This air-supply between the wing-cases and the abdomen is taken in at the spiracles and distributed through the tracheal tubes throughout the body. These tubes branch and subdivide till they end in small twig-like vessels comparable to the capillaries of the human body. They consist of two layers--the inner strengthened by what probably is a spiral fibre, though Packard believes that, in some cases at least, it consists of similar rings. But we must not pursue this subject. It would lead us beyond our appointed limits. * * * * * Another Beetle fairly common in stagnant waters round London and in the southern counties is that to which the name Great Water Beetle (_Hydroph´ilus pic´eus_) of right belongs. This name is sometimes wrongly applied to Dytiscus, with which its rightful owner has little in common, except its aquatic habitat. Its scientific name is _Hydrophilus piceus_; but we shall speak of it as Hydrophilus. It is not a very easy matter to take this Beetle with a net, by sweeping in the ordinary way, for it likes to get into the middle of a mass of vegetation, where it is sure of a good food supply, and is probably safe from the attacks of Dytiscus, who not unfrequently makes a meal of his larger relation. A good plan is to pass the net under a mass of weed and then shake it to and fro in the water. By this means any Beetles in the weed will be dislodged from their hiding-places, and fall down into the bottle. They have, in confinement, the same habit of making a snug place for themselves; and more than once I have fancied that a Beetle of this species had escaped from the aquarium, when all the time it was hidden in a thick patch of water-moss. They are practically vegetable feeders, though Dallas says that they are not such strict vegetarians as to deny themselves a meal of animal food when they meet with a dead mollusc or larva in the course of their wanderings. I have never known them to indulge in animal food, dead or living, but I have known them refuse it. Hydrophilus is the largest British Water Beetle, and, with the sole exception of the Stag-Beetle, the largest British member of the Order. Its total length is very little less than two inches, and across the middle of the back it measures about half as much. It is more slenderly built than Dytiscus, and the contrast in the size and armature of the legs is very striking (Fig. 29). There is also a great difference in their method of progression through the water. Dytiscus moves both legs simultaneously, while Hydrophilus walks rather than swims, moving one leg after the other.