696 Pomona College Journal of Entomology 



It is rciiiarkablc, too, tliat tlu- ])(tr()l(Uin dois not liavi- a fatal effect on the 

 larvae from tiic external contact. In tliis connection the timely paper by Mr. 

 George D. Sliafer of tlie Michigan Agricultural Experiment Station, Lansing, on 

 "How Insecticides Kill" is aijrojios. .Mr. Siiafer has establislied several facts 

 which have lierctofore been mere conjectures or tlicories. Cliief among these 

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

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

 ])lasm 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 possibly. In a personal interview with him a number of facts 

 were learned which are not mentioned in his pajicr. 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 

 tliat tlie liquid enters through tlie spiracles, passes through the tracheal walls 

 and tiuis lias access to the protoplasm of the cells. Very slight, if any, traces 

 were found of the liquid having entered through tlie chitinous cuticiila. 



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 

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

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

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

 some iiaiii 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 liours. 

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

 hours or more. Anv insects, however, immersed in the petroleum died within 

 a verj' few minutes. 



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

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

 the spiracles must be thoroughly protected, and, second, the absorption of the 

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

 these requirements is provided for, first, by the eiliate 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 the spiraeular 

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

 ))robal)ly, by the mass of clear, gelatinous ( ?) tissue beneath the cuticula. This 

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

 U]) by the protoplasm of the inner tissue. 



Tests were made of the larvae in other liquids, to determine what effect 

 these would have on the insects. It was found that in a liquid of a thinner con- 

 sistency than the petroleum the larva' were unable to eonie to the surface, and, 

 therefore, would be killed in a comparatively short time. Even if the liquid were 

 not deep enough to cover the larv*, the latter could not protect its spiracles on 

 account of the inability of the eiliate fans to support the s))iraeular processes wlien 

 the surface tension was lessened. If ))etroleiiin, however, was mixed with other 

 liquids so that the consistencj' of the mixture was thick enough to support the 



