OAK-APPLES. 485 



necessarily variable, being dependent on that of the leaf. I have, 

 for example, many specimens of these galls, of different sizes, 

 from which the insects have escaped, showing that they had 

 attained their fuU size. On the juices of the gall the enclosed 

 insect lives, until it reaches its full term of imprisonment, when 

 it eats its way through the gaU and emerges into the world. 

 In some cases, it undergoes the whole of its change within the 

 gall, but in others, it makes its way out while still in the larval 

 state, burrows into the earth, and there changes into the pupal 

 and perfect forms. 



To the unassisted eye, the insect which forms the leaf-gaU 

 presents no especial attraction, as it is simply, to all appearance, 

 a little black fly. When placed under the microscope, however, 

 it soon proves to be a really beautiful creature, though not pos- 

 sessing the brilliant and gem-like hues which distinguish many 

 of its relatives. The body still retains its blackness, but has a soft 

 tint on account of the white and shining hairs with which it is 

 thickly studded. The eyes are large, stand boldly from the head, 

 and the many lenses of which these organs are composed are so 

 boldly defined, that even in so small an insect they can be dis- 

 tinguished with a very low power of the microscope. Indeed, 

 the inch and a half object glass is quite powerful enoiigh to 

 define them, while the half-inch glass makes them look like the 

 pits in a lady's thimble. 



The chief beauty of the insect, however, lies in the wings, 

 which are very large in proportion to the size of the owner, are 

 traversed by a few, but strong nervures, and glow with a 

 changeful radiant lustre, like mother-of-pearl Uluminated with 

 living light. In order to see these wings properly, the insect 

 should be laid on some black substance, and the light concen- 

 trated upon them by the various means which a microscopist 

 can always employ. 



The oak is a tree that seems to be especially loved by gaU- 

 insects, which deposit their eggs in its leaves, its twigs, its 

 flowers, and even in its roots. One of the most familiar 

 examples of oak-galls is that which is called the oak-apple, 

 and which is produced by a species of insect called Cynips 

 terminalis. Although the insect is not of very great size, the 

 gall which it produces is sometimes enormous, being as large as 



