12 Annals Entomological Society of America  [Vol. VII, 
in the central cavities, but it carries its parasitic habits to the 
extent of mining from cavity to cavity, and having a meal out 
of the occupants. Fig. 12, Pl. Il shows where a larva of Syner- 
gus erinacei has mined from A-B, also one is already breaking 
down the wall at C. Fig. XI, Pl. II shows where a larva of 
Synergus erinacei has mined from a lateral to a central cavity. 
In all, over eighty instances of mining have been observed. On 
eighteen occasions we have fed Dryophanta erinacei and dif- 
ferent Chalcid larva to the inquilines, but only once were we 
able to induce it to attack a larva of its own species. The 
average time required by Synergus erinacei to consume a larva 
was 144 hours. Hence we see that the supposed guest is not 
only a plant feeder, but has shown itself to be a serious parasite 
among the occupants of the gall. 
The Stimulus to Gall Production. 
A. The Relation of the Malpighian Vessels to Gall Formation. 
Investigators have generally agreed that galls cannot be 
produced apart from the presence of insects, but different 
theories have been presented as to the cause of the abnormal 
growth. Adler (1881) points out that in Neuroterus levius- 
clus and Biorhiza aptera, the gall is first caused by the insect 
wounding the surrounding cells with its fine mandibles, and that 
the growth of the gall.is in some way dependent on the presence 
of the larva. Cook (1903) in his publication ‘‘Galls and Insects 
Producing Them,”’ states that the Cynipide stimulate the 
plant to excessive growth by biting, and it is his contention 
that gall formation is primarily the result of mechanical stimula- 
tion. Rédssig (1904) in a paper entitled ‘‘ Von Welchen Organen 
geht der Reiz zur Bildungder Pflanzengalle aus?’’ attributes 
a rdéle to both oenocytes and the Malpighian vessels, and though 
regarding the latter as giving off an effective secretion, he attrib- 
utes the primary source of this secretion to the oenocytes. 
His studies of the Malpighian vessels are purely from the 
morphological standpoint, and are based on a limited and 
poorly selected variety of species. He places great emphasis 
on the size of the cells constituting the vessels, and on the size 
of the vessels as compared with that of the larva. Moreover 
he includes in his discussion species that do not produce galls 
by means of any product poured forth by the Malpighian 
