DEVELOPMENT OF CERTAIN PACHYPSYLLA GALLS 
283 
any one in the study of the initial stages of Hemiptera galls, that throws 
any light whatever upon the profound problem of the nature of the highly 
specific stimulus applied by the insect to the embryonic plant tissue. 
Rosen's (7) "belief" that "the continuous sucking action by the insect 
at one fixed point for fifteen days is the initial stimulus for gall develop- 
ment" will not hold, for there are too many non-gall-making hemipterous 
insects sucking at one place for extended periods without ensuing hyper- 
trophy or hyperplasia. Further, as previously shown, little or no ingestion 
takes place, for in the early days of its gall-making activity no increase in 
size of the larva occurs. It is hardly conceivable that the pumping action 
of the insect's sucking apparatus would function without ingestion going on, 
especially when the proboscis end was in the presence of liquid or semi- 
liquid food. Attention may also be called to the fact that no experimental 
evidence along this line has yet been produced. 
On a priori grounds it would seem much more natural to assume with 
KiAster that such prosoplasmas were Chemomorphosen'' ; that a highly 
specific chemical substance was introduced producing the radial effects 
noted. This conception, however, has not the slightest direct evidence 
for its support, since such a specific chemical substance producing a proso- 
plasma has never been demonstrated. The difficulty of demonstrating it 
may be of course due only to the exceedingly minute amounts of it which 
are formed. Conservatism here as elsewhere is the desirable position to 
assume. 
One who has repeatedly removed these minute, delicate gall-inciting 
organisms from the plastic, watery tissue of the embryonic galls is impressed 
with the possibility of interpreting the situation in terms of correlation 
phenomena. The writer ventures to suggest that the insect may be acting 
as a whole in the matter. 
Though the structural facts concerning gall initiation and early develop- 
ment are of much interest, we must confess that these data give us no 
more fundamental, explanatory information concerning gall ontogenesis 
than do the structural facts of normal growth explain normal ontogenesis. 
We must remain satisfied with having merely demonstrated the leading 
morphological facts involved in the early development of these two Pachy- 
psylla galls. 
Summary 
1. The early stages of two hemipterous insect galls of the hackberry 
leaf were studied. The insects belong to the genus Pachypsylla of the 
Psyllidae. 
2. The newly hatched nymphs after placing themselves upon the upper 
side of the young leaf initiate the following concomitant changes: (a) 
Formation by the cytoplasm of the affected cells of a sheath-like structure 
around the inserted setal proboscis. The cells are not killed. The pro- 
