446 THE POPULAR SCIENCE MONTHLY. 



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 larvse or 



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 



FIG. 5. UNDULATING BACKSWIMMEK. * ' c 



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 suffi- 

 cient 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 0. 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 f^ 

 difference between its temperature and that of the room by dropping it on 

 metal plate, when it assumed the spheroidal state like water on hot iron. 



