May, 1902 .] Galls and Insects Producing Them. 
267 
In P. c. depressa (Fig. 20) and in P. vastatrix (Fig. 21) the 
small larval chamber and general arrangement of the cells is very 
similar to the leaf galls produced by Cecidomyia verrucola (Fig. 2.) 
4. The Cecidomyia Galls. This group of galls shows con- 
siderable variation. C. gleditsiae O. S. (Fig. 22 a. b. c. d.) of 
Gleditscliia triacanthos may be taken as a type of one of the 
simplest. In this the margins of the leaflets are in contact so as 
to form a more or less sperical body. To the naked eye it pre- 
sents no other distortion. Finder the microscope the cells show 
an elongation from midrib to margin, i. e., parallel to the surface 
of the gall except near the margin, where they are irregular. 
C. quercus-pilulae Walsh. (Fig. 23 a. b. ) shows a more highly 
developed gall structure. The epidermal layers are made up of 
smaller cells than the normal leaf. The mesophyll has lost its 
identity and assumed the palisade structure, the long axis being 
perpendicular to the surface of the gall. The larval chamber is 
large and rather irregular and indefinite, and resembles a large 
inter-cellular space. 
C. verrucola O. S. (Fig. 24 a. b. ) on Tilia americana shows a 
much higher complexity than either of the preceding. The epi- 
dermis is made up of small cubical cells. The differentiation into 
palisade and mesophyll is entirely lost, the cells are very irregu- 
lar, but show a tendency to elongation at right angles to the 
surface of the gall. The larval chamber is small and well defined. 
C. q.-pilulae (Fig. 23) and C. verrucola (Fig. 24), especially 
the latter show a striking resemblance to the more highly devel- 
oped Phylloxera galls such as P. c. -depressa (Fig. 20) and P. 
vastatrix (Fig. 21). 
5. The Cynipidae Galls. This family presents the most 
striking series of evolutionary development of any family studied 
and is also apparently the most highly developed. 
The general characters presented by these galls are small, 
cubical epidermal cells ; loss of differentiation between palisade 
and mesophyll cells, all having assumed an irregular character ; 
a differentiation into two well defined zones of cells, the outer 
made up of large, non-staining cells, the inner made up of smaller, 
deeply staining cells and surrounding the larval chamber. 
Fockeu divides these into four zones, which he designates as 
follows: 1. Epidermis; 2. Parenchyma; 3. Protective; 4. 
Nutritive (“Masse alimentaire ”). These four zones may be 
easily traced in most of our American forms, but in some they 
show very indistinctly. 
Neuroterus irregularis O. S. (Fig. 25 a. b. ) is a small, fleshy, 
solid, irregular gall projecting from both sides of the leaf. It is 
covered with dense growth of tricliomes and contains several 
larval chambers. In structure it does not correspond to the pre- 
ceding description, as well as the galls described in the latter part 
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