696 



Pomona College Journal of Entomology 



It is remarkable, too, that tlie petrokum docs not have a fatal effect on the 

 larva from the external contact. In this connection the timely paper by Mr. 

 George D. Shafer of the Michigan Agricultural Experiment Station, Lansing, on 

 "How Insecticides Kill" is ajiropos. Mr. Shafer lias established several facts 

 which have heretofore been mere conjectures or theories. Chief among these 

 facts is tliat insecticides, such as kerosene, creosote, etc., kill the insect not by 

 suffocation by plugging the spiracles or trachese, but by contact with the proto- 

 plasm and resulting poisoning. The entrance to the body, Mr. Shafer states, is 

 through the spiracles and not through the chitinous wall of the insect, except 

 to a slight extent possiblj'. In a personal interview with him a number of facts 

 were learned which are not mentioned in his paper. By immersing an insect in 

 a liquid containing a stain which could be followed in its course through the insect 

 tissue, and then dissecting the insect after a certain length of time, he found 

 that the liquid enters through the spiracles, passes through the tracheal walls 

 and thus has access to the protoplasm of the cells. Very slight, if any, traces 

 were found of the liquid having entered through the chitinous cutieula. 



This is borne out by some tests that I have made of the effect of crude 

 petroleum on insects. A small droplet was placed on a number of plant-lice 

 (Aphids) in such a way as to prevent its entrance into any of the spiracles if 

 possible. In every case the insect was agitated, either by the unnatural and un- 

 comfortable feeling of a heavy, sticky substance clinging to it, or perhaps by 

 some pain occasioned by the oil. After the plant-louse had quieted down it seemed 

 to be more nearly normal, though quiescent, and lived for nearly twelve hours. 

 Leaf-beetles (Diabrotica soror) treated in a similar manner lived for thirty-six 

 hours or more. Any insects, however, immersed in tiie i)etroleuni died within 

 a very few minutes. 



With these facts before us it is much more conceivable that an organism 

 niiglit exist in petroleum, provided that certain adaptations had taken place. First 

 tlie spir.ieles must be thoroughly protected, and, second, the absorption of tlie 

 oil thnuigii the bony wall, however slight, must be counteracted. The first of 

 these requirements is provided for, first, by the ciliate fans which are kept above 

 the surface of the oil by surface tension and thus protect the stigmata from the 

 entrance of oil into the spiracles, and, second, by the telescoping of tiie spiracular 

 processes when the larva goes under the surface. The other requirement is met, 

 probably, by the mass of clear, gelatinous (?) tissue beneath the cutieula. This 

 prevents whatever oil might be absorbed through the body wall from being taken 

 U]) b_v the protoplasm of the inner tissue. 



Tests were made of the larvie in otlur li(iuids, to deterniiiie what effect 

 these would have on the insects. It was found that in a li(iuid of a tliinner con- 

 sistency than the petroleum the larvie were unable to come to tlie surface, and, 

 therefore, would be killed in a eonqiarativcly sliort time. Even if the liquid were 

 not deep enough to cover tlie larva-, the latter could not jirotect its spiracles on 

 account of the inability of the eiliate fans to sujiport the spiracular processes when 

 tlie surface tension was lessemd. If petroleum, however, was mixed witii other 

 liquids so tii.it the consisteney of the iiiixtiire was thick enough to support the 



