July 2, 1894.] 



KNOWLEDGE 



149 



a more correct term, as its properties are more like those 

 of silk than of auy other substance. 



These cottony or silky masses must be extremely useful 

 to the insects, both as a means of protecting them from 

 birds and other predatory creatures, and of assisting their 

 migrations, for when any of the loose masses are blown 

 from one tree to another there are sure to be some insects 

 in them, and these are thus rapidly introduced to new and 

 more extensive pastures from which their own imaided and 

 feeble powers of locomotion would have completely excluded 

 them. Their attacks upon apple trees are much more 

 serious in results than might have been anticipated from 

 the size of the insect. It is not merely that they abstract 

 a good deal of sap from the growing tree, thereby reducing 

 its vitality, but their punctures give rise to a diseased 

 condition of the wood beneath, which becomes soft, pulpy, 

 and swollen, so that the bark splits and exposes fresh 

 surfaces to attack. At the end of summer these moist 

 tumours dry up and thus deepen the cracks, affording both 

 safer anchorage and still more abundant pasturage for a 

 new host of destroyers. 



Many remedies have been suggested against these 

 formidable pests. As the insects do not of their own 

 accord wander much, anything that cuts off the supply of 

 food in the spot in which they are located will be useful ; 

 or, again, anything that clogs up their breathing apertures, 

 which are at the sides of the body as usual, will prove 

 efficacious by suffocating them. Hence washes of some 

 sort or other are the best kind of remedies, and auy lotion 

 in which soap forms a principal ingredient seems to be the 

 most satisfactory in results. There is frequently a little 

 difficulty in applying such remedies, in consequence of the 

 tendency the insects exhibit to lurk iu minute crevices, 

 where it is difficult to get at them with any brush that is 

 large enough to be used where great numbers are concerned. 

 Fortunately, the insects soon give evidence of their presence 

 by the snow-white appearance of the cottony down proceed- 

 ing from them, and as prevention is better than cure, 

 especially in such a case as this, every effort should be 

 made, as soon as the tell-tale flecks appear, to rid the trees 

 of their parasites, and massacre the whole tribe before they 

 have time to follow the characteristic tendency of their 

 race, and multiply indefinitely. 



A similar secretion, though iu much smaller quantity, 

 is formed by those aphides that live beneath the soil and 

 feed upon the juices of underground stems and roots. In 

 this case the waxy material coats their bodies as a sort of 

 mealy powder, and gives them the appearance of having 

 been dusted over with tiour. As the secretion is impervious 

 to moisture, it serves the insects in lieu of a waterproof 

 garment, and keeps them dry, notwithstanding the moistness 

 of their surroundings. 



The same orderof insects, viz., the Homoptera, contains 

 yet another group in which the power of secreting silky or 

 waxy fibres is developed. These are a particular family of 

 the frog-hoppers, which are so called from their wonderful 

 jumping powers. The family in question is called ('i.riidir, 

 and its British representatives are few in number, though 

 two of those few are amongst our common insects. These 

 are called i.'i.rius iiervoftus and <.'. jiilosux. They have 

 brownish black bodies and four transparent glassy wings, 

 and the nervures of the front pair are distinctly studded 

 with black dots. The small head ends in a beak, which is 

 bent under the body in repose, and the thorax is furnished 

 ■with three longitudinal keels. The hind legs are con- 

 siderably longer than the other two pairs, and it is with 

 these that the insects perform their astonishing leaps. In 

 repose the fore wings are placed along the sides of the 

 body, and completely conceal the hind pair, so that the 



Fiy. 9. — Ci.vtus /lilusiix. with cottony filaments 

 at tail. Magnified live diameters. 



shape is something like that of a small moth at rest. 

 Seeing one of these insects sitting thus on a leaf, you go 

 to pick it up with your fingers, but just as you are about 

 to lay hold of it, it vanishes from sight as suddenly and as 

 utterly as if it had been gifted with the property of render- 

 ing itself invisible at will. The explanation is that it has 

 suddenly leapt away with a vigorous stroke of the hind 

 legs, aided by the wings, which are simultaneously out- 

 spread, and the movement is so sudden and tmexpected 

 tlaat it is rarely possible to tell in what direction the 

 creature has gone. 



CixtK.': pihsu-s (Fig. 9), the smaller insect, which is not 

 quite half an inch in expanse of wings, is, of the two, the 

 more noted for 

 the development 

 of the cottony 

 flocks at the end 

 of the body. They 

 very strongly 

 suggest the idea 

 of a parasitic fun- 

 gus, and in fact 

 a similar struc- 

 t u r e in an 

 American species 

 was once described as such. This was in 1769, when 

 Fougeroux de Bonderoy submitted to the French Academy 

 a paper descriptive of insects on which plants were to 

 be found growing — an association of dissimilar organisms 

 which was a great puzzle to the older naturalists, and 

 led some of them to the conclusion that "the passage 

 and mutation of animal species into the vegetable, and 

 reciprocally from the vegetable to the animal," was not 

 merely a possibility but even a frequent occurrence ! 

 After having cited a number of genuine instances of fungoid 

 plants growing on the bodies of insects, the author proeeeds 

 to describe a growth on the body of one of the Cicadaria^ 

 that particular group of insects with which we are now 

 specially concerned. The insect, he says, " is a native of 

 Cayenne, and the plant is a species of fungus, but different 

 from those we have described. It is formed of long and 

 silky filaments, which cover the whole body of the insect, 

 and project about seven to eight lines above and below the 

 belly of the animal." But from the figure that accompanies 

 the description, it is evident that what he had before him 

 was nothing more than filamentous exudations similar to 

 those of our own ''. ^'(/"vh.s-, though on a rather larger 

 scale. Many other exotic species of this order, especially 

 those belonging to the group of " lantern-Hies," produce 

 large masses of similar white tufts and threads, which 

 cover more or less of their bodies, and give them a mouldy 

 appearance. (To be continued.) 



LIQUID AIR. 



By J. .J. Stew^vkt, B.A.Cantab., B.Sc.Lond. 



IT is a familiar fact to all that some substances can be 

 obtained in the varying forms of solid, liquid and 

 gas ; but many substances come under our notice 

 only when occupying one of these states. When 

 granite is mentioned we at once think of a hard, 

 solid rock ; few people have seen granite in a liquid state. 

 Again, the name merciuy calls to our mind the well-known 

 liquid metal. So there are numerous gases and vapours 

 which are known to us only under this somewhat impal- 

 pable and less tangible form. Amongst the substances 

 familiarly known to us, however, there are a large number 

 which, through the action of heat upon them, can be 



