22 



KNOWLEDGE. 



January, 1915. 



CONTINUOUS PICTURES OF THE WEATHER.-- 

 Professor Cleveland Abbe, in a note in the United States 

 Monthly Weather Review on continuous pictures of the 

 weather, said that he is con\inced that it is only by the 

 study of atmospheric conditions o\'er the whole northern 

 hemisphere, as if photographed daily, that we shall ever 

 be able to appreciate the preponderating infliience of the 

 diurnal rotation of the Earth, and the general circulation 

 of the atmosphere as compared with the minor influence of 

 sunshine, radiation, and moisture. That is to say, these 

 last three influences that start the atmosphere in motion 

 are completely overshadowed bj^ the effect of that motion 

 combined witXi the swift rotation of the Earth. The relative 

 importance of these influences on the atmosphere as a 

 whole is quite analogous to their relative importance in the 

 case of a hurricane, where sunsliine, moisture, heat, radiation 

 all come into pla}', and would of themselves start the atmo- 

 sphere into direct lines of motion toward a centre of low- 

 pressure, whereas the rotation of the Earth turns that radial 

 movement into an almost perfect circle. The relative 

 importance is analogous to the influence of gravity on 

 a bowlful of water escaping at the outlet, where the least 

 deviation from symmeti-y converts the straight line into 

 a circular motion. 



Atmospherics is not inerely a study of the physics of the 

 atmosphere on the scale of a laboratory experiment : it 

 is a problem in terrestrial physics in wliich the overpowering 

 influence of the Earth, considered as a small planet, must be 

 fully considered. The lower layers of the atmosphere, 

 being resisted by continents and highlands, move almost 

 independent of the upper layers that have scarcely any 

 connection with the lower layers by way of viscosity of 

 fluid friction, and still less connection due to terrestrial 

 resistances. These upper layeis are affected by radiation 

 and absorption, by density, by the attraction of the Earth, 

 the Moon, and the Sun, by the action of solar electrons 

 and cosmic shooting stars, and by the motion of the Earth 

 in space, as well as its diurnal rotation. Their motions 

 represent the sum total of astronomical and planetary 

 influences, and they react in a most complicated manner 

 upon the lowest layer of the atmosphere, wliich is under 

 the influence of convective circulation. The study of the 

 motions of the centres of high and of low pressure, presented 

 to us every day on these international polar charts of the 

 northern hemisphere, may be conducted either by pure 

 analysis, or by grapliic methods, or by laboratory 

 experiment. 



MICROSCOPY. 



By F. R. M. S. 



ANTENNAE OF THE ICHNEUMONIDAE.— The 

 function of the antennae of insects has long been a matter of 

 conjecture, some entomologists being of opinion that they 

 are organs of perception, others thinking that they may be 

 organs of smell, of touch, or of communication with their 

 fellows ; but as such of these senses which we possess are 

 excited in us by various degrees of vibration, it is quite 

 possible that the antennae are the means of conveying 

 to the insect impressions which, whilst they correspond 

 in method, are more highly sensitive, and respond to 

 vibrations to which none of our own sense-organs are attuned. 

 Careful observation inclines to the belief that the curious 

 retractile terminal joints of the antennae of the female 

 mosquito and the similar organs found on the palpi of 

 a tick enable them to ascertain, with unerring precision, 

 the position of the blood-vessel which they desire to pierce. 

 No one can have watched the tremulous filiform antennae 

 of a tiny ichneumon-fl^-. as it searches for the hidden pupa 

 of the clothes-moth, without being impressed with the idea 

 that they are at least organs of smell by which these restless 

 insects discover their victims, piercing and laying eggs 

 within them, and in this way contributing so largely to the 

 keeping down of what would otherwise be a too numerous 



ii 



pest ; and since, according to " the Cambridge Natural 

 History," some six thousand species of ichneumonidae are 

 known, of which twelve hundred are British, our indebted- 

 ness to them can hardly be overrated. In 

 relation to this subject, a microscopical 

 examination of the mounted specimens in 

 our own cabinet shows that all the ichneu- 

 mons there have in each joint of the 

 antennae, except the terminal and the 

 three nearest to the base, several re- 

 niarkable sigmoid bodies, as represented 

 in Figure 17, the nature of which is at 

 present obscure. Each of these has a 

 very clearly defined elliptical centre, the 

 average number in each joint being three, 

 some of which are straight throughout ; 

 but the majority are curved in opposite 

 directions towards the ends, their measure- 

 ment being from ^ij^ to nj^ of an inch in 

 length, with a breadth of not more than 

 5TRrn °f 3,n inch. The particular antenna 

 from which Figure 17 is taken consists of 

 forty-five joints, and is furnished with 

 short hairs. As mounted in Canada 

 balsam under pressure, it appears of a 

 brown colour, the bodies in question being 

 white in contrast. 



R. T. L. 



THE DRAGON-FLY [AGR ION P UEL LA ) 



{continued). — The nymph undergoes a series 



of moults during its life in the water, and 



the changes during its early periods are 



difficult to describe, owing to this portion 



of its life's history being obscure. 



Some species of Dragon-fly moult im- 

 mediately they leave the egg, as in the 



case of Agrioii puella, and, as mentioned 



earUer, the legs are adherent previous to 



this moult : these organs are eventually, 



however, set free, and the young nymph at 



once makes use of them. 



In Aeschna cyanea the moults have been 



recorded to take place practically every four weeks in the 

 first part of its life ; later on changes occur every few days. 

 In one case the nymph increased in size as follows : — 



July 30th, 6J- millimetres; July 31st, 10 millimetres; 

 August 3rd, 13 millimetres ; August 15th, 18 millimetres 

 (rudimentary wing-cases appeared) ; August 25th, 20 milli- 

 metres ; September 22nd, 25 J millimetres ; after winter 

 rest, January 24th, 29 millimetres ; May 5th, 32 millimetres ; 

 June 6th, 38 millimetres ; July 14th 43 milhmetres ; the 

 total number of moults being twelve, an average of 4 milli- 

 metres for each ecdysis.* 



In Agrion pulcheUum the moults numbered about fifteen, 

 and the number of days required to complete the 

 whole series in one case was over six hundred, whereas in 

 other cases between three hundred to three hundred and 

 thirty were necessary (see Table 6). 



The average length of the nymphs varies somewhat, 

 and reaches 18 millimetres in the final stage ; smaller 

 nymphs are known to measure only 14 millimetres. There 

 is no complete regularity to be observed, for a period of 

 stagnation may take place, which results in drastic changes 

 in the final measurement of body length (see Figure 19) 



The growth of the nymph's antennae is remarkable, 

 and takes place in the segment immediately above the scape, 

 i.e., the third from the base, which divides three times to 

 produce new segments. The actual change is not really 

 instantaneous, but by slow degrees forms under the hard 

 shell the new parts, and these changes are in a sense con- 

 nected with the various moi Its. 



The percentage in the first stage shows an average of 

 twenty-three for body length, but in the final stage this 

 percentage decreases to twelve. 



Figure 17. 



Part of the 

 Antenna of an 

 Ichneumon- 



fly. 



* Knowledge, 1902. East, pages 198, 199, 



