The greater length and more slender build of the legs of Hydrophilus are at once apparent. There is also a marked difference in the tarsal joints of the fore-legs of the male. The disks and cup-like hairs of Dytiscus are absent in Hydrophilus, but in their stead the last joint bears a sub-triangular plate, studded on the inner surface with spines, which probably serve a similar purpose. A great deal of valuable information about organs of this kind and their functions will be found in chapter X of Darwin’s _Descent of Man_. Simmermacher’s paper[13] should be consulted by all who have the opportunity. Our inch magnifier will show us these spines quite clearly; and also a curious little bunch of bristles, which Simmermacher says are probably organs of touch. It is a good plan to take Hydrophilus out of the water, and lay it upon its back, so that the difference between it and Dytiscus may be clearly seen. The Beetle should be handled carefully, for on the thorax is a kind of keel, ending in a sharp spine, which extends over part of the abdomen. This spine is free, and may easily wound the hands of those who do not watch the motions of the creature pretty carefully. The fore part of the abdomen and the thorax are covered with short close hairs, and when the Beetle is in the water these parts entangle a layer of air, which gives it the appearance of being covered with quicksilver. The two Beetles differ also in their method of exchanging impure for pure air. Dytiscus, as we have seen, takes in a fresh supply under its wing-covers behind; Hydrophilus takes in a fresh supply in front, employing for this purpose the antennae, which apparently do not function as feelers, as is generally the case. When Hydrophilus wants to take in a supply of pure air, it rises to the top of the water, slowly and deliberately. Unlike Dytiscus, it is never in a hurry. Then one of the antennae is pushed through the surface film, thus communicating with the air, which descends to the hair-covered thorax, whence it reaches the spiracles on the upper surface of the abdomen. To allow of this the wing-cases are slightly raised in front. The spiracles in Dytiscus are larger at the posterior end of the abdomen: in Hydrophilus the largest spiracles are in front. This is what might be expected, from the method adopted in each case for procuring a fresh supply of air. These Beetles have frequently bred in confinement; but no better account than that of Lyonnet has ever been given of the operation of the female in making her cocoon and depositing her eggs. As his account is not generally available, a condensed translation of it is inserted with his illustration. [Illustration: FIG. 30.--Female Hydrophilus constructing a cocoon. (After Lyonnet.)]