habits of certain species of Eurytomides. 317 



the law of Unity of Habits in insects and other animals, 

 and which he briefly states as follows : — " In the case of 

 all known animals, species belonging to the same genus 

 have the same, or nearly the same, habits, and this is 

 also partially true of genera belonging to the same 

 family, but not unfrequently genera belonging to the 

 same family have very widely distinct habits." " Con- 

 sequently, as the genus and the habits of any particular 

 species of animal are both of them determined by the 

 structure, when the genus of two species is the same, 

 the habits also must of necessity be the same, or very 

 nearly the same." 



In illustration of these principles Mr. Walsh cites the 

 case of Zahrus (a genus belonging to the great carnivorous 

 family of ground beetles, CaraUdcB), which feeds upon 

 living and growing vegetables ; also the genus Oodes 

 (belonging to the same family of ground beetles), which 

 generally makes for the water when endeavouring to 

 escape, crawling under floating rubbish, and the genera 

 Arma and Stiretrus (belonging to the family of the 

 plant bugs, Heteroptera), but which have very stout 

 robust beaks suitable for piercing the bodies of other 

 insects, and cannibal in their habits ; whilst all the other 

 Scutelleridm have tender beaks only for piercing vegeta- 

 tion. Upon minutely examining the perfect joint-worm 

 fly, and comparing it with the other Eurytomides, Mr. 

 Walsh ascertained that the former neither belonged to 

 the genera Eurytoma nor Decatoma, to which the greater 

 portion of these Eurytomides are referrable, but to a dis- 

 tinct genus, Isosoma, AVlk. 



To the genus Isosoma (with 9-jointed antennse in_ both 

 sexes, not counting the minute annuli nor any articula- 

 tion in the terminal joint), Mr. Walsh refers the 

 "notorious joint- worm fly, which I have clearly ascer- 

 tained to be the veritable author of the galls upon the 

 stems of wheat, barley, and rye, figures of which galls 

 are given." — (' American Entomologist,' vol. ii., p. 329). 

 From Harris and Fitch, down to Glover and Packard, all 

 authors have hitherto referred this insect to the genus 

 Eurytoma, from which, however, it differs essentially. If 

 it could with any propriety be referred to that genus we 

 should then have a case of the same genus including 

 both parasitic and plant-feeding species, and I do not 

 believe that any such violation of the great law of the Unity 

 OF Habits can he met ivith anywhere in Nature. As 



