84 



♦ KNOV/I.EDGE ♦ 



[Aug. 1, 1884. 



■were females. The tiny being (Fig. 2) measures no 

 more than -2,\ of an inch in length. It has no nervures 

 in its wings, the hinder pair of which are so narrow as 

 to be scarcely more than linear in shape, and both pairs 

 are fringed round the edges with hairs. It Ijelongs 

 to a group which, like the ichneumon flies, are parasitic 



Fig. 2. — Polynema natans. 



upon other insects, but many of the smaller species attack 

 their hosts, not when the latter are in the larval condition, 

 but actually while they are in the egg, the contents of a 

 single egg being sufficient to furnish nutriment to the grub 

 of the parasite during the whole of its brief larval career, 

 and sometimes even one egg is the home of several para- 

 sites. The present insect, which was named by Sir John 

 Lubbock Pohjnema natans, may, therefore, with much pro- 

 bability be presumed to have been in quest of the larva or 

 eggs of some aquatic creature in which to deposit its own 

 brood. It would seem, however, that this can hardly be 

 the sole cause of the entry of these insects into the water, 

 inasmuch as the males were found swimming as well as the 

 females. The wings did not seem particularly effective as 

 swimming organs, the progress of the insects being but 

 slow, and in a series of jerks ; sometimes, too, the swim- 

 ming was abandoned in favour of crawling over the aquatic 

 plants. Marvellous as it may seem that a creature should 

 use as swimming-organs delicate membranous wings, appa- 

 rently adapted only for aerial flight, the marvel becomes 

 greater when it ia remembered that the little diver is 

 not in any way structurally adapted for an aquatic 

 life, except it be by the fringes round the wings, 

 but these it has in common with other members of the 

 same group which never enter the water at all. There is 

 no flattening of the legs, no tapering of the form in front, 

 no arrangement to provide for subaqueous respiration. The 

 breathing is conducted in the ordinary way by means of 

 spiracles, and all the time the insect is under water, it has, 

 so to speak, to hold its breath, just as one of the higher 

 animals would have to do under similar circumstances. At 

 first thought it would seem, therefore, that the tiny 

 creature, in obeying its maternal instincts, incurs some risk 

 of drowning, but it must be remembered that insects do 

 not require a renewal of air anything like so frequently as 

 the higher animals, and in the present instance the power 

 of endurance seems to be much greater even than usual. 

 Sir John Lubbock found that one of his insects could 

 endure submersion for twelve hours without inconvenience, 

 but that after fourteen hours it was to all appearance 

 dead ; however, on being transferred to a dry spot, it 

 revived, and, after a time, became as lively as ever, so 

 much so, in fact, that, notwithstanding its uncomfortable 

 experience of temporary drowning, it did not hesitate, 

 when an opportunity was again afforded, again to enter 

 the water. Professor Westwood has suggested, however, 

 in explanation of this power of enduring prolonged sub- 

 mersion, that the fringe round the wings may carry down 



entangled in its hairs a small quantity of air, sufficient for 

 the wants of the insect during the time it would naturally 

 remain below. 



Curiously enough, a second aquatic species, a trifle larger 

 than the other, and much less common, was discovered on 

 the same occasion and by the same observer. It swam, 

 however, not by aid of its wings, which were kept still, but 

 by a rowing motion of the legs, and thus progressed more 

 rapidly than its relative. 



Ichneumon flies have recently been bred from the pupa; of 

 a Gyrinus, or whirligig beetle, which, as will be remembered, 

 is, in its larval state, subaqueous. It is not known, how- 

 ever, at what period in the history of the Gyrinus the 

 ichneumon eggs are inserted in the body of the host, 

 though, judging from analogy, it would seem probable that 

 it is the larva that is thus victimised, and in that case 

 either the ichneumon must dive, or the larva must be 

 attacked daring its temporary exposure on the aquatic 

 plant on which it forms its cocoon ; still, however, the eggs 

 may be deposited in the pupa through the walls of the 

 cocoon, the ichneumons possessing ovipositors long and 

 powerful enough for the purpose. 



Certain caddis flies, or water moths as they are sometimes 

 called, and dragon flies have also been known voluntarily 

 to submerge themselves in order to deposit their eggs in 

 appropriate positions. 



(To he continued.) 



RAILWAY BRAKES. 



By " Tkevitheck." 



DURING the past few year.s, a prolonged and, at times, 

 an acrimonious discussion has been carried on con- 

 cerning the advantages and disadvantages of brakes as 

 applied to railway engines, carriages, and trucks. The 

 speed of ti'avelling is now on the average very high, the 

 trains run in rapid succession, and the lines are gradually 

 being called upon to work to the full extent of their capa- 

 city. All this means that if the work is to be got through 

 with anything like efficiency, every possible chance of faci- 

 litating the traffic should be taken advantage of. English- 

 men are, however, more phlegmatic and more inclined to 

 repose implicit faith in the powers that be than they 

 imagine, or would grant to be possible. It is, indeed, a 

 stern reality, that it requires a great calamity to awaken 

 them to the fact that one of man's first duties of citizenship 

 is to see that what is done around him is the best that can 

 be effected for his own comfort and for that of his fellow- 

 creatures. 



Such an awakening has been lately experienced, for 

 during the past few weeks, embracing the longest days of 

 the year, at a season when fogs have no place in our 

 weather " forecasts " or " reviews," we have had our eyes 

 rudely opened to the fact that the railway system of 

 England is one that revels in disaster, torture, and deatL 

 Now it must be patent to the most superficial thinker that, 

 if our climatic conditions are such as to preclude the jury 

 from attributing a disaster, as they did the one near Ox- 

 ford a few years back, to a " freak of nature," such disaster 

 must be due to some cause in the main preventible. Be it 

 observed, further, that the accidents now pro\-iding de- 

 tailed horrors for the daily papers, happen for the most 

 part in the full blaze of the sun, with a good road beneath, 

 a good sky above, and a good light in front. Hundreds of 

 trains seem to carry with them day by day and hour by 

 hour a sure and effectual means for securing their 



* 



