development of certain pachypsylla galls 277 
Initial Stages of the Galls 
Before attacking the problem of primary cecidium ontogenesis, a very 
brief statement of the Hfe history of the insect will be given leading up to 
gall initiation. The following deals with P. mamma. P. asteriscus has an 
almost identical life history. 
The adults, which are formed immediately after the escape from the 
galls of the fully grown nymphs in the fall of the year, overwinter in the 
bark crevices or in ground debris. After mating in early spring, the females 
lay their eggs on the under side of the young leaves as they begin to protrude 
from the buds. The point at which the egg is attached is commonly killed, 
this killing resulting in a prominent hole in the mature leaf. This explains, 
in part, the presence of numerous perforations in gall-infested leaves. 
The minute oval eggs (0.3 mm. long) hatch in 2-3 days, the nymph imme- 
diately migrating to the upper side of the very young leaf, where, after 
reaching a position near a principal vein, it settles down to initiate gall 
development. Once the growing gall has engulfed it, it is a prisoner until 
its escape the following fall as a mature nymph. 
The nymph at the time of gall initiation is a minute, salmon-colored, 
flattened insect, oval in outline, and measures 0.22 mm. in length. On the 
ventral side, the setae, which in this minute insect are perfectly formed, 
extend from the body at a place slightly anterior to the median point. 
The setal puncturing mechanism measures but little over i micron in trans- 
verse diameter. There are no special structural modifications in these 
gall-making nymphs which distinguish them from the large numbers of 
non-gall-forming Hemipterous larvae. 
Under the binocular microscope, the early superficial conditions in gall 
development may be easily observed and are as follows: The insect's 
body is pressed close to the upper side of the leaf; a shallow downward 
evagination forms, lowering the insect into the body of the leaf; when the 
upper side of the insect has been lowered to the level of the leaf surface, a 
very rapid upward growth of the leaf tissue surrounding the insect takes 
place, appearing first like a crater but finally as a closed cone completely 
covering the nymph. Both P. mamma and P. asteriscus galls are charac- 
terized in their initial stages by this combination of the diverticulum and 
walled conditions Umwallungen" of Kiister). The P. mamma gall in its 
further development emphasizes the diverticulum character, the original 
"cover-cone" developing but slightly, while in the case of P. asteriscus no 
prominent evagination occurs but the cover-cone grows into the prominent, 
slender, subcylindric process so characteristic of this gall. 
Histological Phenomena 
At the time when the nymph inserts its setae into the embryo leaf, the 
leaf^cells are not in the primordial condition of undifferentiation but show 
