374 TEXT-BOOK OF ZOOLOGY 



still in the bud, a female gall fly, the mother of the present insect, 

 deposited an egg within it. The introduction of this foreign body acted 

 as an irritant, the effect of which was still further increased on the egg 

 producing a white maggot which fed on the juices of the leaf. In 

 consequence of this, the tissues of the leaf around the affected spot 

 proliferated, resulting in the formation of a gall, from which the 

 perfect insect will now soon make its escape by the help of its strong 

 mandibles. If we cut one of these galls in two, the knife will be stained 

 black, owing to the formation of iron tannate — in other words, ink — which 

 results from a chemical combination of iron with the tannic acid of the 

 gall. Hence, formerly these galls were used for the preparation of ink, 

 but the galls most largely used in the production of ink, and tannic and 

 galhc acids, are obtained from Quercus lusitanica in Asia Minor, and are 

 formed by the larvse of Cynips gallce-tinctorice. The so-called oak-apples 

 are galls formed on the twigs of the oak by another Bpecies of gall-fly. 



The rose-galls, commonly occurring on the stalks of the wild rose 

 and looking as if they were covered with moss, owe their origin to the 

 Rose-gall Fly (Rhodites rosce). They invariably provide food, shelter 

 and protection to several larvse. 



3. The Saw Flies (Tenthredinidce) exhibit the same contrast to their 

 larvse as lepidopterous insects to their caterpillars; for, whilst the 

 perfect insects suck up sweet honey and pursue other insects, their 

 larvae feed upon leaves. Hence, it need not surprise us to find in these 

 latter a repetition of the shape and colours of caterpillars. (See white 

 cabbage butterfly. "Why are the larvse of the other Hymenoptera white 

 maggots ?) They may, however, be easily distinguished from caterpillars 

 by the larger number of abdominal or pro-legs (generally eight pairs) , 

 and also by the peculiar attitude they frequently assume. Thus, they 

 will roll themselves up in a spiral coil, or elevate the posterior part of 

 their body, twisting it into a peculiar shape like a sign of interrogation (?), 

 at the same time moving it rhythmically up and down to frighten off the 

 dangerous ichneumon flies. On account of their resemblance to cater- 

 pillars, they have been called " false or pseudo-caterpillars." They are 

 very voracious insects which frequently do much damage to many of our 

 cultivated plants (gooseberry and currant bushes, plum and cherry trees, 

 roses and pine-trees). 



4. Still more destructive are the Wood Wasps (Uroceridce), the 

 larvse of which are well known under the name of " bore-worms." The 

 most familiar is the Giant Sirex (Sirex gigas). Its larva is often intro- 

 duced into houses with timber, where all of a sudden the perfect insect, 

 conspicuous by its size (up to 1| inches) and wasp-like markings, makes 

 its appearance. 



