190 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
to plants. Where, as largely in Water-Bugs, small creatures are 
selected as victims, these are killed as a result of the feeding 
operations, and the true carnivorous habit is illustrated. But 
Bugs which feed on the blood of relatively large animals may 
be described as external parasites. The most notable example 
is the wingless Bed Bug (Czmex /ectularius), which is fortunately 
a favourite prey of several other insects, including some belonging 
to the same order (e.g. species of Reduvius). The True Lice, 
which live entirely upon the blood of Mammals, are very possibly 
to be regarded as minute Bugs, which have lost their wings and 
become modified in other ways as a result of 
the parasitic habit. Considering the mode of 
life, their small size is clearly an advantage, 
and their remote ancestors were probably 
larger insects. 
Fires (Diptera) as ParasiTEs.—A num- 
ber of these insects are endowed with mouth- 
parts adapted for piercing and sucking, and 
are notable blood-suckers, the habit being com- 
monly restricted to the females. Gnats and 
he Mosquitoes, Midges, Sand -Midges, Breeze- 
Fig. 113. —Swallow-Fly Flies, and Tsetse-Flies, may be cited in illus- 
Oe aa tration. A very interesting series of modifica- 
So ees found within the limits of one family 
(7Zippoboscide), which illustrates the reduction 
of wings resulting from the parasitic mode of life. The feet are 
provided with strong claws for holding firmly to the animals 
attacked, and males as well as females are blood-suckers. One 
of the least modified species is the Forest-Fly (A7zppobosca equina), 
which infests horses. Well-developed wings are present, but not 
much used, as these insects fly unwillingly. Another somewhat 
similar form (Lzpoptena cervz) lives on the Red-Deer, and its wings 
are either shed or bitten off as soon as a host has been secured. 
The wings of the Swallow-Fly (Stexopteryx hirundinis) are small 
and narrow, while in the so-called Sheep-“ Tick” (AZelophagus 
ovis) they are altogether absent (fig. 1135). So also in the 
Bee-‘‘ Louse” (Lrazla ceca), a minute insect that infests bees, 
and some curious little parasites (species of yctevzbza) that have 
been found among the fur of bats. 
Some of the insects of this order live within the bodies of 
