As soon as she has reached the entrance to the cell, which is always below, she loosens her hold of the air-bubble. It at once rises into the cell, and expels a proportionate amount of water. Not many of these journeys are required before the nest is filled with air, and then the diminutive architect spends the greater part of its time in holding on to the mouth of the little diving-bell, and supporting life by means of the air within it.
This nest, as the reader will see, is an exact representation of the various diving schemes in which air-bells are the chief portions of the machinery, although the air is conducted into them after a different fashion.
We now come to another mode of diving, in which the bell is practically superseded by the flexible tube, which allows to the diver far more range than can be obtained by the bell. In this case the diver wears a peculiar dress, the chief part of which is a helmet so constructed that air can be introduced to it from above the surface of the water, and, after respiration, can escape by means of a valve.
Air is pumped into the tube by assistants above water, and, as the tube is long and elastic, the diver can move about with considerable freedom. As is the case with the diving-bell, the diver’s tube is strengthened by an internal spiral wire, so that it is always open, however it may be bent or twisted.
The right-hand figure of the illustration represents the diver examining part of a sunken vessel. The tube through which he breathes is seen passing to the surface of the water, and so is the line by which he gives his signals to his comrades above. In his hand he holds a lamp which can burn for a limited time, being connected by a smaller but similarly constructed tube to a vessel of compressed air.
On the left hand of the same illustration are shown the curious Rat-tail Maggots, as they are popularly called. They are the larvæ of the common Drone-fly (Eristalis tenax), which is so common towards the end of summer, and looks so curiously like a bee.
These creatures pass their larval life buried in the mud and below the surface of the water, and yet are obliged to breathe atmospheric air. This they do by means of the long appendages which have gained for them the name of Rat-tails. These “tails” are very elastic, and are capable of elongation and contraction to a wonderful extent.
When the creature is undisturbed, it lies buried in the mud with its head downwards, and its tail extended so that it reaches the surface of the water. Within this tail are two air-tubes, which are connected with the principal tracheæ, which have already been mentioned. They are wonderfully elastic, and, when the tail is extended to its utmost limit, are nearly straight. When, however, the tail is contracted, the tubes become self-coiled by their own elasticity, and shrink into the base of the tail.
As the tail is very transparent, it is easy to see how these movements are conducted. The larvæ, which may be found in almost any stagnant water, should be placed in a tall and narrow glass. Some mud should be placed at the bottom of the glass, which should then be filled with water to the depth of three inches or so.