272 The Ohio Naturalist. [Vol. II, No. 7^ 



5. The families show parallel lines of development from a low- 

 form of gall structure up to a high form . e.g., Aphididae and 

 Cynipidae. 



6. I am inclined to believe that the modification of the plant 

 tissue is purely mechanical. The loss of differentiation between 

 palisade and mesophyll and the closing up of the intercellular 

 spaces would be a natural result of rapid cell division. The 

 elongation of cells in certain directions would be a natural result 

 of mechanical tension arising from rapid growth. In the famil}^ 

 Aphididae where the gall is primarily a folding of the leaf the 

 elongation of the cells is parallel with the surface of the gall. In 

 those galls where the formation is a thickening of the leaf the 

 long axis of the cells is perpendicular to surface of the formation. 



7. The presence of at least two zones, of which the inner may 

 be considered nutritive, is very common. 



8. The formation of the gall is probably an effort on the part 

 of the plant to protect itself from an injury which is not sufficient 

 to cause death. Both Adler and Fockeu consider that after the 

 first stages of formation the gall becomes an independent organ- 

 ism growing upon the host plant. 



9. Trichomes are far more prominent in galls produced by 

 mouth parts than in those produced by oviposition. 



10. It appears from these studies that the histological charac- 

 ters of the gall will prove very important in determining the 

 characters of the species. 



Part II. Apicai. Bud Galls. 



In my third conclusion in the preceding paper I have expressed 

 a belief that galls produced by the same genus of insects show a 



cided resemblance even though produced on widely different 

 J. nts. Furthermore, this similarity seemed to be due to the par- 

 ticular part of the host plant to which the stimulus was applied. 



The following study of the apical bud galls seem to indicate 

 that when corresponding parts of different plants are stimulated: 

 W insects of different genera that the galls produced have char- 

 acters in common. 



The gall produced by Cecidomyia solidaginis Lw. (Fig. 31) is 

 merely a large bunch of leaves at the end of the stem of Solidago. 

 The cone-shaped gall of Cecidoviyia salicis-strobiloidcsO . S. (Fig. 32) • 

 at the tip of the twigs of Salix is a bunch of leaves reduced in 

 size and so compactly arranged as to produce the peculiar cone 

 effect. A further examination of these two galls shows that the 

 tips of the stems are enlarged and that the larval chamber is in 

 the apex. 



A superficial examination of the gall of Callirhytis clavula Fitch 

 TFig. 33 a. b. c. d.) shows no resemblance to the preceding galls- 

 except in location at the tip of the stem. The gall is apparently 



