494 



AMERICAN FORESTRY 



so abundant as to overlap each other and cover the 

 leaf all over. 



In 1881 Professor Conistock very fully described the 

 insect that caused these spots, his account appearing in 

 the U. S. Entomologist for that year. Mrs. Conistock 

 drew the plate for him and this is herewith rejjroduced in 

 Figure 7. "At the center of each leaf," says Professor 

 Conistock, "may be seen, on the other side of the leaf, 

 an elevated portion. Corresponding to this, on the 

 lower surface of the leaf, there is a pit, within which the 

 larva lives. Larvse that were ])artia]ly grown were found 

 to be held in place in the pit in the leaf by what appeared 

 to be a larval skin. This pellicle covers the body entirely, 

 and is with difficulty removed from it; the edges of the 

 pellicle adhere quite tightly to the leaf. Where the larva 



G.^LLS ON 0.\K TREES .^ND BLACKBERRY BUSHES 



Fiff. 12 Shows vertical, median sections of a spongy oak gall 

 (upper) and the blackberry gall (lower). The latter is the 

 specimen shown in Figure 5 above. It is nearly solid in struc- 

 ture, and riddled with cavities for larvae. Both figures natural 

 size. 



is full grown, it forces itself from under this skin, which 

 then falls back into the cavity or is pushed to one side, 

 where frequently it may be seen adhereing to the leaf, 

 i'lic larva at this time drops to the gorund, into which 

 it enters to undergo its transformation." Following this 

 (lesiripiidii occurs a very full account of the insect, its 



larva, and other important points, all of which is too ex- 

 tensive for quoting in this place, while many of the 

 points described are pretty well illustrated in the plate. 



Of all the trees that are attacked by parasitic insects 

 giving rise to galls of one kind or another, probably the 

 greatest sufferers are the various species of our oaks. 

 No part of the tree, be the species what it may, seems to 

 be immune from such conditions even the leaves devel- 

 oping galls of more kinds than one. We find them not 

 only on the leaves, however ; they also occur on the limbs, 

 branches, twigs and fruit. The nature of some of the.se 

 excresences are well shown in the figures illustrating 

 the present article (Figs. 8, 9 and others). More than 

 three hundred different galls have been described as 

 occurring on the oaks, and many of these have been 

 illustrated. Very familiar to many are the beautiful green 

 spheres we so often see attached to the leaves. Up to a 

 certain time, one of these will be of the same fine shade 

 of green as the leaf to which it is attached ; subsequently 

 it becomes a pale tan color at its distal pole, and this 

 color, in a uniform way, gradually spreads toward the 

 stem, until the entire outer skin of the gall is of the same 

 shade. A ga!I of this kind is of extremely light weight, 

 and when thrown into water it will float like a cork. 

 Cutting one of them open, passing the incision through 

 its center,the plane of it making any angle with its polar 

 diameter, exposes its internal structure. In some species 

 of oaks the entire interior is found to be occupied by a 

 soft, spongy material, fine in texture, and of scarcely 

 any weight. In the center of this there is a small cavity 

 for the larva, the tissue surrounding it being much dark- 

 er than the rest. Now, cutting open other galls of this 

 kind from still other species of oaks, we find, instead of 

 this spongy material, a great number of very fine radi- 

 ating threads, the center from which they spring being 

 the cavity containing the larva. Only a certain propor- 

 tion of these threads are sufficiently long to reach the 

 inner surface of the gall. Without giving their scien- 

 tific names, it should be known that these two galls, 

 although having externally the same appearance, have 

 really very different fillings, as just pointed out, and 

 they are produced by insects of quite different species. 



Another curious oak gall is scientifically known as 

 Biorhisa forticornis, here shown in Figure 7. It looks 

 very much like a wad of raisins squeezed around a twig. 

 Usually it is of a tan color, rather light, while Lutz 

 describes it as being "pale yellow with reddish tinge 

 when fresh. Kernel of each division held by radiating 

 fibers." 



In Figure 9 we see still another most destructive gall, 

 found on the leaves of an oak. These have the appear- 

 ance of green marbles, and are as hard as stones. They 

 completely consume the leaves upon which they are 

 attached or the larvae do after they appear. This gall 

 is probably the Callirhytis palustris of Lutz, judging 

 from his drawing of a specimen that closely resembles 

 the one here shown in Figure 9. 



By far one of the most striking as well as beautiful 



