46 



GENERAL BIOLOGY 



often 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, 

 Avitchhazels 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 Avith 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. 



Fig. 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 



