be, if possible, kept free from 

 such insects. It should be selected early and kept in insect-tight 

 vessels until needed for use. Open corn-cribs should be placed 

 in such a way as to get the benefit of all the cold weather there 

 is, and thus the multiplication of the insects will be checked so 

 early that no serious damage will be done. 



There are other species of Tineids infesting cultivated plants, 

 but few are widely distributed and at the same time injurious. 

 In most cases the application of arsenites to those forms feeding 

 openly upon plant tissue proves more or less satisfactory, while 

 against those feeding upon stored products we can use the 

 bisulphide of carbon. 



CHAPTER VIII. 



THE DIPTERA, OR FLIES. 



The term Diptera means two-winged, and members of this 

 order are separable from all other insects by possessing, nor- 

 mally, only a single pair of wings. These are borne by the 

 middle segment of the thorax, and are, therefore, the first pair ; 

 the second or hind pair are reduced to little club-like processes 

 called " halteres," "poisers," or " balancers." Their function 

 is supposed to be indicated by the last of these terms, — i.e., to 

 aid the insect in directing its flight and maintaining its position in 

 the air. The members of the order are, therefore, easily recog- 

 nizable in most instances, but the reference of individual speci- 

 mens to the proper place in the system is a matter of greater 

 difficulty. They are mostly of moderate or small size, sometimes 

 quite fragile, and the character of the wing venation, upon which 

 much reliance is placed in scientific classification, is not readily 

 made out, except by the special student. Yet a little patience 

 will generally lead to the proper group, and the habits or life his- 

 tory may then be used to determine more closely. 



