April, 1904 .] Galls and Insects Producing Them. 
125 
Pachypsylla celtidis mamma Riley (Figs. looa, b), and P. c. -gemma , 
Riley. 
The study of these mouthparts gave no new anatomical facts. 
The different genera showed considerable variation as to length 
of beak and setae. In general it may be said that the setae tend 
to increase in the distance they may be protruded beyond the tip 
of the beak as the galls approach complexity. This, however, 
cannot be considered an exact rule, since the S. americana, C. 
ulniicola and H. hamamelis have setae of practically the same 
length, although the gall produced by S. americana is much 
simpler than the galls produced by either C. ulniicola and H. 
hamamelis (Part I, Figs. 12, 13 and 15). It was impossible to 
make exact measurements of the distance the setae protruded 
beyond the tip of the beak, since it was impossible to tell whether 
the setae were full)* extended or partiall}’ retracted. The above 
conclusions were reached after the examination of a large number 
of specimens. 
So far as I have been able to determine the insects do not 
remain attached to any one point for a great length of time. The 
P. c. -mamma (Figs. looa, b) has a gall of the greatest complex- 
ity, and the insect has .setae which protrude farther beyond the 
point of the beak than any other examined ; a large number of 
these galls were opened and the position of the insect noted. The 
insect was never found attached and apparentl}" had no definite 
point of attack. 
The preceding observations emphasize Conclusions 6 and 8 of 
Part I and a statement in the first of Part V. That is, the modi- 
fication of the plant tissue to form the gall is pureh' mechanical, 
being a continuous effort on the part of the plant to heal the 
wound produced bj’ the repeated puncturing of the cells by the 
insect. When a branch is cut from a tree a growth is produced 
which tends to cover the wound. In this case a single wound 
and a single stimulus which is purely mechanical but which pro- 
duces rapid growth for the purpose of covering the wound. In 
the case of Aphididae and the Psyllidae galls the wounds are 
more slight but repeated rapidly, the stimulus is mechanical and 
the growth rapid, tending to cover the injury. 
It is possible that the setae of the various genera may stimulate 
different ti.ssues and thus cause galls of var}’ing complexity, but 
upon this question I am not read}- to give a definite statement. 
2. DIPTERA. 
The Cecidomyid galls occur upon a greater variety of hosts than 
any other group of galls, and as previously stated in Part show 
by far the greatest variation in structural characters and the 
smallest number of typical characters. 
