Undescribed Sawfly Gall on Salix humilis

This is a monothalamous gall produced by the abnormal swelling of the leaf petiole of S. humilis. It is conoidal in shape with a long tapering apex which is towards the blade of the leaf. As it is situated at the base of the petiole the uniform enlargement of that organ is often pre- vented by the axillary bud. This causes the gall to project to a greater extent on the outside of the petiole and produces an irregularity in the outline of the gall. The surface is quite glabrous in spite of the fact that the epidermis of the leaf is decidedly pubescent.

Dimensions : — Length of gall 6-9 mm. ; width 3-4 mm.

Nearly the entire mass of this gall originates from the vascular bundle of the petiole which has been stimulated to activity by the in- sect's ovipositor. The small thin-walled cells of the gall substance spring from a cambium layer produced in the pith of the bundle near the ovipositor wound. The cells arising from this tissue force the severed ends of the bundle apart until the vascular elements form only a narrow line of cells between the gall proper and the cortex of the petiole; it is shown at this stage in Fig. 85. This cortex is not materially thickened but shows signs of stimulation in that there appears a small amount of aeriferous tissue located near the place of entrance of the ovipositor. This tissue is shown on each side of the wound in Fig. 85. This tissue was not found in the normal cortex of the petiole. The cuticle is very much thickened.

Euura cosensii, new species

Toronto, Ontario. Described from three females (one, type) and four males (one, allotype) which were reared from galls from the petiole of Salix humilis, material collected and reared by A. Cosens, for whom the species is named. This gall is well described by Cosens, and the description need not be repeated here.

Euura cosensii

Conical, monothalamous, petiole gall, length 6 to 9 mm, diameter 3 to 4 mm.

Euura cosensii

Salix [Salix exigua added as a placeholder]
Leaf galls

Conical petiole gall, length .25 to .33 inch, sawfly

Euura cosensii n. sp.

The sawfly gall on the base of the petiole of Salix humilis Marsh, is common in High Park of this city. and last fall it was particularly plentiful. Induced by this fact to forget several unsuccessful attempts to rear the producer, I collected a large number of the galls and kept them during the winter under as nearly natural conditions as possible. A number pupated a couple of inches below the surface of the soil in the breeding jar, and several adults emerged between June 3rd and 10th, S. A. Rohwer, Washington, D.C., to whom specimens were sent, has not yet published his description, but has sent me a manuscript copy of it. The species which has been named Euura cosensi n. sp., is said to be closely connected with Euura nodus Walsh, a form that produces stem galls on Salix longifolia Muhl. From this species it may readily be differentiated by the shape of the sheath and the different sculpture of the head.