life-history of Fericoma canescens (Psychodidce). 145 
which has comfortably established itself in this position 
will remain feeding beneath the surface for many hours 
together, its body being concealed, and its respiration 
unimpeded. It can travel slowly along the bottom with- 
out closing the cup, but there is little need for locomotion. 
Under natural conditions this tranquil mode of life is 
liable to be interrupted by heavy rain, which may cause a 
sudden deepening of the water, and a great increase in 
the force of the current, if the larva should inhabit 
running water. The larva has several alternatives under 
difficult circumstances. It can cling to the weeds and 
remain submerged for hours. It can leave the water 
altogether and creep upon the wet herbage. Or it can 
float at the surface, if the water is still. In each case it 
can keep up either a free, or at all events a limited, 
respiration, and prevent its spiracles from being wetted 
with water. When the water is made to rise above a 
larva entangled at the bottom of a saucer, the spiracular 
cup closes in, and a large roundish bubble forms, which 
is securely held by the plumose filaments. The bubble 
is large, as compared with the air contained in the 
tracheae, and no doubt suffices to maintain respiration for 
a long time. If the larva is quite free, as, for instance, 
if it is placed in a saucer filled with water only^ the 
bubble brings it to the surface, and then breaks, when 
the floating cup is at once reformed. If a larva, floating 
by means of its spiracular cup, is forcibly submerged, it 
takes a large bubble down with it. 
The floating cup, formed by radiating filaments lying 
in the surface-film, which buoys up the tail-end of the 
larva of Pericoma, finds a tolerably close parallel in some 
other aquatic larvae. The larva of Stratiomys^ exhibits 
an anal coronet, which answers the same purpose. The 
8traUomys larva possesses one advantage, viz., the power 
of quitting the surface at pleasure, which is not shared 
by Pericoma. The larvas of Dixa t and Anopheles { come 
still closer to Pericoma, though the details of the mechan- 
ism are difierent. All the above larvae are Dipterous, 
but among the Coleoptera we find the same contrivance. 
The carnivorous larva of Hydrobius fuscijoes has a float- 
ing basin filled with air at the tail-end, which supplies 
the tracheae, while the head is swept to and fro in search 
Swammerdam, Biblia Naturae, pi. xxxix. 
t Miall, Nat. Hist, of Aquatic Insects, p. 157 (1895). 
1 Meinert, De eucephale Myggelarver, p. 24 (1886). 
TRANS. ENT. SOC. LOND. 1895. PART I. (aPRIL.) 10 
