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 inouthparts.” (Part Vlll, 
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 1 , Con. 3.) 
4. “ Within each family we find certain morphological resem- 
blances.’’ (Part 1 , 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 pre.seuce 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 probably true 
in the highly developed galls of Aphididae, Cecidomyia, and 
Cynipidae, 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, irregularl}^ 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 3" 
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 Sununar3" 2.) 
10. “Variation 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. r.) 
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 camsing a cavity filled with 
trichomes. among which the parasites live, as in the case of man}' 
Phytoptidi (Figs. 8, 9, 10, ii, 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 
