April, 1904.] Galls and Insects Producing Them. 121 



2. GALLS OF THE APHIDIDAE. 



Galls belonging to this family nre usually less pubescent than 

 those belonging to the Cynipidae. The trichomes are usually 

 much shorter and frequently less numerous. Each trichome is 

 usually made up of a single cell (Fig. 88). The place where 

 these trichomes were attached is marked b}- an accumulation of 

 tannin, the .same as in the Cynipidous galls (Figs. 89, 90). 



Examination of the galls of the Phylloxera spinosa Shimer 

 show that the spines were due to the same cause as in the 

 Cynipidous galls (Fig. 87). 



Galls of Pemphigus p.-transversus Riley (Fig. 91) and P. p.- 

 caulis Fitch were perfectly smooth, but the cell walls were much 

 thicker than in any other galls studied. 



CONCLUSION. 



1. The inner layer of cells (i. e., those next to the larva) are 

 always supplied with nutriment until the insect is mature. 



2. The development of the other layers of cells is for the pro- 

 tection of the larvae. These protective devices reach their highest 

 development in the Cynipidous galls. 



3. In the very young galls there is usually little or no distinc- 

 tion between the nutritive and protective zones. The time of the 

 differentiation of the protective zones varies in different species. 



4. The fibro-vascular bundles are most prominent in galls on 

 the petiole and mid-rib. 



5. Most galls are covered with trichomes which disappear as 

 the galls approach maturity. The number of trichomes is varia- 

 ble in proportion to the pubescence of the host plant. 



6. Spines are due to elevations composed almost entirely of 

 epidermal cells. 



Part IX. Ovipositors and Mouthparts. 



One of the most prominent questions concerning the formation 

 of galls which presents itself to the students of entomology and 

 botany and even to the most casual observer, is the exciting factor 

 in gall production. Is the stimulus from the ovipositor or mouth- 

 parts ? Is it mechanical or chemical ? The author believing that 

 the logical method of solving this problem was to first make a 

 careful study of the morphology and development of galls has 

 published the preceding parts of this paper. The author does 

 not claim to have found a complete solution of the problem, but 

 is hopeful that some of the facts stated in this series of papers 

 may lead to more thorough and satisfactorj' studies of the prob- 

 lem. The problem presents many difficulties ; the parasites and 

 inquilines which are usually present are frequently difficult to 

 distinguish from the real gall-maker ; this is especially true when 

 the study is confined to the larvae. In the following studies the 

 author is reasonably certain that the determinations are correct. 



