128 
The Ohio Naturalist. 
[Vol. IV, No. 6, 
CONCIXSIOX. 
1. The fluid secreted by the ovipositor is not an irritant, and 
therefore cannot be the stimulus for gall production. 
2. Since the gall does not form, excepting the Nematus galls, 
until the appearance of the larvae, it is improbable if oviposition 
is a stimulus for gall production ; and in those insects in which 
the egg is not deposited within the tissues of the plant it is 
impossible. 
3. Glandular structures were observed in only a few of the 
Hymenopterous larvae and these were of doubtful character. 
4. Since it has so far been impossible to demonstrate tl;e 
presence of a chemical stimulus except in Xematus, we must 
consider that the stimulus is usualh- mechanical. As previous!}' 
stated (Part I, Conclusion 3) the morphological characters of the 
gall depend upon the genus of the insect producing it rather than 
upon the plant upon which it is produced. The early history of 
all galls except the Cecidomyid is practically the same ( Part 
Con. 2). The shape and external character of the gall probably 
depends upon the following : ( i ) The plant upon which the 
attack is made; (2) Upon the part upon which the attack is 
made ; (3) Upon the tissues affected ; (4) Upon possible results 
of natural selection. 
SUMMARY OF PARTS. 
Next in importance to the problem of a stimulus giving rise to 
a gall is the explanation of specific external characters. This 
question is not easily answered and at the present time any 
explanation must be largely theoretical. 
The gall-producing insects are found in six orders, as follows : 
I. Arachnida (mites); 2. Hemiptera (Aphidae and Psyllidae); 
3. Diptera (Cecidomyidae and Trypetidae); 4. Hymenoptera 
(Cynipidae and Tenthrenidae); 5. Lepidoptera, and 6, Coleop- 
tera. The gall-producing habit must have originated independ- 
ently in each of these orders and in some orders (Diptera and 
Hymenoptera) it must have originated independently in each of 
the two families represented. 
The formation of the gall is due to two primary factors ; a 
stimulus, usually mechanical, given by the insect, and nourish- 
ment furnished by the plant. 
Conclusions reached as results of previous studies and bearing 
on this subject are as follows : 
I. “Galls maybe classified into two general groups, viz.: 
those produced by mouthparts and those produced by oviposition. 
Those produced by oviposition may be considered the more highly 
developed.’’ (Part I, Con. i.) 
