EARLY STAGES IN THE DEVELOPMENT OF CERTAIN 
PACHYPSYLLA GALLS ON CELTIS 
B. W. Wells 
One of the most interesting angles from which the insect gall problem 
may be attacked is that involving the early stages of gall formation. The 
present study is a morphological one dealing with the beginning stages of 
two Pachypsylla (Fam. Psyllidae of the Hemiptera) galls as they develop 
on the leaves of the hackberry (Celtis). 
The Galls 
The galls concerned are common ones in their respective regions. 
Pachypsylla mamma Riley is found on Celtis occidentalis L. It is a 
hemispheric to subcylindric outgrowth projecting from the under side of 
the hackberry leaf (PI. XVIII, fig. 2). Above, on the opposite side, is a 
prominent depression, in the center of which is a minute conical process. 
Interiorly there is an inverted dome-shaped larval cavity lined by soft 
nutritive tissue; this latter in turn is invested by masses of hard, "pro- 
tective" sclerenchyma. For a detailed discussion of the adult gall, the 
reader is referred to an earlier paper of the author's (8) . 
Pachypsylla asteriscus Riley occurs on Celtis mississippiensis Bosc. 
(fig. I, c, and fig. 7). It projects from both sides of the leaf, the part on 
the under side assuming the shape of a much abbreviated Convolvulus 
corolla, the part above consisting of a slender, straight process. These 
processes are attached to a blister-like enlargement in the plane of the leaf, 
in the interior of which is the lens-shaped larval chamber. Well defined 
layers of sclerenchyma occur in this gall, bounding the nutritive tissue. 
Both of these galls are prosoplasmas or "higher" galls characterized 
by definite constitution and growth period and possessing highly specific 
forms and differentiation structures, which latter unusual characters are 
induced to appear under the action of the specific stimulus developed by 
the insect larva. 
Earlier Work 
No ontogenetical studies of the beginning stages of the galls formed by 
this genus of insects have heretofore been made. A few studies, however, 
have been made on the galls of other genera of the Hemiptera which may 
properly be presented here. 
Prillieux (6) finds, in his study of the woolly apple aphis {Schizoneura 
lanigera Hausm.) gall on the apple twig, the following facts: No change 
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