April, 1904 .] Galls and Insects Producing Them. 
I 2 I 
2. GALLS OF THE APHIDIDAE. 
Galls belonging to this famih- are 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). 
Examinatio:i of the galls of the Ph^dloxera spinosa Shinier 
show that the spines u'ere 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. 
COXCLUSIOX. 
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 la3’ers of cells is for the pro- 
tection of the larvae. These protective devices reach their highest 
development in the Cimipidous galls. 
3. In the very j’oung 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 entireh' of 
epidermal cells. 
Part IX. Ovipositors axd Mouthparts. 
One of the most prominent questions concerning the formation 
of galls which presents itself to the students of entomolog}' 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 stud}- 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 
ma}- lead to more thorough and satisfactory studies of the prob- 
lem. The problem presents many difficulties ; the parasites and 
inquilines which are usually present are frequently difficul t ta 
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. 
