29 
It feeds on algae, diatoms, and parts of phanerogamous plants, which 
are brought to the mouth by means of the currents set up by two 
broad fan-like organs situated upon the head. In colour the larva 
varies according to the species, and perhaps also to some extent in 
accordance with its food, from deep shining black to yellow or dark 
green. When mature, the larva spins a silken cocoon within which 
it pupates, and in which the pupa remains motionless, breathing by 
means of a pair of branched respiratory filaments, which project 
from behind the head. The pupal stage lasts for about a week, and 
the perfect insect, making its escape through a rent in the back of 
the thorax, ascends to the surface in a bubble of air, and makes its 
way to the stem of a rush or some similar support on which it rests 
until its tissues are sufficiently hardened to enable it to fly. 
Genus 
SIMULIUM, Latreille. 
Simulium reptans, Linn. 
Plate 10. 
So far as present experience goes, this would appear to be 
essentially a northern species, since all the British specimens of it 
in the Museum collection come from beyond the Tweed. A very 
similar species, which is common in the midland and southern 
counties of England, is distinguished from 5. reptans by the middle 
tibiae of the male being wholly brown, or, at any rate, not con- 
spicuously silvery-yellow at the base, and by the hind tarsi in the 
female being less clear yellow on the basal two-thirds. Well- 
preserved females of S. reptans show on the anterior half of the 
thorax a whitish-grey blotch on each side above the anterior angles, 
which unfortunately does not appear in the plate ; besides this, the 
thorax is clothed with a closely-fitting coat of minute golden hairs, 
the tibiae, with the exception of the tips, are in reality conspicuously 
