Page:Popular Science Monthly Volume 45.djvu/464

This page has been proofread, but needs to be validated.

together with a great quantity of other forms of pond life, and placed them all in glass aquaria to study their feeding habits. The bugs seemed to feed most voraciously upon the larvæ or PSM V45 D464 Undulating backswimmer.jpgFig. 5.—Undulating Backswimmer. nymphs of dragon flies. These were captured continually, and their juices greedily sucked out. The next most abundant victim was the common undulating backswimmer (Notonecta undulata) shown in Fig. 5. In one aquarium, in which a large amount of pond material, including half a dozen zaithas, had been placed, four of the latter were in sight at one time, each with one of these backswimmers grasped in its front legs and the beak inserted in the body.

Small fresh-water snails occasionally contribute to the diet of this insatiable creature, and young mayflies are also commonly eaten. Flying insects which fall upon the surface of the water are sometimes caught and killed.

The giant water bugs are typical examples of the true bugs. They belong to the group called by naturalists Heteroptera, the members of which are characterized by having two pairs of wings, the front pair being thickened at the base and thin at the tip, and mouth parts fitted for sucking rather than for biting. During their development they do not undergo so complete a series of changes as do the caterpillars, which transform into butterflies, but grow more like the grasshoppers, the young resembling the adults in general appearance but having no wings. These bugs can be dipped out of ponds and ditches almost anywhere by means of a net, and are easily kept in aquariums, where they form interesting objects for study.

Illustrating, in one of his juvenile lectures, the liquefaction and solidification of gases, Prof. Dewar said that ether is evaporated to produce, by abstraction of heat from the gas, solid carbonic acid, which, though a white substance like snow, is boiling at 80° C. below zero. If the pressure is reduced by the air-pump, it boils at a lower temperature, and -110° C. may thus be reached. This is sufficient to liquefy nitrous oxide, which boils at -90° C.; and liquid nitrous oxide under the air-pump produces cold enough to liquefy ethylene, which boils at -100° C. The last stage is to liquefy air under pressure by the cold made by evaporating ethylene. In practice all these stages are not used, but they illustrate the gradational method which must be employed. The lecture was illustrated by liquid air being handed round in a flask inclosed in a vacuum jacket; though at a temperature of -180° C, it was boiling gently away. An idea was given of the difference between its temperature and that of the room by dropping it on a cold metal plate, when it assumed the spheroidal state like water on hot iron.