374 



GENERAL PRINCIPLES 



764. The galls SO often seen on oak leaves and twigs, and also 

 on many other plants, are abnormal developments of the plant 

 tissue due to a stimulation produced by insect parasites. The 

 female Cynips, a wasp-like insect, deposits her eggs in the tissues 

 of the plant, and during the development of the young the tissues 



Fig. 237. — Dog flea, Ctenocephalus canis. A, Larva; B, adult. 



after Kunckel d'Herculais.) 



(From Folsom 



Fig. 238.— Oak galls (A) made by the gall wasp, Holcapsis globulus (B). 

 A, natural size; B, magnified X 2. (From Folsom.) 



are irritated in such a way as to cause the abnormal develop- 

 ment of the surrounding tissues. The gall forms a shelter for 

 the young brood and the juices of the plant provide food. 



765. Protozoa As Parasites.— Many species of amoeba 

 (Entamoeba) are found, as parasites, in the digestive tract and 



