I02 AN ECONOMIC ENTOMOLOGY. 



The mouth structure is in some respects intermediate between 

 that of the true bugs, as already described, and that of the man- 



dibulate type ; but there are no 

 true mandibles, and the insects can- 

 not chew their food, hence come 

 naturally, for our purpose, into this 

 order. 



The species of Thrips are often 

 seriously injurious to vegetation. 

 They sometimes attack onions in 

 great numbers, puncturing the suc- 

 culent leaves, and everywhere leav- 

 ing a small yellow dot. As these 

 increase in number the leaf loses 

 vitality, the top itself turns yellow, 

 and in serious cases the rich green 



Thrips tritici.—b, antenna ; c, a tarsus, of a normal Ouion field is changed 



to a peculiar, sickly yellow. The 

 insect is yellowish when immature, and blackish-brown when the 

 wings are developed. 



Cabbages sometimes suffer in a similar manner. The insects 

 in this case are yellow, and congregate in large numbers on the 

 lower side of the outer leaves, and as these become dry and 

 wither they move to those nearer the centre. 



Grass is often attacked by them, and here they cause one 

 form of what is known as "silver top." Wheat, oats, rye, and 

 other grains are often victimized, and, indeed, there are few 

 plants not liable to infestation, even tobacco appearing among 

 the sufferers. 



These insects thrive best in hot, dry weather, hence become 

 more dangerous as the plants are less able to resist them, and 

 our effort must be in the direction of aiding the crop as well as 

 destroying the insects. Contact poisons are indicated here, as 

 against the true Hemiptera^ and either the kerosene emulsion, 

 diluted ten times, or the whale-oil soap, one pound in four gal- 

 lons of water, is satisfactory. Plenty of cold water, where it is 

 available, acts very well on a small scale ; but no application is 

 feasible on large fields of cereals or grass. Here stimulation only 

 can be resorted to to give the plants additional vigor, and har- 



FiG. 73- 



