1912] Morphology and Biology of Insect Galls 317 



Fam. Trypetidse. 



Eurosta solidaginis Fitch. 



The classification is as far as possible in accordance with Aldrich's 

 catalogue of North American Diptera, Smithsonian Institute, Washing- 

 ton, D.C., 1905. 



Cecidomyia bulla Walsh. 

 Tj ( Helianthus decapetalus L. 

 [Helianthus divaricatus L. 



"Galls found usually on the stem, often from leaf axils, occasionally 

 on petiole and midvein of leaf, rarely on flower disc, protruding from 

 between scales of involucre. 



"The galls are attached by an ample base and are very irregular in 

 form and position, usually somewhat compressed, varying from nearly 

 spherical to flask and cone shaped and from equilateral triangular to 

 spur-shaped. 



"Dimensions: — The average of twenty galls was, base, 5.5 mm. 

 thick and extending 8 mm. from stem." — Brodie.^^ 



On the side of the stem from which the gall originates the vascular 

 bundles are very irregularly arranged and elongated transversely in the 

 direction of the gall axis. From these bundles vascular strands pass 

 out into the gall mass. The principal part of the tissues in this gall 

 originates from the medullary rays, as can be seen in Fig. 39. When 

 the cortex of the stem passes into the abnormal cortex it becomes con- 

 siderably thicker ; this is due chiefly to the increase in size of the cells as the 

 number of rows remains approximately the same as in the normal cortex. 



Glands are found in the normal cortex of Helianthus; they are 

 arranged in such a manner that a gland is placed opposite each fibro- 

 vascular bundle. These glands are very much larger in the abnormal 

 cortex of the gall. Besides these glands there are others that have 

 not a counterpart in the normal stem. These are elongated in the 

 direction of the gall axis and are most abundantly produced in the 

 vicinity of the fibro-vascular strands. 



There are practically only two zones represented in this gall, the 

 epidermal and the parenchyma. The cells of the latter become slightly 

 smaller towards the larval chamber but a well defined nutritive layer is 

 not differentiated. 



Cecidomyia halsamicola Lintner. 

 Host Abies balsamea (L) Mill. 



A monothalamous gall formed by a folding of the leaf with the upper 

 surface on the inside (Fig. 29). An enlargement ellipsoidal in shape is 

 thus produced. The needles affected are near the apex of the stem and 

 the galls are situated close to the base of the needles. 



