﻿THE EVOLUTION OF INSECT-GALLS. 75 



more than flattish swellings of the leaflets. The galls of R. 

 globosus, n. sp., are globular, and very little attached to the leaf ; 

 and R. spinosellus, n. sp., makes globular spiny galls on the 

 leaves, like those of R. bieolor in miniature. Of rose-galls on 

 the stems we get first fusiform or irregular roughish galls on the 

 stems and branches, formed by R. fusiformans, n. sp., and 

 occurring at West Cliff. Next are globular smooth galls, the 

 product of R. ignota, Osten Sacken, and larger and roughish 

 globular galls formed by R. tuberculator, Eiley. Finally, the 

 galls of R. bieolor, Harris, are globular and excessively prickly, 

 while those of R. roses, Linn., develop moss-like hairs. 



Some sawflies, as " Phyllcecus integer, Norton, are simple 

 internal feeders in twigs ; but species of Euura and Nematus 

 form true galls. In North America nine species of sawny galls 

 occur on the willow. 



Turning to Diptera the same gradations of types are to be 

 observed. The swelling produced by Cecidomyia destructor, Say, 

 is hardly to be called a gall, and is quite inconspicuous — yet how 

 destructive ! Compare this with the white, woolly, conspicuous 

 galls of Trypeta bigelovice, n. sp., produced in abundance at West 

 Cliff, Colorado, on Bigelovia, and yet apparently not injuring the 

 plant seriously at all. 



Cecidomyia, which reaches its maximum of destructiveness in 

 the non-gall-making species, as C. destructor, yet has myriads of 

 forms producing genuine galls. Thus on Carya are eight species 

 of galls produced by Cecidomyia. 



The Dipterous willow-galls of West Cliff, Colorado, are very 

 instructive. A species doubtfully referred to Cecidomyia salicis- 

 nodulus, Walsh, produces an elongate fusiform hard woody 

 swelling of the stem, hardly a true gall at all. C. silicis-saliqua, 

 Walsh, makes broader reddish fusiform swellings, which are 

 very conspicuous. Then C. salicis -batatas, Walsh, or an allied 

 species, forms large irregular potato- shaped galls at the end of 

 the twigs. 



Among Lepidoptera we have any number of internal feeders, 

 which there is no occasion to enumerate ; but Gelechia gallce- 

 solidaginis, Riley, forms genuine galls on Solidago. Many other 

 Lepidoptera have been bred from galls, but in most cases the 

 evidence points to their being mere inquilines. 



I think I have now given sufficient evidence to show that we 

 have all the gradations from mere stem-borers to complicated 

 galls, and the idea that they evolved in the way I have indicated 

 is irresistible. 



The study of galls and the rearing of gall-insects is a most 

 fascinating pursuit, offering much chance of new discovery. 

 Galls may be collected in the early spring, and as the warm 

 weather comes on they yield their inhabitants without any 



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