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 he peculiar to 
our islands, are, not only apparently, hut 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 munda. — 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) — This 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 
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