136 ARTICULATES: INSECTS. 



feeds upon the substance of the body, but attacks no 

 vital part, so that the insect does not die till the ich- 

 neumon is ready to enter upon the pupa state. 



GALL-FLIES. 



These are very small insects, and the females have 

 a long, slender ovipositor, with which they insert their 

 eggs into leaves and other parts of plants. These punc- 

 tures cause outgrowths called galls, which vary in size, 

 form, and solidity, according to the nature or part of 

 the plant that is wounded, and ac- 

 cording to the kind of gall-fly that 

 makes the wound. Some are shaped 

 like an apple, as the gall of the oak ; 

 some like a bunch of currants ; some 

 are almost as hard as iron ; and some 



length the eggs hatch, and the larvae 

 feed upon the vegetable matter which surrounds them. 

 Some galls have only one tenant, others contain many, 

 and usually these insects undergo all their changes 

 within the galls, and, gnawing through the shell, fly 

 away ; but some kinds gnaw through at the end of 

 their larval life, and enter the ground to go into the 

 pupa state. Some produce no galls themselves, but 

 live in galls made by others, and are called Guest 

 Gall-Flies. The nut-galls used in making ink, in col- 

 oring, and in medicine, are caused by an insect which 

 punctures a species of oak common in Western Asia. 

 The Rose-bush Gall-Fly punctures the stems of rose- 

 bushes, producing excrescences, or woody galls, upon 

 them. One of the largest species is- the Willow Gall- 

 Fly ; its galls are found at the ends of the twigs of the 

 basket- willow. 



