16 Annals Entomological Society of America __[Vol. VII, 
From a further study of Table II, it will be seen that between 
the first and fourth larval stages there has been a considerable 
increase in the size of the cells which constitute the tubules. 
The greatest increase was between the third and fourth larval 
stages, which was coincident with the greatest growth of the 
gall, when it doubled in size. During this period the larva 
gives off the greatest amount of secretion from the Malpighian 
vessels. After this time, as already noted, the larval form 
changes, and the amount of secretion diminishes rapidly, so 
that a larva taken, say two weeks later, that is about the first 
of September, would not pour forth any secretion when placed 
on a foreign substance. The lining of the cavity is by this 
time quite dry, brittle, and deep yellowish brown in color. 
Now, from the development of the Malpighian vessels, and 
the amount of secretion poured forth by them coincident with 
the gall development, also in view of the effect of this secretion 
when applied to the plant tissue in the experiments, it is evident 
that the Malpighian vessels have elaborated some product 
which when poured forth by the insect stimulates the surround- 
ing plant tissue to rapid growth. In a few instances, we have 
found urate crystals in the lumen of the tubules, but urates are 
present in the Malpighian vessels of all insects, and, as Réssig 
has shown, chemically pure urates do not produce galls. Hence 
an additional factor is without doubt present in the secretion of 
the Malpighian tubules of Dryophanta erinacei, and this 
produces the effective stimulus. 
The Malpighian Vessels of the Inquilines. 
The Malpighian vessels of the inquilines were dissected out. 
They were white in color, and consisted of two slender tubules. 
These arise at the point of union of the mid and hind-intestine, 
having a broader attachment than that found in Dryophanta 
erinacei, (Fig. 60, Pl. VIII). The cells are smaller than those 
of D. erinacei, the nuclei more regular, and the lumen quite 
distinct. They show no evidence of great secreting activity, 
and in a longitudinal section appear as in Fig. 61, Pl. VIII. 
The larva when placed on a glass slide does not pour forth a 
secretion as does Dryophanta erinacei. Further, the species, 
though an inhabitant of a gall, does not emerge from the egg 
until the gall has attained considerable growth. Its eggs are 
