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.

- A Cosens: (1912) A contribution to the morphology and biology of insect galls ^©

Reference: https://www.biodiversitylibrary.org/item/99818#page/49/mode/1up