334 



. KNOWLEDGE ♦ 



[Oct. 24, 1884. 



method of viewing the matter, the tonsumjitioii in that 

 year will be .373.', millions of tons ; accordiug to the 

 latter, the consuiuptiou would be no less than 1,446 

 millions. 



Startling as this result may seem, the commi.«sioners 

 found that in the last year of the five during which their 

 labours continued, the consumption corresponded much 

 more closely with the anticipations of Mr. Jevons than with 

 the theory of an arithmetical rate of interest. And they 

 remarked thereon that, though "every hypothesis must 

 be speculative, it is certain that, if the present rate of 

 increase in the consumption of coal be indefinitely con- 

 tinued, even in an approximate degree, the progress towards 

 tke exhaustion of our coal will be very rapid." 



This will readily be believed when we mention that, 

 according to Mr. Jevons' method of calculation, adopting 

 3-26 as the rate per cent, of increase, 150,000,000,000 of 

 tOQs as the total available supply of coal, and 120,000,000 

 as felie present annual consumption, our coals would not 

 last us quite 127 years from 1872. 



I venture, however, to indicate reasons for believing 

 that the rate of increase here contemplated cannot possibly 

 continue during many years, and that even the assumption 

 of an arithmetical rate of increase at the present mean rate 

 over-estimates the annual consumption for any time far 

 removed from the present. 



(_To be continued.) 



THE ENTOMOLOGY OF A POND. 



By E. a. Butler. 

 ABOVE THE SURFACE. 



THOSE insects that make their home in the pond in 

 their early life only, when fully grown still haunt 

 the neighbourhood, never straying far from the scene of 

 their juvenile associations. Their movements will, of 

 course, be very largely determined by the functions they 

 have to perform. Those whose only business it is to found 

 new families, and which so thoroughly devote themselves 

 to this work as to have but little thought about their own 

 sustenance, will evidently have no temptation to leave 

 the arena of their family labours, but will hover about 

 close to the pond till they have fulfilled their task, 

 and then, after but a brief stage of enjoyment in 

 the performance of parental duties, either die of old 

 age, or more probably fall victims to the voracious 

 appetites of predatory insects, or of insectivorous birds or 

 fish. Such as these will either be found executing merry 

 dances just above the surface, lurking amongst the rank 

 vegetation that fringes the banks, or resting on over- 

 hanging trees, from which positions they may easily be 

 dislodged by disturbing their place of retreat. But those 

 that are destined for a longer existence, and so have to 

 make vigorous onslaught upon nature on their own account, 

 as well as to give birth to new generations, will often take 

 foraging excursions into neighbouring woods and fields, 

 though the procreative instinct will with them, too, suggest 

 a speedy return to the water which is to be the nursery of 

 their expected brood. 



Coming under the former category are first the Ephe- 

 meridcc, or May-flies, whose larvse formed the subject of 

 part of our last paper. There is no mistaking a May-fly 

 (Fig. 1). A most fragile creature, with four membranous 

 and reticulated wings, of which the hinder pair are very 

 much smaller than the others, and sometimes, indeed, 

 absent altogether, with two or three long filaments at the 



tail generally ujuch longer than the insect itself, and with 

 the organs of the mouth in the most rudimentary con- 

 dition possible — such is the insect that, in consequence 

 of the sudden appearance and the as sudden disap- 

 pearance of its myriad swarms, and the extraordinary 

 brevity of its adult life, has attracted popular atten- 

 tion from the times of Greeks and Romans, to the 

 present day, and has, more often than almost any other 

 insect, been used to point a moral to human kind. The 

 name Ephemerid;e, a Greek word meaning living for a day, 

 indicates what is usually considered to be the extreme 

 duration of their perfect existence. In captivity they 

 have, it is true, been kept alive for upwards of a week ; 

 indeed Stephens reports having kept one for more than 

 three weeks ; but there is no doubt that, in a natural con- 

 dition, this longe^-ity would not have been attained, and in 

 the majority of cases the perfect existence seems to last 

 little more than a few hours. They have no power what- 

 ever of taking food, and are utterly defenceless, so that they 

 have no chance of holding their own against the numerous 

 foes that with hungry eyes are looking longingly upon 

 them. Their sole business is to become mated and lay 

 their eggs, and this exhausts all their energies. The 

 antenna; are extremely short, but, as if to counterbalance 

 this, the forelegs are sometimes extraordinarily long. 

 The wings have a number of nervures running longi- 

 tudinally at short intervals, and the spaces between 



Fig. 1. 



them are divided into tiny quadrilateral figures by num- 

 bers of short transverse nervures, so that the whole surface 

 of the wing is composed of minute enclosed areas which 

 are largest in the centre and smallest at the outer edge. 

 Several species of these insects are known to inhabit the 

 British Isles, but their preservation is a matter of con 

 siderable difliculty. Their texture is so fragile that such 

 parts of the body as are of most importance in the sepa- 

 ration of the sjjecies shrivel up to mere shapeless masses 

 after death, if they are simply allowed to dry as is usually 

 done with insects. A collection of them, therefore, is of 

 but little value to a systematist unless they are preserved 

 in spirit into which they have been plunged when fresh 

 and soft. 



They are remarkably regular in their times of appearance, 

 and immense swarms arrive at maturity at almost the same 

 instant, and few phenomena have excited more admiration 

 and wonder than the sudden appearance of myriads of these 

 insects where a few moments before not one was to be seen, 

 and where their presence would be hardly likely to be 

 expected ; for, owing to their mud-loving propensities, they 

 are seldom noticed during their long larva- and pupa-hood, 

 and thousands may be close at hand without anyone except 

 those who specially search for them being any the wiser. 

 To the naturalists of a century and a half ago, coming 

 fresh to the observation of nature at first hand, when, as 

 yet there was but the most scanty literature on the subject, 

 the sudden appearance of these gigantic swarms was matter 

 of such intense interest, excitement, and delight, as in these 

 more matter-of-fact days of ours, with the charm of novelty 

 destroyed by an abundant literature, can scarcely be credited, 



