Sl\Rcn 1. 1901. 



KNOWLEDGE 



51 



explosions of gun cotton. In our own experience just 

 cited, however, tlie lightning flashes which appeared to 

 coui-se between cloud and cloud rather than between 

 cloud and earth were like typical mountain storms fol- 

 lowed only by a single short report, and in this resembled 

 phenomena twice observed by Mr. Wise in America. 

 This observant aeronaut on two separate occasions speaks 

 of thunder as " rattling like small arms without any of 

 the rolling reverberations that are heard below.'' But 

 another significant observation was made by Mr. Wise in 

 both his experiences just referred to. " The thunder pack 

 itself developed uprising cloud columns whose motion 

 resembled that of ebullition in a vast cauldron from 

 whence electric flashes were discharged." With almost 

 the same language he describes the effect presented to 

 him on each occasion when he approached from aloft the 

 neighbourhood of a thunder storm ; and Mr. Green in 

 his Newbury ascent already alluded to, uses words that 

 are hardly dissimilar. " I observed,'' he says, '' among 

 other phenomena af every dischai-ge of thunder all the 

 detached pillars of cloud within the distance of a mile 

 round became attracted and appeared to concentrate 

 their force towards the first body of clouds, leaving the 

 atmosphere clear and calm beneath and around us." 

 Neither of the above bvgone aeronauts describe to my 

 knowledge any occasion when they have actually found 

 themselves in the heart of a thunderstorm, and in our 

 own case, had there been but fair warning, I think 

 there would have been but little difficulty in avoiding 

 the storm by simply rising above it; but, as I have 

 already sufficiently explained, we were practically with- 

 out warning from the peculiar manner in which from 

 our restricted point of view the thunder cloud seemed to 

 develop about us out of thin air. 



I do not imagine that the smart impact of the hail 

 stones necessarily proved that their origin was far over- 

 head. It is my impression that the hail was being 

 forcibly swept down upon us by a violent downrush of 

 icv air, and as we were counteracting the descent of the 

 balloon by a discharge of ballast our own motion would 

 not reduce the pelting of the storm. 



It needs no pointing out that hail is chiefly a pheno- 

 menon of the warmer hours of a summer day suggesting 

 that warm moist currents are answerable for its 

 formation, and in this view we may fairly regard the 

 thunder cloud itself as its cradle, and this very fre- 

 quently at no higher altitude than a mile above the 

 surface of the ground. 



The tendency of hail to form with exceptional fre- 

 quency and severity over certain ai-eas is a local pheno- 

 menon which has perhaps hardly met with the attention 

 it deserves, but no facts are more striking or better 

 established. In KxowLErcE some years ago it was stated 

 that '' within a radius of 12 miles round Somersham 

 Railway Station Tfluntingdonshire) hailstorms are so fre- 

 quent and destructive that all the insurance companies 

 charge double the ordinary rates per acre for crops giow- 

 in? within that district.' 



The general meteorological conditions prevailing at 



the time of the storm above described are given in the 



accompanying charts. 



. — ♦ 



THE INSECTS OF THE SEA. II. 



Bv Geo. H. Caepenter, b.sc.(lond.), Axuintant iti the 

 Mmeum of Srience mul Art, Diihlln. 



SPRING-TAILS. 

 " Wh7 the Collembola should be neglected," wrote Dr. 



Sharp,* in 1895, " when the Thysanui-a attract so much 

 attention, is as inexplicable as many other fashions .ai'e." 

 During the last few years, however, the Spring-tails or 

 Collembola have been receiving a fair share of attention 

 from naturalists, and it seems that the entomo- 

 logical fashion to which Dr. Shai'p referred, is destined 

 to show its likeness to other fashions by experiencing 

 a marked change in the near future. The naturalist by 

 the sea-shore at least ought not to neglect the Spring- 

 tails. Smaller and less conspicuoiis than the Bristle-tail, 

 Mnrhiliit inarifiiiia (described in KNowLEncE for 

 January), the marine Spring-tails may be, but they are 

 fairly luimcrous in species, and some of them arc more 

 perfectly adapted than Machilis for a marine life. 



Spring-tails resemble Bristle-tails in being entirely 

 without a trace of wings, and have often been united 

 with them in the order "Thysanura (to which it has lately 

 been suggested to restore the Linncan title Aptera). 

 But the divergence between the two groups is in many 

 respects so striking that Lord Avcbury's scparationf of 

 the Spring-tails as a special order, to which he a])plied the 

 name Collembola, seems to be fully justified. The name 

 (KiA/.a, i/liii', and Ja/SoX?) ii /liniin'ng in) refers to the 

 supposed function of the " ventral tube " — a very 

 characteristic organ of these insects, situated beneath 

 the first segment of the hind body (Fig. 1, v. t.) ; it 

 shelters two protrusible processes which may help the 

 insects to cling to smooth surfaces, or may possibly, like 

 the abdominal sacs of Machilis, serve as breathing 

 organs. This tube is unknown among the Bristle-tails, 

 and there are other and more conspicuous differences 

 between the two groups. The hind-body of a Spring-tail 

 has never more than six evident segments, and long 

 " tail-bristles " like those of the tenth abdominal segment 

 in the Tliysanura are never present. The feelers, always 

 with very numerous segments among the Bristle tails, 

 have never more than six distinct segments among the 

 Spring-tails. 



The reduction in the number of abdominal segments 

 from ten to six marks the Spring-tails as a less primitive 

 group than the Bristle-tails. And their specialization is 

 further shown by the possession of that very remarkable 

 organ — the " spring " — which gives them their English 

 name. The spring (Fig.l, .s), which may be borne beneath 

 either of the fourth or fifth abdominal segment, roughly 

 resembles a two-pronged fork, consisting of a single basal 

 piece— the " handle " {niavnhriinii), to which arc attached 

 two long, flexible tapering "teeth " (denten) ; at the end 

 of each tooth is articulated a little " point " or tip 

 {tinicro) which is not always sharp. It is likely that 

 this spring has been formed by the fusion of a pair of 

 abdominal limbs. It may be tucked beneath the body 

 pointing forwards, as the insect runs along; when it is 

 released and straightened out so that the tip points 

 backwards, the Spring-tail leaps into the air. Let the 

 reader lift a stone under which a colony of Spring-tails 

 are sheltering, and he will have full demonstration of 

 their leaping powers. (See Fig. 1.) 



There is a family of Spring-tails, the Sminthurida;— 

 having, however, no"^ marine representative— with compact 

 globular hind-body ; these insects breathe by means of a 

 set of air-tubes opening by paired air-holes on the head 

 —an altogether exceptional position. But most Spring- 

 tails have no air-tubes at all; they breathe entirely 

 through the skin. Adaptation to an aquatic lifejs, 



* " The Cambridge Natural History," \'ol. V., p. 190. 

 t Sir Jolm Lubbock. " Monogi'aph of the Colleml)..]^ and 

 Thysanura. 



London {.Uay Socie/i/), 1873. 



