366 BOTANICAL GAZETTE [ay 
~ Ironrpiwak (Diptera) (pl. XXII).—Numerically this is the lar- 
gest group of gall producers. Many of the galls attain high evo- 
lutionary level, but not as high as many cecidia of the Cynipidae 
(Hymenoptera), which rivals this group in number and complexity. 
The most primitive gall of the Itonididae (Cecidomyiidae) is the ~ 
simple leaf fold, a principal vein constituting the gall axis (1). 
Conceivably all others may be derived from this, if we grant the 
possibility of the necessary changes in the insect’s instinctive 
behavior so that other plant parts are persistently attacked and 
used. Certain of these primitive galls, however, might have 
appeared independently. Both situations are shown in the 
diagram. 
From the initial leaf fold was evolved the variable ill-defined 
pocket type (2), which in turn gave rise to the simple prosoplasma 
of (3). This advances either through the differentiation within it 
of a sclerenchyma layer (4), or through the partial transfer of the 
gall toward the upper side of the leaf (5). From this type we have 
a striking advance through the sclerenchymatized cecidium (6) to — 
the type characterized by a dehiscing larval cell formed by the 
lignified tissue together with the lining of parenchymatous nutri- 
tive tissue. In (8) we have the attainment of the full up-walled 
(“umwallungen”’) condition which is believed to have been derived, 
as indicated, from the diverticulum or pouch form. The majority 
of up-walled galls show the advanced state, however, through their 
containing a scleride zone (9). From the up-walled type (8) 
there has also been derived the interesting double chambered 
form (11), with the type shown in (10) as an intermediate con-> 
dition. Excellent examples illustrating all stages of this latter 
evolution series have been described by the writer (11) from the 
hickory (Carya). 
Originating either independently or from the itonid insect which 
produces the simplest leaf type, we have the burrowing form 
which causes the “blister” gall (14). A form of this kind attack- 
ing the embryonic fruit gives the situation shown in (15), oF 
becoming more specialized on the leaf gives us the prosoplasma 
(16), or attacking the stem initiates a series of stem galls (1 7-20), 
going over into the prosoplasma region with the attainment of 
