452 
THE STUDY OF INSECTS . 
The females of many species suck blood and are well* 
known pests. Unlike mosquitoes and midges, the black-flies 
like heat and strong light. They are often seen in large 
numbers disporting themselves in the brightest sunshine. 
The larvae are aquatic; and usually live in swiftly-flowing 
streams, clinging to the surface of rocks in rapids or on the 
brinks of falls. They sometimes occur in such large num¬ 
bers as to form a moss-like coating over the rocks. There 
is a disk-like sucker fringed with little hooks at the caudal 
end of the body by means of which the larva clings to the 
rocks; and just back of the head there is a fleshy proleg 
which ends in a similar sucker fringed with hooks (Fig. 535). 
By means of these two organs the larva is able 
to walk with a looping gait similar to that of a 
measuring-worm. It also has the power of 
spinning silk from its mouth, which it uses in 
locomotion. The hooks on the caudal sucker 
and at the end of the proleg are well adapted 
to clinging to a thread or to a film of silk 
spun upon the rock to which the larva is 
clinging. Respiration is accomplished by means of three 
much-branched tracheal gills which are pushed out from be¬ 
tween the last two abdominal segments. The head bears 
two large fan-shaped organs, which aid in procuring food. 
The food consists of microscopic plants and bits of tissue of 
larger plants. 
When full-grown the larva spins a boot-shaped cocoon 
within which the pupa state is passed 
(Fig. 536). This cocoon is firmly fast¬ 
ened to the rock upon which the larva 
has lived or to other cocoons, for they 
occur in dense masses, forming a carpet¬ 
like covering on the rocks. The pupa, 
like the larva, breathes by tracheal gills; 
but in this stage the tracheal gills are Fig . S36 ._ L arva and co- 
borne by the prothorax. coon ' 
Fig. 535 — Head 
of larva. 
