136 
HALF HOUKS WITH INSECTS. 
[Packard. 
aquatic. We know of no other water spider. Certain mites 
are aquatic, but do not differ from land species in their mode 
of respiration. The Tardigrades (Fig. 97, Macrobiotus 
Americanus) are low microscopic mites, which live in water 
and are called water bears. They have no spiracles nor air 
tubes, and respire solely through the skin. The Pycnogo- 
nids, which live at all depths of the ocean, from low "water 
to several hundred fathoms, and which are probably related 
to the mites, also breathe through the skin. 
It is among the two-winged flies (Diptera) and Neuroptera 
Fig. 98. 
Ephydra and pupa case. 
that we find insects with gill-like appendages penetrated by 
air tubes which supply fresh air to the blood. There are 
quite a number of aquatic larvae of flies, but with few excep¬ 
tions, such as those of the mosquito and black flv, they 
belong to families which also contain closely allied genera 
which live in the earth. For example, most crane fly larvae 
are terrestrial, a few are adapted for aquatic life ; some lar¬ 
val horse flies live in mould, some are known to be aquatic ; 
8 
