INSECTS AFFECTING PARK AND WOODLAND TREES 245 



direct service by intentionally driving; away various enemies of one kind or 

 another, but it also draws wasps and bees, insects which, while they may not 

 intentionalh' render a service, in all probability aid to some extent In scaring 

 away parasites which might otherwise be attracted to the leaves in search 

 of prey. 



The honeydew is a disadvantage in certain respects, since it occa- 

 sionalh' collects on the trees in immense quantities and ma\- become the 

 medium for the growth of a black sooty fungus which clogs the leaf pores 

 and interferes with the growth and development of the tree, somethin"- 

 which can hardly be considered favorable to the ajjliids. Occasionally 

 this honeydew is excreted so copiously that it collects in large drops on the 

 leaves, evaporates and forms semisolid gelatinous masses. This has been 

 repeatedly observed by the writer on Norway maples in particular, infested 

 by a species of Chaitophorus. 



Protective devices. Plant lice or aphids resort to a number of protective 

 devices of one kind or another. Xot a few s[)ecies are covered with a 

 whitish, powdery matter which is evidently an excellent shelter from the 

 wet, since it is almost impossible for moisture to penetrate this excre- 

 tion. This is ot the greatest \'alue jjerhaps to the inhai)itants of galls, 

 where the insects are overcrowded and tliere is considerable dampness 

 resulting not only from the excretions from the plant lice themselves but 

 also from the interior walls of the gall. rhr-,e particles of moisture 

 become coated with this powdery matter and roll about among the crowded, 

 struggling plant lice and yet do not wet the individuals because of the 

 enveloping e.xcretion. This is imdoubtedl}' o{ \er\' great importance to gall 

 inhabitants since it is a fact well known to collectors that almost any insect 

 placed in an empty corked vial will soon become stuck to its sides by the 

 accimiulated moisture and |)erish much sooner than if placed in one with a 

 piece of grass or other footing on which it can sustain itself and prevent 

 contact with dampness. In other words, the constant presence of moisture 

 on the bodies of the insects is certainl\- very unfavorable to life and may 

 result in speedy death, either from suffocation or possibl\- by de\eloi)ment 



