Forest entomology . bs of the leaves. They formirregular thickenings of the leaf-surface, and cause the leaf to twistinwards (fig. 144). Sometimes we find the extreme tip of the twigaffected by the gall-formation, and, as a consequence, a swellingtakes place where the terminal shoot bends, and grows at right angles1 From this point the alternation of generations is not followed. HYMENOPTERA—OAK GALLS. 151 to the more upright twig. The portion thus affected seldom does anymore good, but, when this occurs on minor twigs, the tree is notappreciably injured (fig. 145). When, however, several galls
Forest entomology . bs of the leaves. They formirregular thickenings of the leaf-surface, and cause the leaf to twistinwards (fig. 144). Sometimes we find the extreme tip of the twigaffected by the gall-formation, and, as a consequence, a swellingtakes place where the terminal shoot bends, and grows at right angles1 From this point the alternation of generations is not followed. HYMENOPTERA—OAK GALLS. 151 to the more upright twig. The portion thus affected seldom does anymore good, but, when this occurs on minor twigs, the tree is notappreciably injured (fig. 145). When, however, several galls are onone leaf and very abundant on a young tree, the leaves are arrested intheir development, and the result in the aggregate would diminishgrowth and cause disfiguration. The gall itself is an irregular swell-ing, of a green colour, and poddike in structure. When opened, thecentral cavity is comparatively large, and contains a small brownishseeddike pod, which is the inner gall, wherein is found the larval form. Fig. 14d.—Galls of Aphilothrix quudrilineata on Jiowtrs of oak. of the gall-fiy. This inner gall is not attached to the wall of the outergall. The galls may be looked for in May, and the flies appear in fly is about to 2 mm. in size. Body and antenna? shiningblack; legs lighter in colour; wings more or less hyaline. The two following species of the Aphilothrix group are notassociated by Adler as connected with the alternation of generations,but are nevertheless more or less common. Aphilothrix quadrilineata (Htg.) The gall of this species, fig. 146, is found on the flowering catkinsabout the end of May or beginning of June. It is very small in size, 152 FOREST ENTOMOLOGY. and may not inaptly be compared to a miniature rifle-bullet withlongitudinal ridges added on. I have not succeeded in rearing the fly, which Adler says is verydifficult, and does not appear until the following April. It is verycommon in Alnwick parks. Aphilothrix albopunctata (Schlt
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