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 tlieir mode 

 of respiration. Tlie Tardigrades (Fig. 97, Macrohiotus 

 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 breatlie through the skin. 



It is among the two-winged flies (Diptera) and.Neuroptera 



Fig. 98. 



Ephj'di-a 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 fl}-, 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 



