GALLS FOUND NEAR BOSTON. 65 



The gnat larvae, when first hatched, are colorless, but later they 

 become yellow, orange, or red. They are usually flattened, with 

 an inconspicuous head, and no legs ; some of them go under- 

 ground to transform, and some change within the gall ; in the 

 latter case, one can often see the white pupa skin protruding 

 from the gall, after the perfect insect has flown away. Some- 

 times several larvae inhabit one gall. 



The deformations which they produce on plants are numerous 

 and varied, — sometimes being little more than a spot on a leaf, 

 like those which have so injured the foliage of our tulip trees this 

 summer ; sometimes a lenticular thickening around the larva, — a 

 swelling in a twig, leaf-stalk, or midrib, — a folding over of the 

 edge of a leaf, — a plaiting up into a crested ridge, — or a regular 

 gall, attached to the leaf only by a small portion of its surface. 



Plant lice, like the green flies of our greenhouses, form galls 

 on elms, witch-hazel, and other plants. Sixteen different kinds 

 have been found on the hickory, made by Phylloxera. 



The formation of the cock's-comb elm gall is thus described by 

 Professor Riley : 



" The eggs are found on the bark of the tree, and the young 

 hatch about the time that the leaves unfold, and crawl nimbly 

 over the tree till they come to a young leaf, when they settle on 

 the under side, and begin to fret the leaf -surface with their long 

 beaks. The galls show at first as slight elongated ridges on the 

 upper surface, with corresponding closed depressions on the 

 lower ; upon drawing apart the lips of the wrinkle beneath, the 

 louse is seen constantly running back and forth in the cavity, and 

 inflicting rapid punctures with her beak, the inner surface of her 

 dwelling being smooth and glossy, with a slight blistered appear- 

 ance, in contrast with the normal more rough and pubescent 

 texture of the under surface of the leaf. In about two weeks the 

 gall is fully developed and young lice begin to appear in it, and 

 in two or three weeks more it becomes crowded with them ; they 

 are quite active within the gall, exploring its cavities and obtain- 

 ing their nourishment through its walls ; they finally issue from a 

 slit on the lower surface of the leaf, which opens for their exit 

 about the time they become fledged." This brood has not been 

 found to produce galls. 



At Magnolia, I have found the " bead-like poplar gall" quite 

 abundant on leaves of the Balm-of-Gilead poplar. They form a 

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