Au<i. 1, 1884.] 



• KNOWLEDGE ♦ 



83 



squares of 8 to 10 kilometres a-side, liaving each a large 

 steam-motor, we may supply a horse-power at 0-25 franc 

 hourly as against 032 franc, which would be the cost of a 

 gas-motor, showing a considerable economy in favour of 

 electricity. 



There are numerous cases where local conditions render 

 it impossible to set up a motor at the place where the 

 power is required, and only certain systems of transmission 

 can here be employed. Thus in mining and tunnelling, air 

 and electricity only are applicable, and if we suppose that 

 there is need for 10 horse-power, we see, on comparing the 

 price of this power transmitted by compressed air and by 

 electricity, that the advantage is greatly in favour of the 

 latter. For more considerable transmissions of power the 

 prices agree fairly well up to 5 kilometres, but beyond this 

 the advantage of electricity becomes very decided. In 

 addition, an electric transmission is more easily established 

 than the conduction of compressed air, and it is much 

 easier to extend a system of the first kind than of the 

 second. 



Certain boring-machines with compressed air often 

 sutKce for ventilation, whilst an electric transmission T)f 

 power requires to be accompanied by especial appliances 

 for this purpose. Still the advantages of electricity as 

 regards convenience and economy are so great that we 

 cannot hesitate to employ it whenever there is no fear that 

 sparks from the dynamo machines may occasion explosions, 

 especially as electricity can at the same time serve for 

 lighting. 



In conclusion, in cases where telo-dynamic cables are not 

 applicable, electric transmission is much preferable to trans- 

 mission by water or compressed air. It is more economical 

 than gas-motors for transmissions up to o kilomttres. 

 Where transmission by cable is applicable it is more 

 economical up to 1 kilometre. From 1 to 5 kilometres 

 electricity has the advantage. — Revue des Mines. 



THE ENTOMOLOGY OF A POND. 



By E. a. Butleb. 

 THE MIDDLE DEPTHS {continueiX). 



PASSING on now to the stouter-bodied, shorter-horned 

 flies, our only example will be the insect called 

 Stratiomys c/iameleon, the common chameleon fly, which 

 belongs to a family containing several aquatic representa- 

 tives. It is a broad, flat-bodied insect (Fig. 1), with a 



Fig. 1. — Chameleon Fly. 



velvety black body, adorned with yellow markings, and is 

 a near relation of those lovely, glossy, metallic-looking flies, 

 with long, dark wings, and bodies of a greenish, purplish, 

 golden, brassy, or bronzy tint, that are often seen sucking 

 the honey of flowers in damp places, or sunning themselves, 

 and displaying their beauty on the leaves of trees. The 

 eggs are not launched in rafts, like those of gnats, but laid 

 in overlapping rows, like roofing slates, on the underside of 

 the broad leaves of the water plantain, Alisma i^lantago. 



The larva, which is of an elongate form, tapering greatly 

 towards the tail, is chiefly remarkable for the perfect star 

 of about thirty feathery hairs it carries at that ex- 

 tremity. As usual, this circlet of hairs is intended 

 to assist in the respiratory function. To breathe, the 

 insect slowly rises to the surface by serpentine wrig- 

 glings, and remains suspended there, the coronal hairs 

 acting as a float, and by their capillary attraction causing 

 the water to recede from the respiratory orifice which is 

 situated in their centre, so that air can be taken in at 

 pleasure. When this has been efl'ected, the insect closes 

 its hair star somewhat as one would shut an umbrella, 

 and slowly descends to the depths again, carrying with 

 it. the spoils of the outer world in the form of a silvery 

 globule of air entangled in its plume. Its jaws and 

 other appendages of the head are in constant motion, 

 creating currents which bring to it the minute creatures 

 on which it feeds. During larvahood, then, it does not 

 very greatly depart from the general style and method 

 of life of the gnats and other long-horned flies, but when 

 we come to the next stage we notice a great difierence. 

 Hitherto we have found the pupa shaped like a large 

 comma, and breathing by appendages attached to the 

 thoracic region. In the chameleon fly, however, a totally 

 difierent arrangement is made. The true pupa is formed 

 within the old larva skin, which retains its form so 

 that but little change, except an inflexibility of body, is 

 apparent outwardly. The pupa itself, however, reveals all 

 the organs of the future insect, and with its wings and legs 

 folded lengthwise along its breast looks like a miniature 

 Egyptian mummy. It is much smaller than the larva, and 

 so does not occupy nearly the whole of the space the old 

 skin afibrds, the long tail-like part being converted into 

 an air-chamber to supply with aerial nutriment the im- 

 prisoned mummy, which has its spiracles situated in the 

 usual position down the sides. When the time for emer- 

 gence arrive", a portion of the case near the head is re- 

 moved, and the fly makes its exit through the opening. 



There is a small family of moths whose caterpillars are 

 aquatic, and may be found feeding on plants below the 

 surface ; but we will reserve a notice of these till we treat 

 of the perfect insects, which are abundant amongst the 

 rank vegetation fringing the edges of the pond. 



Besides the bugs, beetles, and fly larvie, which are the 

 legitimate inhabitants of this part of our pond, certain 

 perfect insects belonging to orders that one would assuredly 

 not expect to find represented in the water — at least in the 

 adult state — may occasionally be detected paying flying 

 visits to these regions. About twenty years ago, Sir John 

 Lubbock discovered that some minute insects allied to the 

 ichneumon flies, and therefore belonging to the order 

 Hymenoptera, are aquatic in habits. This was a most 

 surprising discoveiy, for though the Hymenoptera form an 

 enormously large order, the number of species having been 

 estimated even at 30,000, not a single member of this vast 

 host had previously been known to have any connection 

 with water. Sir John Lubbock describes the discovery as 

 follows : — " Great was my astonishment . . . when I 

 saw in the water a small Hymenopterous insect, evidently 

 quite at its ease, and actually swimnimg by means of its 

 wings. At first I could hardly believe my eyes, but having 

 found several specimens, and shown them to some of my 

 friends, there can be no doubt about the fact. Moreover, 

 the same insect was again observed, within a week, by 

 another entomologist, Mr. Duchess, of Stepney .... It 

 is a very curious coincidence that, after remaining so long 

 unnoticed, this little insect should thus be found almost 

 simultaneously by two independent observers." Twenty- 

 one specimens in all were seen, and two-thirds of these 



