DEVELOPMENT OF CERTAIN PACHYPSYLLA GALLS 
285 
EXPLANATION OF PLATE XVIII 
Fig. I. Leaf of Celtis mississippiensis showing three distinct types of Pachypsylla 
galls: (a) Variety of P. mamma, (b) Compound gall of P. venusta. (c) Gall of P. asteriscus. 
Fig. 2. Galls of P. mamma (typical form) on Celtis occidentalis leaf. 
Fig. 3. Earliest initial stage of the P. mamma gall. 
Fig. 4. Early stage of P. mamma gall. 
Fig. 5. Outline of longitudinal section of early stage of gall of P. asteriscus (i mm.). 
Fig. 5a. Tissue study of lower right-hand portion of section shown in figure 5. 
Fig. 5&. Cell study of region around mouth parts shown in figure 5a. 
Fig. 5c. Multinucleate cell from same gall as figure 5 showing two nuclei, one of which 
has partially disintegrated. 
Fig. 6. Detail from section of very young P. mamma gall ; setae partially withdrawn 
from sheath. 
Fig. 7. Gall of P. asteriscus, external and internal views. 
Fig. 8. Detail from section of very young P. mamma gall showing the forked sheath 
formed through withdrawal and reinsertion of the setae. 
Fig. 9. Section of very young P. mamma gall (i mm.). 
Fig. 9a. Cell from half grown P. mamma gall showing multinucleate condition. Note 
nuclei which show a degenerate condition. 
Fig. 10. Detail from section of very young P. asteriscus gall showing three sheaths 
(more than one is extremely rare) ; setae still inserted in one to right. 
