INSECTS AFFECTINC PARK ANT) WOODLAND TREES 615 



ri.ANT (.AM..-. AM) (.AI.I. 3IAKKIIS 



The abnormal i^rowths frequently found on plants and known as insect 

 galls, excite considerable interest and have led to much speculation as to 

 their orii^MU and manner of development. The species producint,^ these 

 peculiar structures do not compose a natural };rou|), but an- found in six 

 ortlers and two classes of the animal kinotlom. Certain of the insects 

 present most remarkable features in their life histories and all ha\e attained 

 the position souy^ht by many of a hii,di<-r race, in that they secure both food 

 anil shelter with little or no labor. 



Development of galls. Galls produced by insects or by the; somewhat 

 closely allied mites, may be found on |)ractically every portion of the plant. 

 The roots are affecteil by a number of species of plant lice and mites ; 

 trunks or stems are attacked by certain 4-winired gall llic^s, beetles or Cole- 

 optera, 2-winged gall flies and plant lice, while branches, foliage and truit 

 or seeds are infested b\- various species producing most remarkable and 

 diverse growths. These abnormal structures may vary from the compara- 

 tively simple galls of certain plant lice, which consist of litth- more than a 

 folded leaf to the nearly .solid, hard cynipid galls on stems. The many 

 celled bud galls may be contrasted with the beautiful fuzzy swellings adorn- 

 ing the stems or leaves of oaks and various plants. These structures, as 

 previously stated, are caused by several insects belonging to widely sepa- 

 rated groups, and it is probable that no general law can account for their 

 production. Certain galls like those of the Tenthredinidae or sawtlies, are 

 probably produced by the irritation incident to oviposition or the presence 

 of the e^g, since, according to several observers, the gall develops before 

 the young hatches from the egg. The larvae of the 4-winged gall flies or 

 Cynipidae, are probably responsible in most cases for the development of 

 the abnormal growths caused by this family, as their activity results in 

 abnormal stimulation followed by excessive cell formation and the develop- 

 ment of a mass of unhealthy tissue. The plant lice illustrate another and 

 in certain ways a more remarkable method of development, in that the 

 tissues of the affected plant grow around and inclose the insect. The plant 



