4 6 



GENERAL BIOLOGY 



of ten in great numbers: and each of these species has its 



inevitable train of parasites. 



All these forms together constitute a 

 miniature animal society, dependent on 

 the overgrowth of willow tissue that re- 

 sults from the attack of the gall midge. 



Study 6. A study of common galls. 



Apparatus needed: A scalpel, or 

 knife, a lens, and a basket, bag, or 

 very capacious pockets. 



Collect afield a large number of 

 galls, bringing into the laboratory 

 enough to fairly represent each kind 

 found. Search such trees as oaks, 

 hickories, lindens, hackberries and wil- 

 lows; such shrubs as sumach, roses, 

 witchhazels and dogwoods and such 

 herbs as goldenrods, ox-eyes, and 

 touch-me-nots. 

 The record of observations. Select a dozen or more species 

 that represent best the general phenomena outlined in 

 the preceding pages, and write down their characters in a 

 table prepared with the following column headings : 

 Name of plant. 

 Part of plant affected. 

 Position of gall on this part (upper or lower 



surface of leaf, etc) . 

 Gall type. 



Aggregation, solitary, clustered, or compound. 

 Cavity of gall (shape, close fitting, etc.). 

 External coat, armature, etc. 

 Special structural features, if any. 

 Defences against foraging animals. 



Fio. 36. Diagram illus- 

 trating the distribu- 

 tion of the inhabitants 

 of the cone gall of the 

 willow: a, the gall 

 maker, b, moth larva. 

 c, sawfly larva. d, 

 meadow - grasshopper 

 eggs. e, guest gall- 

 midge larvae. 



The Gall 



