The work is now rendered much easier. There is nothing to obstruct. As soon as the larvae reach the bottom they will walk round and round, giving us a good opportunity of watching them. In swimming--which may be backwards as well as forwards--the abdomen is waved from side to side. To see the backward motion one need only put a dipping-tube or a pencil, or the like, in front of the larva, so as to bar its progress. The creature will retreat a step or two, and then, with a flourish of the abdomen, dart back through the water. The larva will sometimes wave the abdomen up and down, just as one may see a Chironomus larva do when it has affixed its dwelling to the side of the glass, and this motion probably assists the process of respiration. When the larvae have been watched under the conditions above described, I have never been able to detect independent motion of the gills. But it is better that every one should observe for himself, and draw his own conclusions from what he sees. Now let us examine a specimen more closely with the lens, or under the dissecting microscope. The mouth parts may be broken up separately, or a little careful manipulation will enable us to see the chief features without injuring the specimen. The head is strong and massive, and the group of ocelli, or simple eyes, may be clearly made out. The antennae bear a small pencil of hairs, no doubt sensory in function, at the extremity, but careful management of the light will be required to distinguish them. The mandibles are extremely business-like instruments, and each bears two strong teeth on the inner side. Next come the maxillae, with their palps, each of which has an appendage, while each maxilla carries three strong spines. The labium, with its palps, and the mentum, with its saw-like notchings, may be plainly seen. The three segments of the thorax offer no difficulty. The legs are worth more than a cursory examination from their apparatus of spines and double fringe of hairs. Nine body-segments succeed to the thorax, and behind these is a long tail-like organ, which some authors consider represents a tenth segment. [Illustration: _h_ _t_ 1 2 3 4 5 6 7 8 9 tail -- ----- -- -- -- -- -- -- -- -- -- ------ | | | | | | | | FIG. 86.--Diagram of Sialis larva, showing arrangement of gills (represented by vertical lines). _h_, head; _t_, thorax; 1–9, segments of abdomen.]