10 BULLETIN 938, IT. S. DEPARTMENT OF AGRICULTURE. 



mass of eggs (both recorded as hatched) were closely observed, more 

 larvae had emerged from the former than from the latter. The ones 

 sprayed with nicotine sulphate were slightly retarded in hatching, 

 and the eggs in most of the masses which did not hatch contained 

 aborted embryos, while the remainder of them were totally unde- 

 veloped. 



DISCUSSION AS TO HOW NICOTINE SULPHATE ACTS AS AN OVICIDE AND 



LARVICIDE. 



It is difficult to determine accurately how any insecticide kills a 

 larva or an adult insect, and in the light of our present knowledge 

 along this line it would perhaps be impossible to ascertain definitely 

 how an ovicide affects insect eggs. For this reason no attempt was 

 made in the present investigation to determine how nicotine sulphate 

 killed the eggs tested. Nevertheless a little theory on this subject 

 may not be out of place here. 



Before attempting to explain how nicotine sulphate acts as an 

 ovicide, it is first necessary to know something about the coverings of 

 an insect egg. It is well known, as compiled by Packard (£, p. 520), 

 that a ripe insect egg has two coverings. The external one, the egg- 

 shell or chorion, is usually thick and hard; it is pierced by one or 

 more minute canals, the micropyles, and consists of two chitinous- 

 like laminae kept in close apposition by means of numerous minute 

 pillars. The internal covering, the vitelline membrane, is always 

 thin and delicate. The developing embryo inside the egg is supposed 

 to breathe directly through these coverings as does a chick embryo 

 through the shell of a hen's egg. 



According to the results already presented, it was ascertained that 

 freshly laid eggs, sprayed with solutions of nicotine sulphate, were 

 affected more than were older eggs likewise treated. The chorions 

 of these eggs were not as hard as those of the older eggs. This fact 

 alone may explain the difference in the mortality recorded. Since it 

 is scarcely possible that the spray solutions passed through the egg- 

 shells, there remain two possible methods of explaining the death of 

 the eggs sprayed: (1) Upon evaporation of the spray solutions there 

 was usually left a thin film on the leaves sprayed, and perhaps also 

 on the eggs. This film on the eggs might have decreased the aeration 

 through the eggshells and thus caused death by suffocation. (2) 

 Since practically all of the aborted embryos appeared to have died 

 during the last stage of development, the preceding view does not 

 seem to have much weight. As the sprayed surfaces continued to emit 

 a nicotine-like odor for some time after the spray solutions had been 

 applied, it seems reasonable to suppose that the embryos were killed 

 by breathing the exhalation from the sprayed surfaces through the 

 eggshells. Nicotine is a nerve poison, and the more highly developed 



