THE INSECTS. 339 



covered with the median seta abbreviated, they were ranked 

 between those with three long setae and those with two 

 only. Subsequently, in addition to the setae, the number 

 of the wings was employed as a leading clue to the arrange- 

 ment of the genera. But it is now well known that these 

 criteria are serviceable, at the most, for nothing more than 

 the distinguishing of genera very intimately related to each 

 other, belonging to various subordinate alliances comprised 

 within the family ; while one of them (the number of setae) 

 is not always available even for this purpose, varying, as it 

 does in some forms, with the sex or with the individual." 

 A variety of distinctions of modern writers, such as the 

 cross veinlets in the wings, the structure of the trachea! 

 branches of the nymph, and other modifications of organs, 

 can hardly fail, he thinks, to be unnatural and arbitrary. 

 "And it is only," he says, "by taking cognisance of points 

 of difference and agreement in many details, in the anatomy 

 and the mode of development, and the habit of leading 

 representatives of the various alliances of genera, at dif- 

 ferent periods of their lives, before and after their exclusion 

 from the egg, that the mutual affinities of the several as- 

 sociations of genera to one another can be demonstrated 

 adequately. Until such comparisons can be, and shall 

 have been, carred out, the whole question of their arrange- 

 ment can only be dealt with in a tentative and experimental 

 manner ; and it will be fortunate if error be avoided in the 

 necessary grouping of genera into provincial alliances of 

 apparently kindred forms, preparatory to the study of their 

 affinities. It is more easy to demonstrate defects in pro- 

 posed methods of classification than to devise a trustworthy 

 system in their stead ; and possibly extended observation 

 in the future may eventually show that some of the bases 

 of arrangement adopted in this present work are mere tem- 

 porary expedients worthy of mention in this paragraph." 



Mr. F. M. Halford has made and carefully recorded a 

 great number of observations on the Ephemeridce, which we 

 have every reason to hope will eventually do much to clear 

 away the obscurity which surrounds the various genera and 

 species of this family of insects. 



