Active as the Water Boatman is, it often falls a victim to the Water Scorpion, if both are kept in the same aquarium. I learnt this fact by experience, for having put two Water Boatmen into a small tank in which was a Water Scorpion, I found both the former dead in the morning. It was evident that their destroyer had had a good meal. The only point to which attention need be called in examining the Water Boatman is its method of taking in a supply of air. In looking at a dead specimen we shall see a ridge or keel running down the middle of the under side of the abdomen, and fringed with hair on each side. A similar fringe runs along each side of the abdomen, thus forming two passages along which the air taken in at the end of the body is conveyed to the spiracles, the largest of which are on the thorax. Corixa (Fig. 47) swims with its back uppermost, and when kept in the aquarium may generally be seen foraging for small creatures--larvae or worms--among the sediment at the bottom. Bateman, who kept a number of these insects, says that he fed his specimens on garden worms and pieces of raw meat. Mine have always foraged for themselves, and done very well without feeding. They may often be seen to rub the short fore-legs alternately across the front of the head, probably for the purpose of producing a call-note. I have often watched them at this practice, but have never been able to detect any sound. The defect is evidently in my sense of hearing, for the sounds rest on undoubted authority, and are coincident with the rubbing of the fore-legs across the head. [Illustration: FIG. 47.--Corixa, with wings expanded.] Where sound-producing organs exist it is more than probable the capacity for receiving sound-impressions also exists. Graber made some interesting experiments to test the sense of hearing in Corixa. His results are thus summarized by Sir John Lubbock[28]: ‘He placed some Water Boatmen (Corixa) in a deep jar full of water, at the bottom of which was a layer of mud. He dropped a stone on the mud, but the insects, which were reposing quietly on some weeds, took no notice. He then put a piece of glass on the mud, and dropped the stone on to it, thus making a noise, though the disturbance of the water was the same. The Water Boatmen, however, at once took to flight.’ CHAPTER IV SPIDERS, MITES, AND MYRIAPODS In this chapter we shall take examples from two Classes--the Arachnoi´dea and the Myriap´oda. To the first-named Class belong also the Scorpions, which, with the Book Scorpions, the Harvestmen, and some others, may be neglected here. This will leave us only the Spiders and Mites to deal with.