104 
Journal of Agricultural Research 
Vol. VII, No. 3 
aphids were fixed with the mixture of absolute alcohol and phospho- 
molybdic add and were further treated as described on page 102. No 
precipitate was found in the tracheae nor dsewhere inside the integu¬ 
ments. 
Some of the caterpillars of the catalpa sphinx (Ceratomia catalpae ), of 
Atteva aurea, of Datana sp., and larvae of the lesser wax moth, Achroia 
grisella , that had been sprayed with a solution of nicotine sulphate 
(1:64) were fixed in Carnoy's fluid. This fixative is a mixture com¬ 
posed of equal parts of absolute alcohol, chloroform, and glacial acetic 
acid, saturated with mercuric chlorid (HgCl 3 ). Mercuric compounds are 
among the general alkaloidal reagents and the mercury in mercuric 
chlorid readily predpitates nicotine. The sections of the above larvae 
were stained with Ehrlich's hematoxylin and the crystals of the mercuric 
chlorid remaining in the tissues after the fixative had been washed out 
were removed by 95 per cent alcohol containing tincture of iodin. The 
iodin unites with the mercuric chlorid, forming a compound which readily 
dissolves in alcohol, but iodin apparently has no effect on the gummy 
precipitate resulting from the union of nicotine and Carnoy's fluid. For 
this reason, if the spray solution passed into the tracheae, the nicotine in 
it should have been precipitated and should not have been affected by 
the iodin, and all of it certainly could not have been washed out while 
the slides were being run through the reagents, At any rate, after the 
sections had been treated with tincture of iodin, no precipitate of any 
kind was observed inside the integuments of these larvae. This indicates 
that a spray solution of nicotine sulphate does not enter the spiracles 
nor pass through the integument; but this fiiethod is not fully reliable, 
on account of having to remove the crystals of mercuric chlorid. 
Since it has been shown that spray solutions, as applied under prac¬ 
tical conditions, do not pass into the tracheae, a study of the spiracles 
of the aphids, coccids, and larvae that have been used in the experi¬ 
ments shows that it is practically impossible for aqueous solutions to 
enter the spiracles. The mouths of the spiracles of all these insects, 
except the coccids, are guarded by pseudohairs, which are outgrowths 
from the chitinous linings of the spiracles and by the rims (PI. 1, fig. J, r) 
turning inward and sometimes downward. The spiracles (PI. 1, fig. H, 
sp) of the coccids are practically unprotected, while those of aphids 
(PL 1, fig. B, sp) bear a few hairs. The small size of these spiracles 
seems to be the best reason why aqueous solutions can not readily pass 
into the tracheae. The hairs guarding the spiracles of some of these 
insects vary from short, stout ones, as in the larvae of Atteva aurea (Pl.i, 
fig. I, sp), in the lesser wax moth (PI. 1, fig. J, sp) and in Datana sp. 
(PI. 1, fig. K, sp) to long, stout ones, as in the caterpillar of the catalpa 
sphinx (Pl. 1, fig. E, sp) and in the larva of the Colorado potato beetle 
(PL 1, fig. M, sp). The hairs (Pl. 1, fig. N, hr) in a spiracle of the fall 
