HYMENOPTERA (GALL-FLIES). 171 



it can easily eat its way to the open. The adults have extremely 

 hard and strong biting mouths, and leave behind a large round 

 hole in the trunk that we very often notice on pine-trees. 

 Numbers of larvsB usually inhabit each tree, so that it is soon 

 killed. It is always advisable to cut down and burn any tree 

 showing symptoms of their attack in the winter when the borers 

 are at home. 



Gall- Wasps oh Cynipid^. 



Gall-wasps produce a great variety of deformities upon trees 

 and plants. There are many interesting features in these insects, 

 and their life-histories are often very complex. They are mostly 

 small Hymenoptera, with apodal grub-Uke larvae (fig. 75, d). 

 They are subject to great numbers of parasites, and have other 

 harmless guests (inquilines) called Synergi living in their galls. 

 The thorax is humped and abdomen short, often laterally com- 

 pressed. The females, which have the ovipositor curved up- 

 wards from the ventral surface, deposit their eggs in plant tissue. 

 The eggs are sometimes placed on long stalks or peduncles, 

 usually in groups or clusters, but sometimes singly (b). Those 

 here figured are of Briorhiza termincdis laid in the terminal buds 

 of the oak. When the eggs are laid, a clear liquid is poured 

 over the openings through which they have been forced, to 

 protect them. The larvae, which are curved, white, footless 

 grubs, pupate in the galls, some developing in a few days 

 (catkin galls), others taking a long time to mature (woody 

 galls). The " galls " were at one time said to be formed by an 

 irritating venom ejected into the wound with the egg. This 

 is not the case. The galls are produced by the irritation set 

 up by the larvse when they are hatched. We find, as a rule, 

 two forms of most gall-insects, a sexual form and an agamic 

 form. The galls of each differ. Those whose galls appear in 

 the autumn are always asexual. We thus get an alternation 

 of an asexual autumn generation and a sexual spring genera- 

 tion, Parthenogenetic reproduction then takes place in these 



