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 

 Gleditschia 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- 

 stnts no other distortion. Under the microscope the cells show 

 an elongation from midrib to margin, /. e., parallel to the surface 

 of the gall except near the margin, where they are irregular. 



C. qncrciis-pihdae Walsh. (Fig. 23 a. b. ) shows a more highly 

 developed gall structure. The epidermal layers are made up of 

 smaller cells than the norrrxal leaf. The mesophyll has lost its 

 identity and assumed the palisade structure, the long axis being 

 perpendicular to the surface of the gall. The larv^al 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: I. 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. 



Neiiroteriis 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 trichomes 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 



