524 Journal of Agricultural Research voi. x, no. io 



more than one-half of those sprayed with a solution of quassia extract 

 usually fall from the plants until several hours after they are dead. 



The preceding symptoms exhibited by aphids do not indicate that the 

 nervous system is the first tissue to be vitally affected, but a critical 

 study of the behavior of pea aphids sprayed with a quassiin solution 

 shows one symptom which indicates a nervous affection. Several sweet- 

 pea plants bearing many aphids were sprayed with a solution containing 

 a definite proportion of the extract from quassiin powder in water. An 

 hour after being sprayed, a few of the aphids stood up and trembled 

 vigorously for two or three seconds; afterwards they gradually became 

 less active when touched and weaker until they died. The trembling 

 behavior was never observed in unsprayed aphids. 



2. HISTOLOGICAL METHODS OP TRACING QUASSIIN IN TISSUES 



To determine, if possible, what tissue is vitally affected when aphids 

 are sprayed with solutions containing extracts of quassiin powder and 

 quassia chips, the insects, after being treated with these solutions, were 

 fixed in a fluid containing a precipitant. By this means one or more 

 constituents in each spray solution were precipitated wherever they had 

 gone into the insects ; and after carefully studying the microscopical sec- 

 tions made from these aphids, it was usually easy to trace the precipitated 

 particles. 



(a) Tracing Water Extracts of Quasshn Powder into Apmos 



Of the few reagents that precipitate quassiin, not one of them pre- 

 cipitates it satisfactorily so that the above procedure might be followed. 

 Owing to the unsatisfactory precipitation, it was necessary to add to 

 the spray solution some chemical which under normal conditions is never 

 used, in order that this chemical might be thrown down wherever the 

 spray solution had carried it into the insect. 



The senior writer (22, p. loi, 103) submerged aphids for 45 minutes 

 in, and also sprayed them with, a nicotine solution colored with indigo- 

 carmine. By using the above method it was found that the solution had 

 passed into a few of the larger tracheae of the aphids submerged, but 

 never into the tracheae of those aphids sprayed, and only once in the 

 latter had a little precipitate lodged in a spiracle. These results indicate 

 that any solution almost totally water sprayed upon aphids would not 

 pass into the tracheae, unless the permeability of this solution had been 

 considerably increased, or its surface tension had been rendered very 

 weak by the addition of some other substance not already in the solution. 



Despite the supposition that water containing quassiin has practically 

 the same permeabiHty and surface tension as does water not containing 

 this substance, it was desirable to know whether this solution would pass 

 into the tracheae. After failing to obtain a satisfactory precipitate by 



