128 The Ohio Naturalist. [Vol. IV, No. 6, 



CONCLUSION. 



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 the 

 presence of a chemical stimulus except in Nematus, we must 

 consider that the stimulus is usually mechanical. As previously 

 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 earl}- history of 

 all galls except the Cecidom3'id is practicalh^ the same (Part V, 

 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 easil}' answered and at the present time any 

 explanation must be largel}^ theoretical. 



The gall-producing insects are found in six orders, as follows : 

 I. Arachnida (mites); 2. Hemiptera (Aphidae and Psyllidae); 

 3. Diptera (Cecidomj-idae and Trypetidae); 4. Hymenoptera 

 (Cynipidae and Tenthrenidae); 5. I^epidoptera, 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 independenth- in each of 

 the two families represented. 



The formation of the gall is due to two primary factors ; a 

 stimulus, usually mechanical, given b>- 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 tho.se produced by oviposition. 

 Those produced by oviposition may be considered the more highly 

 developed." (Part I, Con. i.) 



