April, 1904.] Galls and Insects Producing Them. 129 



2. " The gall does not form until the appearance of the larvae. 

 Therefore all galls are produced by mouthparts." (Part VIII, 

 Con. I.) The Nematus galls are an exception. 



3. "The morphological character of the gall depends upon 

 the genus of the insect producing it rather than upon the plant 

 on which it is produced." (Part I, Con. 3.) 



4. ' ' Within each f amil}' we find certain morphological resem- 

 blances." (Part I, Con. 4.) 



5. " The families show parallel lines of development from a 

 low form of gall structure up to a high form." (Part I, Con. 5.) 



6. " The presence of at least two zones, of which the inner 

 may be considered nutritive." (Part I, Con. 7. ) 



7. "The formation of the gall is probably an effort on the 

 part of the plant to protect itself from an injur}^ which is not 

 sufficient to cause death. Both Adler and Fockeu consider that 

 after the first stages of formation the gall becomes an independ- 

 ent organism growing upon the host plant. This is probabl}' true 

 in the highly developed galls of Aphididae, Cecidomyia, and 

 Cj-nipidae, but the writer is doubtful if this is true in the less 

 complex galls of Acarina, Aphididae and Cecidomyia." (Part I, 

 Con. 8 and Part V, Con. 6.) 



8. "In the formation of all leaf galls except the Cecidomyia 

 galls the normal cell structure of the leaf is first modified by the 

 formation of a large number of small, compact, irregularly shaped 

 cells. In the galls of Acarina and Aphididae this is followed by 

 a development of trichomes, especially in the former. In all 

 galls the mesophyll is subject to the greatest modification. Man}' 

 small fibro-vascular bundles are formed in this modified meso- 

 phyll." (Part V, Con. 2.) 



9 . " Trichomes are far more common in galls produced by 

 mouthparts than in those produced by oviposition." (Part V, 

 Con. 9, and see Summary 2.) 



10. " \"ariatiou in galls is due to their being produced by 

 insects of different orders, to their working upon different parts 

 of the plant and upon different tissues of these parts." (Part 

 III, Con., and Part IV, Con. i.) 



I. ARACHNIDA. 



The Arachnida galls are of four types : ( i ) A modification in 

 the epidermis of the leaf as in the Phytoptus galls on maple and 

 elm ; (2) A fold in the plant tissue cau.sing a cavity filled with 

 trichomes, among which the parasites live, as in the case of many 

 Phytoptidi (Figs. 8, 9, 10, 11, 43, 44, 45, Parts I and V) ; 

 (3) A swelling with an exposed surface covered with trichomes, 

 among which the parasites live, as in the case of Erineum 



