Euura S. ovum Walsh.

"On Salix cordata [eriocephala]. An oval or roundish, sessile, monothalamous swelling, .30 to .50 inch long, placed lengthways on the side of small twigs, green wherever it is smooth, but mostly covered with shallow longitudinal cracks and irregular rough scales which are pale opaque brown. Its internal substance fleshy in the summer like that of an apple, but with transverse internal fibres. When ripe in the autumn filled with reddish-brown spongy matter, with close-set transverse internal fissures at right angles to the axis of the twig. On cutting down to the twig at any time a longitudinal slit about .20 inch long becomes plainly visible." — Walsh .

As already noted the host of this gall in this locality is Salix humilis, it remains to be determined whether there are two distinct species of producers or one species with two hosts [Zinovjev and Smith imply this is more likely Euura salicisovulum]. Walsh's description of the gall on Salix cordata corresponds to the form occurring here on S. humilis.

The ovipositor of the producer has in this case made a longitudinal cut in the stem. A transverse section at the place where the gall is located shows that this wound extends in from the epidermis to the boundary of the pith. The activity of the young tissues, abnormally stimulated, soon fill this fissure with a mass of small, angular parenchyma. The rapid division of these cells forces the exposed edges of the cortex and central cylinder apart so as to form a wedge-shaped opening which is filled up with the gall mass (Fig. 71). It should be stated that the newly formed cells originate mainly from the division of a cambium bordering the pith at the bottom of the fissure. But other tissues also respond to the stimulation initiated by the ovipositor of the insect. Thus a section of the stem at a short distance from the gall shows that the outlying cambium has become abnormally active and has produced a layer of bast nearly one-third thicker than that found in the normal stem. Likewise the activity of a cork-cambium layer has thrown off a strongly cuticularized epidermis present in the earlier developmental stages.

S ovulum n. sp.

On Salix humilis

Undistinguishable both internally and externally from S. ovum n. sp. But for the fact that the larvae differ in color, and that, of the five species of willow found near Rock Island, IL., this type of gall occurs only on S. humilis and S. cordata [eriocephala] — two willows which are, according to Mr. Bebb, very distinct — and never, as I have carefully observed, on S. discolor — which is on the same authority, very closely allied to S. humilis, the species on which S. ovulum is found — I should not suppose S. ovulum to be a distinct (phytophagic) species from S. ovum. Possibly, indeed, it may not be so and to determine doubts it is better to await the discovery of male female imago. Besides the three bushes of S. discolor mentioned above, (Proc. Ill, p. 589,) I have since discovered and examined closely several dozens. This gall was rare in 1864 — 5 and 1865 — 6, but common in 1863 — 4. Not local, but generally distributed. Described from 30 specimens.

Euura ovum

Host: Salix humilis

An elongated oval swelling on one side of a twig constitutes the gall in this case.

Range: Toronto, Ontario

Euura salicisovulum

Valid name. — Euura salicisovulum Walsh.

Notes. — The galls are the same as those of Euura salicisovum; the larvae, according to Walsh, are distinguished from those making galls on Salix eriocephala being "all decidedly pale greenish," while those of E. salicisovum were "all decidedly yellowish." No galls were found by Walsh on Salix discolor which is closely related to S. humilis.

The taxonomic status of this species and its host plant specificity are not quite clear. Galls of this type occur not only on Salix eriocephala and S. humilis (including var. tristis), as stated by Walsh, but also on S. discolor and S. petiolaris. All such galls seem to be associated with a bud, which is however left unmodified, the swelling always situated just below the bud.

Specimens that seem to be conspecific with E. salicisovum are known from Illinois and Virginia, but all of them are either captured or reared from undetermined willows. We are not sure if they belong to E. salicisovum or to the complex of the sibling species involving E. salicisovulum. Thus, we cannot designate a neotype yet.