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 ccUidis-mannua Riley (Figs. 59 and 60, a, b, c) of 

 the Celtis occidentalis the j^oungest 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: (i) 

 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 gleditsiac O. S. (Fig. 61, a, b) the two halves of 

 the leaflet never have an opportunity to unfold, Init there is a 



