PLANT-LICE AND GALL-FLIES 17 



cone" of the spruce-fir or other coniferous trees. This gall is 

 caused by an aphid named Chermes, which so affects a bud or 

 young shoot of the tree that it fails to develop in the ordinary 

 manner and gives rise instead to an abnormal growth resembling 

 a small fir-cone. Other galls of different form, but caused by 

 the presence of plant-lice, are found on the yew, elm, hawthorn 

 and on many native herbaceous plants. 



Plant galls are, however, caused by many other creatures 

 than aphids, for many species of dipterous flies, hymenoptera, 

 beetles, a few small lepidoptera, eel-worms, mites (relatives of 

 spiders) and some fungi are known to give rise to them by the 

 irritation set up by their presence in plant tissues. A very large 

 number of different kinds of galls are found upon the oak alone, 

 the majority of them being due to the attacks of small four- 

 winged hymenopterous insects belonging to the family Cynipidte. 



The female cynipid is provided with a very long and slender 

 ovipositor, which is doubled back on itself within the body, so 

 that only a short portion projects posteriorly, except when the 

 organ is actually in use. The extremity of the ovipositor is 

 armed with saw-teeth, which the insect uses to cut or pierce a 

 passage into the leaf, stem, bud or root attacked. The egg is then 

 pushed in by the ovipositor, and left in or close to the layer of 

 growing cells in the plant. A fresh passage is usually pierced 

 for each egg. Sometimes the gall begins to form immediately after 

 the egg has been introduced, but is usually delayed for some time 

 until the larva is hatched, and the emergence of the larva is in 

 turn deferred until the plant resumes its activity after the winter. 

 It is not known with certainty why the presence of the developing 

 cynipid or other larva should give rise to these abnormal growths. 



It is a singular fact that many of the Cynipidcz cause two 

 distinct kinds of galls to be formed, and they themselves exist 

 in two distinct forms in the course of their life-cycle. Indeed, 

 there are several instances where the insects previously regarded 

 as two quite separate species are now known to be but two genera- 

 tions of one and the same. In such cases the individuals of one 

 generation reared in one variety of gall are all females, and pro- 

 duce their eggs parthenogenetically ; but the larvae resulting 

 from these eggs cause a totally different gall from that of their 



VOL. II.- 



