Conical traps with galvanized-iron frames are satisfactory, but they are less easily rescreened. These, of course, can be constructed only by shops with considerable equipment. Traps constructed with a wooden disk about the base of the cone, and a similar disk around the top to serve as a frame, or those with a square wooden frame at the bottom and top, with strips up the corners, are fairly satisfactory. It should be borne in mind, however, that the factor which determines the number of flies caught is the diameter of the base of the cone, if other things are equal. Therefore, the space taken up by the wooden framework is largely wasted, and if it is too wide it will have a deterrent effect on the flies which come toward the bait. For this reason it is advisable that the wood around the base of the cone should be as narrow as consistent with strength--usually about 3 inches.
Under no condition should the sides or top of the trap be of solid material, as the elimination of light from the top or sides has been found to decrease the catch from 50 to 75 per cent.
Where large numbers of traps are to be constructed, and especially if they are to be used for trapping screw-worm flies on the range, it is desirable to make them with metal rather than wooden frames. Traps with galvanized-iron frames are now being used extensively on ranches in central and western Texas in combating the screw-worm fly. Mechanics may wish to vary details of construction from those given in the specifications, but the dimensions and general features of the trap should not be altered. The cost of the metal-framed traps varies from about $3.50 to $5 each according to the number ordered, the workmanship, and the kind of materials used. For the guidance of those who desire such traps the following directions, together with a working drawing (fig. 4), are given:
_Wire_--14-mesh galvanized or preferably copper screen wire.
_Cylinder_--Wire gauze, to be soldered completely around inside of top ring and at intervals of 2 inches or less in groove of bottom ring. Vertical seam to be soldered entire and placed behind one leg. Where shipment of traps is not contemplated the diameter of the top of the cylinder may be the same as that of the bottom.