INDIRECT. INJURIES CAUSED BY INStCTS. 211 



the purgation of the air * ! ! Perhaps you may not be 

 aware that it is a secretion of Aphides, whose excrement 

 has the privilege of emulating sugar and honey in sweet- 

 ness and purity. It however often tarnishes the lustre 

 of those trees in which these insects are numerous, and 

 is the lure that attracts the swarms of ants which you 

 may often see travelling up and down the trunk of the 

 oak and other trees. The larch in particular is inhabited 

 by an Aphis transpiring a waxy substance like filaments 

 of cotton: this is sometimes so infinitely multiplied upon 

 it as to whiten the whole tree, which often perishes in 

 consequence of its attack. The beech is infested by a 

 similar one. Some animals also of this genus inhabiting 

 the poplar, elm, lime, and willow, reside in galls they 

 have produced, that disfigure the leaves or their foot- 

 stalks. Perhaps those resembling fruit, or flowers, or 

 moss, produced by the Aphis of the fir [Ajohis Abietis), 

 the different species of gall-gnats [Cecidomyia)^ or oc- 

 casioned by the puncture and oviposition of the various 

 kinds of gall-flies [Cynips)^ may be regarded rather as 

 an ornament than as an injury to a tree or shrub ; yet 

 when too numerous they must deprive it of its proper 

 nutriment, and so occasion some defect. And probably 

 the enormous wens, and other monstrosities and defor-» 

 mities observable in trees, may have been originally pro- 

 duced by the bite or incision of insects. 



Besides exterior insect enemies, living trees are liable 

 to the ravages of many that are interior. I'he caterpillaf 

 of the great goat-moth {Cossus ligniperda^), of the hor- 

 net hawk-moth [Sesia crabroniformis, F.), and of two 

 beetles {Nitidula grisea, and Crijptorhynchus Lapathi)^ 



• Hist. Nat. 1. xi. c. 12. " Curtis Brit. Ent. t. GO. 



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