These latter, like many of the insects that inhabit fresh water, are well adapted to withstand prolonged immersion. Their bodies are not capable of being wetted, a covering of short hairs effectually preventing the water from coming into actual contact with the body. The openings of the breathing tubes (spiracles) are also guarded by closely set hairs which prevent the water from entering; and, in some cases, the creatures are provided with special air-sacs in which a supply of air is stored for use while the insect is shut off from the external atmosphere. The lowest order of insects includes the so-called Bugs (_Rhynchota_), which are parasitic on plants or animals. Quite a number of these are to be found inhabiting fresh water, but only one is truly marine in its tendencies. This one is a small insect, only about an eighth of an inch in length, and named _Æpophilus_ (fig. 222). It has never been seen except between the tide-marks, and occurs so near low-water level that it is submerged during the greater part of its existence. But little is known of this peculiar creature. Even its food has not been ascertained. As with the other Rhynchota, but little change of form takes place during growth, the young being very much like the adult in appearance. It has been observed that the larvæ live crowded together under the protection of stones. The reader is probably acquainted with those fresh-water bugs that are popularly known as ‘boatmen’ on account of the oar-like action of their long, fringed hind legs; and although none of these may be described as marine, yet certain species may often be seen in salt and brackish water, living in company with creatures that are decidedly inhabitants of the sea. <div class="figcenter" style="width: 500px;"> <a name="fig222" id="fig222"></a> <img src="images/i_329.jpg" width="500" height="320" alt="" /> FIG. 222.--SEA SHORE INSECTS 1. _Æpophilus._ 2. _Machilis maritima._ 3. _Isotoma maritima._ 4. _Cœlopa_ </div> We frequently meet with a pretty, slender-bodied insect, measuring about half an inch in length without appendages, creeping over the rocks in the sunshine, generally very near the crevices in which they hide, and leaping from place to place when disturbed. These are the Bristle-tails (_Machilis_), belonging to the order _Thysanura_, the members of which, like the bugs, scarcely undergo any metamorphoses. This insect (fig. 222) has long antennæ, and also a long, stiff, and elastic bristle extending backwards from the tip of the abdomen; and this bristle is the means by which the creature leaps. Occasionally the machilis may be found resting on the surface of the still water of a rock pool, in which case its body is not wetted, its weight not being sufficient to break the surface film of the water; and, in fact, the film is even sufficiently firm to enable the insect to leap on the surface just as it would on a solid body.