268 OAK GALLS AND GALL INSECTS 



stalks of the cabbaj^e, belongs to the order Coleoptera (beetles). 

 In the order Diptera (true flies) gall-making species are found 

 chiefly, but not entirely, in the family Cecidomyidae, an example 

 being the hairy nail-gall, Hormormyia piligeva, found on the upper 

 sides of the leaves of the beech. In the order Hemiptera we have 

 the family Aphidae including such species as Chevmes abietis, which 

 causes the common but remarkable cone-like gall on the Norway 

 spruce, and Schizoneiira ulmi which forms a gall on the elm. 

 The order Hymenoptera includes the family Tenthredinidae 

 (sawflies) some of which form well known galls on the leaves of 

 various willows. To this order also belong the Cynipidae, 

 which produce galls on the rose and other plants, but chiefly 

 upon the oak. There are many other gall-makers besides those 

 which I have enumerated, but in the following record I am deal- 

 ing only with those Cynipids which form galls upon the oak. 



Cause of the formation of Oak-Galls. 



A gall is a peculiar growth of the tissue of a plant due to 

 irritation set up by some internal animal agency. This irritation 

 acts upon the cambium layer of the plant in which the gall- 

 maker lays its egg. The abnormal formation is not due to any 

 poison injected by the insect at the time of oviposition, but the 

 egg must be laid in the cambium ring, or growing meristamatic 

 tissue of the plant, in order to form a gall. Morbid growth 

 results either from irritation caused by the swelling of the egg 

 after oviposition and the subsequent hatching of the larva, or 

 more commonly by the latter alone, in which case the gall does 

 not conmience to form until the larvae have emerged. Growth 

 of a gall only occurs while the juices of the plant are active, no 

 growth taking place in the winter. 



The gall-makers themselves are usually small and insignifi- 

 cant looking insects, but the galls which they form differ greatly 

 botli in size, shape, colour and texture. The position in which 

 they appear on the tree also varies as they may arise from the 

 buds, leaves, stems, or roots. In the centre of each gall 

 there is a hollow cavity in which the larva feeds and grows, 

 subsisting on the juices of the surrounding tissue; here it 

 pupates, the perfect insect subsequently boring its way out through 

 the side of the gall, the hole thus formed being in many cases 

 exceedingly small compared with the size of the insect. The hole 



