no 
Journal of Agricultural Research 
Vol. VII, No. 3 
resulting from the union of the fumes of nicotine sulphate and phospho- 
molybdic acid. It is practically impossible to find an aphid or perhaps 
any other insect that does not carry at least a little organic matter on the 
integument. Bees, for instance, when placed into silicotungstic acid or 
into any other alkaloidal reagent soon become more or less covered with 
a white precipitate, showing that the hairs are full of organic matter. 
It is supposed that some of the nicotine fumes which had passed into 
the tiacheae had not changed into liquid by the time the insects were 
fixed, and in order to prove that the fixative precipitates nicotine, whether 
in a liquid or in a gaseous state, a test tube was filled with the fumes from 
the nicotine sulphate. Immediately after a small quantity of the fixative 
was poured into the test tube a yellowish precipitate w r as thrown down. 
Upon examining the sections that had not been stained, a few of them 
were observed to have a light-tan color and the chitin in places assumed 
a darker tan color. All the aphids after being fumigated assumed a light- 
tan color; this color was particularly noticeable when the insects were 
embedded in white paraffin. The same species of aphids, not fumigated, 
had a whitish appearance. The light-tan color in most of the sections is 
caused by minute particles of tan-colored precipitate on the integument, 
in the tracheae, and to a limited degree in the tissues, but in a few cases 
the tan-colored tissues contain no perceptible precipitate. In such 
instances the fumes must have penetrated the cells and mixed with the 
protoplasm before the cell constituents were coagulated. 
Most of the tracheae contain more or less tan-colored precipitate, but 
very little of it lies outside the tracheal walls. Plate 3, figure A, represents 
a large trachea ( tr ) cut both crosswise and longitudinally near a spiracle, 
showing the precipitate (pr) inside the tracheal walls and some scattered 
in the fat cells (jc) which surround the trachea. Tracheae may be traced 
for short distances between the cells of any tissue, but the precipitate is 
never found further from the tracheae than that shown in Plate 3, figure 
A. It is quite probable that some of it which lies in the cells has been 
dragged there from the tracheae by the microtome knife. 
Baker (2) has well described the respiratory system of the woolly apple 
aphis (Eriosoma lanigerum Hausm.), and he gives two drawings, one repre¬ 
senting the dorsal tracheal system and the other the ventral tracheal system. 
To illustrate how well a gaseous or vaporous form of an insecticide may be 
distributed to all tissues, these two figures are again given (PI. 3, fig. D, E). 
It is to be noted that there are seven pairs of abdominal spiracles and two 
pairs of thoracic ones and that in the abdomen a short distance from 
each spiracle the trachea divides into two smaller branches, one of which 
passes dorsally to help form the dorsal tracheal system (PL 3, fig. D) 
and the other passes ventrally to help form the ventral tracheal sys¬ 
tem (PL 3, fig. E). In the ventral system of the thorax there are two 
ventral arches, while in the dorsal system of the abdomen there is only 
one, the dorsal arch (PL 3, fig. D, da). The anterior ventral arch (PL 3, 
