Pachypsylla celtidis-vesiculum n. sp.
This gall appears upon the upper side of the leaf merely as a flat blister of yellowish or reddish-yellow color and of irregular outline. It is generally rounded, but often influenced and limited by the larger leaf nerves, which are rarely crossed by the gall. On the under side of the leaf the gall is still less conspicuous, and is visible only as a discolored spot with a small rounded nipple in the center. The sculpture of the surface of the gall is the same as that of the leaf, and the walls are not thickened. This gall often occurs in very large numbers on one and the same leaf, crowding one another, and often confluent. The full-grown pupae break through the wall of the gall either on the upper or lower side of the leaf. The species is most readily recognized from the very inconspicuous appearance of the gall, and more especially from the fact that it is the only one which is hardly developed on the under side of the leaf, whereas all the other leaf-galls assume there a more or less conspicuous form.”
- CV Riley: (1890) Insects injurious to the hackberry©