Comparative Studies of Metabolism in Insect Galls 



and Normal Tissues 



E. H. NEWCOMB 



THE striking abnormalities produced by plant tissues in response to 

 the stimuli of gall insects are familiar to all of us. On the leaves 

 of a single hickory tree there may be, for example, as many as eight 

 or nine different types of galls caused by as many closely related species 

 of gall midges of the family Cecidomyiidae, while four or five kinds of 

 midge galls can be found on the upper surface of a single leaflet. These 

 may be tubular, conical, flask-shaped, or globular, and covered with 

 trichomes, waxy bloom, or sugary exudate. On oaks, there is an even 

 larger and more remarkable series of growths induced by the larvae of 

 gall wasps or Cynipids. 



The great diversity shown by galls in their tissue differentiation and 

 arrangement, pigmentation, compartmentalization, and shape appears 

 the more remarkable because it may be exhibited on a single plant organ 

 by closely related species of insects ovipositing on the same surface at 

 about the same time. Such facts have led some investigators to assume 

 that each species of gall-forming insect must elaborate specific morpho- 

 genetic hormones to which the plant tissue responds by producing a 

 highly specific structure. Such a viewpoint is common in current litera- 

 ture on galls. It must be emphasized, however, that no growth substances 

 have been isolated and identified from the gall insects, not to mention 

 morphogenetic compounds, and that attempts to induce gall formation 

 by insect parts or extracts have met with Uttle success. Recent contribu- 

 tions to this field have been made by several investigators whose work 

 will be reviewed briefly. 



Beck (2), studying the gall-fly Eiirosta solidaginis, could not obtain 



