CHAP. XVI.] 



THE BRITISH ISLES. 



337 



selection, and giving due weight to the facts of local distribution 

 as they are actually presented to us, I am forced to differ from 

 the opinion held by our best entomological authorities, and to 

 believe that some considerable proportion of the species which, 

 in the present state of our knowledge, appear to be peculiar to 

 our islands, are, not only apparently, but really, so peculiar. 



I am indebted to Mr. Robert McLachlan for the following in- 

 formation on certain Trichopterous Neuroptera (or caddis-flies) 

 which appear to be confined to our islands. The peculiar 

 aquatic habits of the larvae of these insects, some living in 

 ponds or rivers, others in lakes, and others again only in clear 

 mountain streams, render it not improbable that some of them 

 should have became isolated and preserved in the mountain 

 districts of our western coasts, or that they should be modified 

 owing to such isolation. In these insects the characters depended 

 on to separate the species are wholly structural, and the care 

 with which Mr. McLachlan has studied them renders it certain 

 that the species here referred to are not mere varieties of known 

 continental forms, however closely they may resemble them in 

 form and coloration. 



Trichoptera peculiar to the British Isles. 



1. Setodes argentipunctella. — This species is known only from the 

 Lakes Windermere and Killarney. It has recently been described by Mr. 

 McLachlan, and is quite distinct from any known species though allied to 

 S. punctata and S. viridis, which inhabit France and Western Europe. 



2. Rhyacophila mun da.— Described by Mr. McLachlan in 1863, A 

 very distinct species, found only in mountain streams in Wales and 

 Devonshire. 



3. Philopotamus insularis. (? A variety of P. montanus. )~T\us can 

 hardly be termed a British species or variety, because, so far as at present 

 known, it is peculiar to the island of Guernsey. It agrees structurally 

 with P. montanus, a species found both in Britain and on the continent, 

 but it differs in its strikingly yellow colour, and less pronounced markings. 

 All the specimens from Guernsey are alike, and resident entomologists 

 assured Mr. McLachlan that no other kind is known. Strange to say, 

 some examples from Jersey differ considerably, resembling the common 

 European and British form. Even should this peculiar variety be at some 

 future time found on the continent it would still be a remarkable fact that 

 the form of insect inhabiting two small islands only twenty miles apart 

 should constantly differ ; but as Jersey is between Guernsey and the coast, it 

 seems just possible that the more insular conditions, and perhaps some 



Z 



