May, 1903 .] Galls and Insects Producing Them. 
425 
galls grow older the cells near the outer epidermis become elon- 
gated as in the Phytoptus galls. 
In Pemphigus iiltnifusus (Walsh) Oestland (Fig. 49, a, b) on 
U. Americana, we have the gall originating first as a fold in the 
leaf which becomes developed into a conical structure. The struc- 
ture of the gall shows that the characteristic structure of the leaf 
is at first modified into a large number of small, irregular- shaped 
cells (Fig. 49, b). The tendency for the cells near the outer 
surface to elongate parallel to the surface begins with the. further 
development of the gall. In the very young galls the tannin is 
in very small quantities, but increases as the gall grows older. 
In Colopha uhnicola Fitch (Fig. 50, a, b) we have a condition 
almost identical with P. ulmi-fusus. The gall first appears as a 
slight fold in the leaf and later develops into the characteristic 
cockscomb gall. The cell structure is the same as in P. ulmi- 
fusus. 
In Phylloxera carya-fallax Riley (Figs. 51, 52) on H. ovata, I 
secured the youngest galls possible to detect and identify. These 
galls showed a slight projection from both surfaces of the leaf, 
but at first the gall was not so conical as at a later period of its 
development. However, the youngest galls showed the charac- 
teristic structure described in Part I of this series. The first 
effect of the parasite attack appears to be the formation of a large 
number of irregular cells. The arrangement of these cells is the 
same in the young gall as in the more mature, but the fibro-vas- 
cular bundles of the older specimens were not observed in the 
young galls. 
I was not so successful in securing young specimens of P. 
c.-globuli Walsh (Fig. 53), but, so far as I w T as able to observe, 
the line of development coincided with P. c.-fallax. However, 
the upper wall of the gall is at first very thin and grows in thick- 
ness as the gall approaches maturity. 
Phylloxera cajya-catclis Fitch of Hickory ovata was studied very 
carefully from a very complete series of specimens. The material, 
especially the younger galls, did not cut well, and so w r as not 
satisfactory for drawings. However, the development and struc- 
ture were of the typical Phylloxera type corresponding very 
closely with that just described for P. c.-fallax. The only marked 
peculiarity was the close association with fibro-vascular bundles, 
the galls always occurring on very young green twigs, on mid-rib 
or on prominent veins of the leaf. 
Pemphigus populi-transversus Riley (Figs. 55, a, b, and 56, a, b) 
and P. p.-caulis Fitch (Figs. 57, a, b, c, and 58, a, b, c) of the 
Populus are galls growing on the petiole ; the former at some 
point between the blade and stem, the latter at the base of the 
leaf. In both cases the attack is made from the outside, the same 
as in other Aphididae galls and in the Acarina galls. A careful 
