Sept. 15, 1882. 



♦ KNOWLEDGE ♦ 



269 



ever attends to such matters). [It is little when, in a 

 report, an elliptic tide is called an ecliptic one, but really 

 matters have reached a pleasing pass when Sir W. Thom- 

 son gravely tells the editor of the Times that it is not to 

 l>e ^\ ondered at, if in a leading article which is even con- 

 descendingly explanatory, an electric tide appears upon the 

 scene. — Ed.] The moon is alternately nearer to and farther 

 from the earth by a difference of about 26,000 miles 

 (13,000 miles on either side of her mean distance) a little 

 more than one-tenth of her average distance, in [very 

 nearly] the monthly period of her revolution in her elliptic 

 orbit 



Lastly, Sir W. Thomson mentioned the experiments 

 carried on by Messrs. G. and H. Darwin on a pendulum 

 so delicately suspended as to be sensible to the smallest 

 influences. They found incessant vibrations, which show 

 that the earth's surface is constantly palpitating, so to 

 speak, chiefly from local disturbances. These palpitations 

 were in fact minute and unceasing earthquake tremors, 

 completely masking the delicate regular periodical variations 

 which their pendulum would have shown had the earth 

 been worthy of its name — terra firma. 



A STUDY OF MINUTE LIFE. 



By Hen-ry J. Slack, F.G.S., F.R.M.S. 



POSTPONING for the present a further consideration 

 of the microferments, the wants of young microsco- 

 pists may be met by a dip in a waterbutt, or shallow pond 

 in a sunny situation. Besides other creatures likely to 



reward the experiment, it is probable that the eggs, 

 larvie, and pupai of the common gnat may be caught, and 

 will amply reward careful examination. The female gnat 

 lays eggs of long, conical shape, and glues a number of 

 them together so as to form a floating raft. They are too 

 small to be readily recognised by an unpractised eye, with- 

 out a hand magnitier, but if any specimen of water contains 

 larva; and pupa', small brown masses floating on the 

 surface may prove to be eggs. When hatching-time 

 arrives, they split lengthwise, and out comes a most 

 curious creature, which grows pretty quickly, changing 

 its skin by frequent moultings. A few ounces of water 

 obtained as mentioned, will probably exhibit larva? in 

 various stages of growth. A two-ounce wide-mouthed 

 bottle, just filled from a water-butt, contains scores of 

 them, many about on(veighth of an inch long. Hold- 

 ing the bottle up to the light enables the naked eye to 

 see them as wormy-looking things, with big heads and a 

 many-jointed abdomen, curiously forked at the lower end. 

 Many move about briskly by Hicking what may be popu- 

 larly called their tails ; others are suspend(Kl upriglit, head 

 downwards, and quiet, just kecjiing one of the forked ends 

 out of the water ; others are in a similar position, but com- 

 pletely immersed. The moving ones are violent in their 

 jerks, which indicate great muscular power in proportion 

 to their size. Closer examination with a magnification of 

 about fifty times shows the internal structure through the 

 transparent integument, and amongst other things it is seen 

 that the creature is provided with the tracheal breathing-tubes 

 belonging to insects, and that the prong of the terminal 

 fork, which is often thrust just through the surface of the 

 water, can open a way for tliem to receive the air. Pro- 

 bably the four leaf-like expansions attached to the other 

 fork may serve as gills. At any rate, the active creature 

 can stay for some time under water, and when a swarm of 

 them are watched in a bottle, they will be found at various 



depths, some at the bottom, and, as in the case before us, 

 few caring to come often to the surface for a gulp of fresh 

 air. When not jerking about, they either float head down- 

 wards, almost stationary, or slowly sink, and then with 

 a kick dash the water behind them, and rise up again. 

 Their motions are in all directions, and though scores are 

 moving briskly in our bottle, which is less than two inches 

 in diameter, collisions are rare. 



After well examining their ways in the bottle with the 

 naked eye and pocket lens, one or two should be trans- 

 ferred to a small shallow glass cell, and a power of about 

 a one-inch objective employed. The great roundish head 

 is thus seen to be broadest near the base, with two con- 

 spicuous black eye-spots, and to be furnished with two large 

 antennre, and numerous bristles. It is a lierce-looking 

 object. At about one third from the tip, each feeler has a 

 large tuft of bristles, a smaller one above it, and a terminal 

 bunch. Next to the head, which is very movable, comes a 

 wide thorax, followed by a ten-jointed abdomen, bristled 

 ornamentally on both sides of each joint. If the light 

 from the suVistage mirror of the microscope is thrown a 

 little slantingly through the creature, two large vessels of 

 a satiny aspect are seen to run one on each side of the 

 thorax through the abdomen, and into a tube that springs 

 from the eighth joint of the abdomen. From the thorax: 

 smaller objects of the same sort spring from the larger 

 ones, and enter the head. These objects are the breathing- 

 tubes {trachea), and their peculiar sheen in glancing light 

 comes from the coils of wire-like material that keep them 

 open. For collecting food, the mouth is furnished with 

 two tufts of bristles, fan-like when expanded, and used, 

 like the so-called wheels of rotifers, to lash the water into 

 whirlpools, and bring floating particles into its gullet. 

 Only very small objects are swallowed, but when they aue 

 massed together in the intestinal tube, they make a, 

 considerable show. 



Besides the tracheal tubes and intestinal canal, a long 

 vessel is seen rhythmically pulsating — acting like a heart. 



After these objects have been well examined with an 

 inch-power, a half-inch may be used with advantage. This 

 enables the structure of the tracheal tubes to be more 

 distinctly seen in their minute ramifications, taking air to 

 all parts of the little animal. I\Iany of the bristles will 

 also be found elegantly feathered, and the muscular bands 

 with which the vigorous motions are eflected may be dis- 

 cerned. 



What is called dark ground illumination gives tliese 

 creatures a splendid appearance under a low power. They 

 look self-luminous, and shine with brilliant pearly and 

 opalescent tints. The principle of this illumination is that 

 no light reaches the eye straight through the object, V>at 

 only such rays as are refracted by it. With a little prac- 

 tice, it can be managed by a slanting action of the substagt 

 mirror, but a finer cfl'ect is obtained by a spot-lens, that is, 

 a lens which has a patch upoit it to stop the passage oS' 

 light excepting through tlie margin which is left clear. 

 This is placed at a suitable focus under the object, which >? 

 made to glow brilliantly upon a dark ground. 



Besides the great larva, the pupa must be looked for, 

 and if a lucky dip is niade, it is sure to attract attention 

 from its grotesfjue oddity, though no one not in the secret 

 would imagine it had any connection with the larva, or 

 would ultimately disclose a winged gnat It is a queer, 

 bigheaded thing, with two hornlike projections and a 

 slender, tail like abdomen, sometimes curled close to the 

 body, and then vioUntly jerked to make it dart through the 

 water. The breathing tubes of the larva terminate at the 

 tail end, as we have seen ; those of the pupa end in the 

 two horns which it thrusts through the surface of the fluid 



