xxxvi Introduction. 



throw out a blastem, and only those larvae survive 

 to hand down their art, which emerge from an tg^ 

 so cunningly placed as to excite the growth of a nu- 

 tritive blastem. It is not always possible to keep 

 the besiegers from using the waters of the moat, although 

 there is no disinterested thought of the besiegers' 

 wants when the ditches are planned. So in the war- 

 game that goes on between insect and plant, natural 

 selection directs the moves of both pla3Trs, but there 

 is nothing generous or altruistic on either side. 



The means of defence against parasites which have 

 been evolved by galls are many of them very curious. 

 Aphilotrix Sieholdi and some others ^ secrete a sweet 

 glutinous secretion which, like honey dew, is particu- 

 larly attractive to ants, and leads them to act as senti- 

 nels in guarding the galls from parasites. Occasionally 

 ants will go so far as to cover the secreting galls over 

 with a concrete made of sand, leaving only a tunnel by 

 which they pass up to reach the stores of honey dew. 



A glutinous secretion is sometimes found on long 

 tufts of hair growing from the gall. A defence of this 

 kind is used by Andricus ramuli^ and forms a stockade 

 which entangles parasites before they reach the gall. 



Ala3^er of thick- walled parenchyma affords a protection 

 to some larvae, such as Dryophanta longiventns ; in 

 others a thick layer of spongy parenchyma, as in Teras 

 ierminalis, serves the same purpose ; in Cynips Kollari 

 both these layers are present ; and in addition, these and 

 many other galls, have an inner gall of stony hardness 

 which guards the larva like a shirt of mail. 



^ Cynips glutinosa glues small insects to the gall ; and a sweet 

 secretion is found on Cynips calycis, growing on Q. pedunculata. 

 Giraud, Verh. z.-h. Ges. su Wien, 1859: Dr. E. Kkihay, Nature, vol. xlv. 

 p. 546, 1892. 



