November 1, 1891.] 



KNOWLEDGE 



219 



Fio. 4. — Pupa of 

 Daddy-longlegs, with 

 upper pai't projecting 

 above ground. 



projecting half-way out of the soil (Fig. 4). These empty 

 cases may often be seen at the right season, still stand- 

 ing upright in the holes, as silent witnesses to past 

 resurrections. The newly-extricated fly is at first soft, 

 but soon hardens by exposure to the air, and proceeds 

 to its appointed task — the perpetuation of its kind. 



The breathing arrangements of the 

 pupa are as peculiar as those of the 

 larva. We have already mentioned 

 that the spiracles of the larva are 

 reduced to a single pair, which are to 

 be found on the last segment of the 

 body. Similarly, the pupa has but two, 

 but these are at the opposite extremity 

 — a very desirable arrangement, as this 

 is the part that is uppermost, and 

 therefore nearest the air, when the 

 insect works its way to the surface. 

 They form two horn-like prominences, 

 which project considerably from the 

 head. Thus, in the daddy-longlegs, 

 the same restriction and reversal of 

 respiratory structure takes place as 

 in the aquatic larva and pupa of the common gnat. 



As these insects are so destructive, an important question 

 arises as to the best means of reducing their numbers and 

 checking their ravages. As damp soils, with plenty of loose 

 straggling vegetation, especially grasses, are particularly 

 favourable to their multiplication, it is evident that drainage 

 and the clearance of weeds are two of the most important 

 means of prevention. Untidy, shady corners will often 

 harbour scores of specimens, while on open and cleared 

 spaces in the immediate vicinity they may be sought for in 

 vain. Tbe larger insectivorous birds are certainly of great 

 service in clearing the soil of the " grub "; indeed, accord- 

 ing to the belief of Mr. Verrall, the English dipterologist, 

 rooks are by far the best remedy. But perhaps we 

 cannot do better than refer those who are practically 

 interested in the matter to Miss E. A. Ormerod's " Manual 

 of Injurious Insects," where they will find variety enough 

 to make a choice rather embarrassing, since most of 

 the remedies suggested have been found useful on 

 occasion. 



But we must caution our readers against putting too 

 much faith in specific remedies, or expecting too much of 

 them, since it is extremely difficult, if not impossible, to 

 obtain an " infallible cure " for insect depredations of any 

 kind. The circumstances of each different visitation, even 

 of the same insect, often vary considerably, and what 

 would be efUcacious in one instance might altogether fail 

 in another. Again, just as the satisfactory working of a 

 small model of machinery is by no means always a 

 guarantee that the same thing would work well on a larger 

 scale, so insecticides that may be perfectly efficacious and 

 very rapid in action when administered direct in the labora- 

 tory to an individual specimen, may fail altogether when 

 the attempt is made to apply them promiscuously, and on 

 such a scale as would be necessary in a large field, to say 

 nothing of the great expense often involved in attacking 

 such large areas. Of this principle, indeed. Miss Ormerod 

 gives some remarkable illustrations in the book above 

 referred to, appending to them the following pertinent 

 remarks : " The above experiments are of much value by 

 showing how little these remedies can be depended on, 

 some of which have often been tried, and time, valuable 

 for checking the attack at the beginning, thereby lost. It 

 will be observed that the application that caused the most 

 rapid destruction of life experimentally, failed to have any 

 decided efi'ect on the grubs in the ground, even when 



applied at a strength which, without the greatest care in 

 using, would be destructive to the crop." 



Probably natural agencies are, after all, the best to depend 

 upon ; but even in this category there are several that 

 might have been thought available, but yet fail through 

 the hardiness of the grub. For example, frost, which kills 

 many insects, has little effect upon the hardy " leather- 

 jacket." They may be frozen hard and stiff, and yet 

 recover on being thawed, and proceed about their business 

 as usual. In some experiments carried out for Miss 

 Ormerod at Kew, specimens were exposed to an artificial 

 cold of - 10- F. , or forty-two degrees below freezing point, and 

 some were found to resist even this extremely low tempera- 

 ture, though most of them died. Therefore, evidently the 

 frost of an ordinary winter cannot be expected to make 

 much difference to their numbers. Again, it was found 

 that grubs which had remained immersed in water for 

 fifty-eight hours, and looked quite lifeless, recovered on 

 being restored to the air, though an immersion of twice as 

 long proved fatal. Hence floods, to be effectual in killing 

 the grubs, must be of long continuance, and there must 

 be no means of escape to drier quarters. Drought appears, 

 however, to be much more reliable as a destroying agency. 



THE CANALS OF MARS. 



By E. Walter Maunder, Umi. Sir. 11. A. f!., Siipcrinlfmlrnt 

 (if the P/ii/.iical Ih'partincnt, Hiiij<il (Him'rrnldiij, (rnrniric/i. 



SEVENTEEN years ago our knowledge of Mars 

 appeared to be in a very satisfactory state. The 

 principal markings had been often and long 

 observed, and had been found to be perminent. 

 The Kaiser Sea, the Oculus, the Marakli Sea, had 

 been observed by Hooke and Cassiui, Hersohel and 

 Schroeter, Beer and Miidler, Dawes, Loekyer, Knobel 

 and Green. The inference, from the annual waxing and 

 waning of the white polar caps, that cloud, snow, and rain 

 were features of the meteorology of the planet, had been 

 confirmed by the testimony of the spectroscope, and that 

 the white caps themselves were composed of ice or snow 

 was a natural conclusion. It followed necessarily that 

 there must be water on the surface of the planet, and the 

 dark spots were considered as seas, leaving the brighter 

 districts to be regarded as land. Mars was, in short, a 

 smaller copy of our own world. " The analogy between 

 Mars and the earth " was pronounced to be " by far the 

 greatest in the whole solar system." 



Schiaparelli's discovery of the " canals " — to use a term 

 which, however misleading it has been, has now been too 

 strongly sanctioned by custom to be easily chan,u:ed — was 

 the beginning of a new epoch in Martian observation, and 

 its chief and most patent result has been to disturb our 

 old conceptions of the analogy between Mars and tbe 

 earth, and consequently to unsettle our notions of the 

 physical condition of the planet. The reaction has gone 

 so far that Prof. Schaeberle has reversed the old identifi- 

 cation of the dark spots as seas, and claimed that the 

 bright districts are to be thus regarded. The grounds for 

 this change of view have not been generally accepted, and 

 more is to be said for an objection raised by Prof. W. H. 

 Pickering, that just as snow is not the only substance to 

 give a bright white reflection — hoar-frost or cloud would 

 serve as well — so water is not the only surface which 

 would appear dark ; forests and prairies would appear as 

 at least sombre districts. 



The " canals," however, have been the great element of 

 disturbance. The report of their discovery was received 

 in 1877 with very considerable mistrust. But time has 



