348 GROWTH OF PLANTS 



attachments of the appendages. While it is conceded that the pyrethrum 

 could penetrate to a greater depth than the stain, it seems improbable that 

 the reverse could be true. 



As pyrethrum extract is slightly soluble in water, it is probable that it 

 "would be soluble in the body fluids and be carried to the nerve ganglia. 

 Portions of the ventral nerve cord were dissected from insects that had been 

 killed '\\dth pyrethi'um extract. The tissue was fixed in 95 per cent alcohol 

 and stained for 5}^ hours with 0.1 per cent aqueous toluidine blue, and 

 imbedded and sectioned by the usual paraffin-xylol method. Cross-sections 

 of the ventral ganglia and nerve cord stained blue throughout in all cases, 

 except in larvae that had been killed with pyrethrum. Scattered among the 

 blue-stained cells, in this case, were areas in which the cells stained violet. 

 In addition there were other areas that appeared vacuolated, the margins 

 of which stained dark blue (Fig. MOB). The contrast was so striking that 

 one could readily distinguish between the controls (Fig. 140A) and larvae 

 treated with pyrethi-um extract. Insects killed with nicotine sulphate, 

 rotenone, and lead arsenate did not show visible differences from the con- 



trols.20- 43. 44 



Kriiger ^^ noted morphological changes in the nerves, muscles, and hypo- 

 dermis of Corethra larvae that had been treated ^^^th suspensions of pyreth- 

 rum flowers in water. The ventral nerve ganglia of the treated larvae 

 showed vacuoles that were not present in untreated larvae. 



It seemed desirable to pursue this investigation further and determine 

 more precisely the region of the nervous system affected. Lesions were 

 found throughout the main part of the central nervous system, in the brain, 

 suboesophageal ganglion, thoracic ganglia, abdominal ganglia, and the con- 

 nectives of meal worm larvae and adult grasshoppers {Melano-plus femur- 

 ruhrum De Geer) killed by pyrethrum.^ From this histopathological study 

 it was concluded that death is caused by the destruction of the central 

 nervous system, followed by paralysis. That pyrethrum causes nerve 

 lesions in insects was later confirmed by Klinger,^^ who made a histological 

 study of the nerve lesions of the larvae of the gypsy moth (Porthetria 

 dispar L.) after application for 24 hours of a 15 per cent pyrethrum extract. 

 He reports that the nerves appeared to be isolated and surrounded by 

 spaces, as contrasted with the check where the tissue was not distorted. 

 A similar histological change was reported by him for moribund insects. 



The use of activators with pyrethrum in housefly sprays presented new 

 problems in the mode of action of the combined spray. The percentage of 

 fUes that are knocked down is fully as important in fly sprays as the per 

 cent killed. Interest was therefore centered on the action of pyrethrum 

 and activators on the nervous system of moribund flies and in individuals 

 that fully recovered after spraying. 



The foremost effect of pyrethrum^'* by the Bodian technique consists 

 of drastic destruction of the fiber tracts of the brain. The general disorgani- 

 zation is apparently due to actual dissolution of the fibers. In addition to 



