426 
The Ohio Naturalist. 
[Vol. Ill, No. 7 , 
study of an excellent series of both galls shows a cell structure 
and development very similar to other Aphididae galls ; i. e., a 
large number of small, irregular cells. In P. p.-transversus (Fig. 
55, a, b) the gall originates as a swelling on the petiole and 
within this swelling is a large cavity opening to the outside 
through a slit. In the P. p.-caulis the same condition is true 
but the attack of the insect causes a one-sided growth, resulting 
in the petiole being twisted at right angles to the blade (Figs. 
57, a, b, c, and 58, a, b, c). 
A careful examination of the cell structure of P. p.-transversus 
(Fig. 56, a, b) and a comparison with the unaffected petiole (Fig. 
54, a, b) indicated a very rapid growth, resulting in the very 
large number of small, irregular cells. The character of the 
young and of the mature gall was practically the same, and not 
different, as in the more highly developed galls of other orders. 
The fibro-vascular bundles were very slightly affected. 
P. p.-caulis showed the same cell structure and development, 
and, judging from these points alone, one would be unable to 
separate these two galls. 
3. GALLS OF PSYLLIDAE. 
In Pachypsylla celtidis-mamma Riley (Figs. 59 and 60, a, b, c) of 
the Celtis oceidentalis the youngest galls did not show a cavity, 
but showed a modification of the leaf by which there is formed a 
large number of small, irregular cells which can be readily sepa- 
rated into two zones ; the upper made up of small, and the lower 
of somewhat larger cells (Fig. 59). I was unable to secure speci- 
mens intermediate between this stage and a later stage, showing 
the true form of the gall (Fig. 60, a, b, c) The youngest galls, 
showing the true form, exhibited four -well-defined zones: (1) 
epidermis, (2) zone of large, irregular-shaped cells, (3) zone of 
elongated cells, (4) zone of irregular-shaped cells next to the 
larval cavity. Adjacent to zone (3), but derived from zones (2) 
and (4), are cells which even in very young galls show schleren- 
chyma characteristics. As the gall approaches maturity this 
tissue increases until in the mature gall it may be found in great 
abundance. This gall is undoubtedly the most highly developed 
of any of the Hemiptera galls which I have studied. 
4. GALLS OF CECIDOMYIA 
Although I have a large number of Cecidomyia leaf galls, I 
have succeeded in getting a series of only two species. Since the 
Cecidomyia show by far the greatest variation in structural char- 
acters and the smallest number of typical group characters, two 
species are not sufficient to draw a very definite conclusion. 
In Cecidomyia glcditsiae O. S. (Fig. 61, a, b) the two halves of 
the leaflet never have an opportunity to unfold, but there is a 
