32 Galls, Gall-makers, and Cuckoo Flies, [Sess. 



stance, so that it is no longer capable of doing harm. Such is 

 the method adopted by the oyster : the formation of a pearl 

 depends on the intrusion of some foreign substance — possibly a 

 parasite — into the mollusc's shell, followed by the secretion of 

 films of carbonate of lime around the objectionable particle, 

 which serve to protect the tissues of the animal from constant 

 irritation. 



In a somewhat similar way, when an insect punctures the 

 growing tissue of a plant and introduces its egg into the 

 wound, a gall is formed. Galls may thus be regarded as 

 exemplifying the vis medicatrix naturce, or effort of a living 

 tissue to expel a foreign body. An exception to this rule is 

 seen in fungi : a toadstool often grows round and encloses 

 blades of grass and similar objects. The substance of a fungus 

 is not, however, a true tissue ; perhaps for this reason galls 

 are never produced on fungi. In the case, at least, of the 

 more perfect galls, the egg or larva of the insect inside is so 

 encysted that its power to injure the plant is comparatively 

 limited. Among insects injurious to crops for this reason, 

 gall-making species do not occupy the most prominent place. 

 Vegetation has worse enemies than the gall-makers, for the 

 formation of a gall is to a certain extent a protection to the 

 plant ; at the same time the requirements of the insect are 

 fully met, the larvae of the gall-fly being provided with the 

 food and shelter necessary for their development. 



The most remarkable circumstance connected with gall for- 

 mation must now be stated with all the prominence it deserves, 

 for no thoughtful observer can fail to be impressed with the 

 wonder and significance of the fact that each species of gall- 

 maker gives rise to a particular style or definite pattern of 

 gall. The galls made by closely allied species are often quite 

 dissimilar, each insect always producing its own characteristic 

 form of gall. 



In how many curious ways do insects manipulate the leaves 

 of plants ! One caterpillar constructs an elegant hammock by 

 doubling a leaf at the midrib and swinging it from two twigs 

 with threads of silk ; leaf-rolling Tortrices either curl up a 

 single leaf scroll-fashion, fastening it with silken threads, or 

 bind several leaves together to form a retreat, where they 

 undergo their metamorphoses ; leaf-miners bore their long and 



