Aeentropns. 151 



There is one point to which I have not referred, 

 because no use has hitherto been made of it, for the dis- 

 crimination of the supposed species of Acentropus. I 

 mean the shape of the genital organs and anal appendages, 

 in which Trichopterists so much delight. Nolcken finds 

 fault with the figures given both by Kolenati and Brown ; 

 and they are certainly wanting in detail. Hagen and 

 Douglas have remarked upon the certainty which an 

 examination of these organs would give, but to arrive at 

 this certainty it is desirable that fresh specimens should 

 be examined. From such examination of a few dried-up 

 examples as I have been able to make, I find nothing 

 which, in my view, warrants any separation into difierent 

 species ; but with newly captured insects, the result 

 might be different. * 



Nolcken himself, to whom we are indebted for the 

 greatest amount of subdivision, admits that amongst the 

 males of all the forms reported to hQ A. niveus, he could 

 not find any trustworthy diS'orences. In the case of 

 forms so nearly allied, I think the onus probandi ought 

 to lie upon those who assert their specific distinctness. 

 And believing that, by simply asking an abstract ques- 

 tion, I am less likely to provoke investigation and 

 discussion, than by expressing an opinion which can be 

 contradicted and disproved, I will conclude by expressing 

 an opinion — to which I am not wedded, and from which 

 T shall be glad to be converted — but still an opinion 

 founded on such evidence as I have been able to obtain, 

 namely, that all the forms of Acentropus heretofore 

 attempted to be distinguished are, in fact, referable to 

 one and the same species, for which, in the present state 

 of our knowledge, I shall retain the name that is in 

 vogue, Acentrojnis niveus. 



* Since this paper was read, M'Lachlan has examined the anal appen- 

 dages of specimens from various localities. See the result stated* in the 

 next following paper. 



M 2 



