Through a pocket lens

by Henry Scherren

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.

Fig. 30.—Female Hydrophilus constructing a cocoon. (After Lyonnet.)

Lyonnet[14] wanted to find out how the female made the cocoons (Fig. 30), and this is how he set to work. He put some of these Beetles into a large aquarium, with a good quantity of water and some duckweed. On May 31 and the following day he noticed that one of the females was swimming about in every direction, as if in search of something. Thinking that this was because she had not the proper materials for her work, he then put into the aquarium some thread-like alga of a kind which he had seen attached to some cocoons, and on June 3 the Beetle began to make a cocoon, but soon gave up the task, apparently because she was troubled by other aquatic insects which had made a home in this weed. These intruders were removed, and the Beetle set to work once more. Lyonnet then noticed that, like a spider, she had her spinning apparatus at the posterior end of the body. She extended the last segments slightly, and opened the hindmost one, when he saw a nearly circular opening, in which was a whitish disk (Fig. 30A, a). On this disk were two little brown tubercles side by side, nearly at right angles to the longitudinal axis of the body. From each there projected a blackish-brown conical tube, about a line long, stiff towards the base, but flexible and elastic towards the tip. These tubes were the spinnerets, which acted together with a parallel movement, and from each proceeded a separate thread.

And this is how she made her cocoon. She lay near the surface of the water back downwards, the under part of the body and the second and third pair of legs buried in the thread-like weed. The front legs were free, and with these she shaped the weed over her abdomen. Then she spun a covering of white silk against the under side of the weed. While she was spinning, from time to time she used her front legs to press and flatten the work against her body (Fig. 30B), giving it the shape of a flattened arch, to which her body gave the requisite curve. This, forming the top of the cocoon, was finished in about half an hour. Then she turned (Fig. 30C), and spun the bottom of the cocoon, moulding this, like the top, on the curve of her abdomen, and uniting the top and bottom with silk which she spun. The work occupied about an hour and a quarter.

The Beetle then remained nearly in that position for some two hours. At first she was hidden in the cocoon quite up to the thorax. The body, however, was withdrawn almost imperceptibly. During this time she was busy laying her eggs in regular order, with the pointed ends upwards.

After this she came out of the cocoon, and closed the mouth (Fig. 30D), making the opening smaller by degrees. Then she made a little mast (Fig. 30D, b), of the use of which Lyonnet admits his ignorance, suggesting, however, that its construction may serve to use up the silky matter remaining after the work is finished, lest it should acquire harmful qualities in the body of the Beetle. The true explanation seems to be that it serves to convey air to the eggs inside the cocoon.