Professor Miall[59] has verified Burgess’s observations, and carried them a step further. He found by actual experiment that ‘the mouth-lock acted automatically, opening when the mandibles opened, and closing when they closed.’
Both these authorities stand very high. Yet, with all respect to them, it will be well to test these statements before accepting them.
Something of this mouth-lock may be seen in any well-prepared slide of a Dytiscus larva. If we hold the slide up to the light and examine with a power of 10, we shall see a dark line—in some cases two nearly parallel thinner lines—running from the base of one mandible to the base of the other. The dark line is the closed mouth-lock. The two nearly parallel thinner lines are the edges of the lips drawn asunder in preparing the specimen or by the pressure of the cover-glass. If we get to see this much, we shall have advanced one step.
Fig. 76.—Dytiscus Larvae.
Next we may verify Professor Miall’s experiment, though not quite in his way, for such section-cutting as he speaks of is beyond our powers. Larvae as large as possible should be chosen, and killed by dropping them into boiling water. The mandibles of one should be allowed to remain closed, and those of the other plugged open with pith or a small piece of wood—a bit of a match will serve. By putting each in turn into an excavated slip with water, carefully covering this with a plain slip, and holding these slips together with an elastic band, we may examine the larvae as we did the slide.
The result will be practically the same. Where the mandibles are closed, we shall see the dark line; and where they are open, we shall see the two thinner ones nearly parallel.
One caution may be necessary. The mandibles bear at the base a rounded process, which fits into a chitinous cup. It is not difficult to mistake this for the mouth-lock, with which, however, it is not connected.
There is still one other method which we may adopt to see the mouth. The head of one of these larvae may be cut off, dropped into a strong solution of caustic potash, where it should remain for a day or two. After washing it well in water, cut it in half lengthways, and turning either half upon its side, so as to expose the part cut, examine with the pocket lens.
These experiments are by no means difficult. But if carried out step by step, it will be easy to understand how the larva can suck out the juices of its prey, and how it can open its mouth to swallow some of the solid parts.