112 
Journal of Agricultural Research 
Vol. VII, No. 3 
Two of the recently emerged house flies that had been fumigated were 
fixed with the mixture of absolute alcohol and phosphomolybdic acid, 
and were sectioned. Most of the sections were not cleared well and con¬ 
sequently are not reliable for a study of this kind, but a feW of them are 
fairly reliable. Plate 3, figure M, represents a spiracle and a portion of a 
trachea taken from the abdomen of a fly. The neck of the spiracle was 
almost closed with the precipitate ( pr ), while scattered particles of it 
were seen along the walls of the trachea. Plate 3, figure L, represents 
two medium-sized tracheae ( tr ) and a large fat cell (/c), taken from another 
section through the abdomen, showing fine particles of precipitate (pr) 
inside the tracheae- and in the fat cell outside the nucleus. 
In conclusion, under this head a few more remarks may be made. A 
ganglion is composed usually of two more or less round or oblong halves 
which are securely united to one another. The outer or cortical layer is 
cellular, while the center of each half never shows definite cell walls or 
nuclei like those in the cortical layer. There is also usually a difference 
in coloration between these two portions after being stained, although 
this difference in the aphids stained with safranin was scarcely noticeable, 
and the cortical layer was not cellular. This was true not only for those 
that had been fumigated, but also for the controls that had been fixed 
and stained the same way. In the illustrations the two portions are dis¬ 
tinguished by a difference in stippling. At no time was any anatomical 
change observed in any insect that could actually be attributed to the 
effect of nicotine. The failure to see such changes, if they existed, is not 
significant, because the physical changes effected by the fixation probably 
mask the smaller physical changes brought about by the nicotine. In 
the higher animals, however, it has been observed that nicotine causes 
slight anatomical changes in the cortical layer of the brain. 
In cross sections of caterpillars the tracheae are easily traced into the 
ganglia. Most of them penetrate the neurilemma and pass between the 
two halves, where they ramify considerably by sending minute branches 
through both portions of a ganglion. Occasionally a small tracheal 
branch may enter a ganglion near or at the base of a nerve. The ramifi¬ 
cations of tracheae inside the ganglia of aphids are not so easily observed, 
but they seem to exist, although perhaps not so abundantly. 
All the preceding histological work has shown that nicotine spray solu¬ 
tions, and even nicotine used as a fumigant, do not penetrate chi tin. To 
determine whether pure nicotine, undiluted, is able to penetrate chitin, 
larvae of house flies, of the lesser wax moths, and aphids (Aphis rumicis) 
were submerged in this fluid for 35 minutes. A study of the sections 
made from these insects showed that the nicotine had passed into the 
newly-formed chitinous wails of the tracheae in the larvae of the house 
flies and wax moths, but had not passed all the way through them. Plate' 
3, figures J and K, representing cross sections of small tracheae, well illus- 
