INSECTS AFFECTING PARK AND WOODLAND TREES 617 



inhabited, footless, comparatively helpless maggots. The Tciithretliiiicl 

 gall flies are small creatures with many veins in their four wings. Their 

 galls are likewise closed, and when inhabited are easily recognized by the 

 many footed, somewhat c\lindric larvae within. 



Coleopterous larvae [)roduce swellings in trunk and limb, which have 

 been classed as galls, and these are in most cases due solely to the 

 mechanical irritation caused by the larvae. 



The Diptera or 2-winged llies, include several distinct groups of gall 

 flies. Certain Trypetidae produce solid, closed galls, the one on goldenrod 

 or Solidago being an excellent example. The Cecidomyidae or gall gnats, 

 are the most important gall makers of the family, and deformities may be 

 produced by them in stem, branch, leaf or fruit. Their galls are usually 

 closed and when inhabited can easily be recognized by the character of the 

 larva, which has the somewhat unique power of propelling itself some 

 distance by bringing the two extremities together and then suddenly 

 extending them. Some gall flies also belong to the Mycetojjhilidae, but 

 the habit is abnormal for this group. 



The Hemiptera include a number of important gall insects, some of 

 which are quite destructive. The Psyllidae or jumping plant lice, include a 

 few gall insects belonging to the genus Pachypsylla, while the majority of 

 galls referable to this group are the work of plant lice or aphids whose 

 characteristics are well known. The deformities caused by these insects, 

 may be easily recognized by the mouth of the gall being open, and when 

 fresh by the peculiar inhabitants. 



The Acarina or gall mites, produce many deformities in vegetation, 

 particularly on leaves. These creatures are usually pyriform in shape and 

 remarkable among mites because of their possessing but four legs. Their 

 galls are always open and are frequently ornamented with hair growths. 

 They are sometimes very abundant on foliage and occasionally somewhat 

 injurious. 



Gall insects display a marked preference for certain trees, various 

 oaks, the willows and rose-bushes suffering specially from their attacks. 

 Fortunately this group is comparatively harmless to vegetation. 



