166 



KNOWLEDGE 



[September 1, 189S. 



out the crumpled leaves, a number of little orange maggots 

 may be found, which are the larvae of one of our 

 commonest gall-gnats. Here the gall consists of little 

 more than a tuft of distorted leaves, but this must not be 

 taken as a universal type ; there is every variety of form 

 as well as iwsitiou, and the plants attacked are as varied 

 as the galls. Thus we have swellings on the midribs of 

 ash leaves and the stalks of aspen leaves ; warts, pads, 

 and other deformities on the margins of willow leaves ; 

 tufts and rosettes of leaves on willow and yew, and on the 

 pretty little blue speedwell and yellow St. John's wort ; 

 bud-hke bodies on broom and furze ; distorted tiow-ers on 

 cabbage and other cressworts and on the yarrows ; blisters 

 on bedstraw, water-cress and box ; woody swellings on 

 ■willow twigs and bramble stems ; spongy galls on barberry ; 

 woolly galls on weasel-snout; hairy galls on nettles; horns 

 on beech leaves and cylindrical tubes on ground ivy, 

 together with many others too numerous to mention. 



As in other cases, so also with the gall-gnats, the most 

 remarkable facts concerning them are connected with their 

 reproduction. We have already seen that amongst the 

 true gall-flies parthenogenesis is an extremely common 

 phenomenon, so that the continuance of the race is doubly 

 secured — first, by the fact that the plant responds to the 

 touch of the insect, as if it had been that of a fairy's wand, 

 and forthwith there upsprings with mushroom-like rapidity 

 a structure combining board and lodging ; and secondly, by 

 the fact that there is no need to wait for the meetmg of a 

 suitable pair of individuals before fertile eggs can be 

 produced, but that the female, often the only representa- 

 tive of the species present, can proceed at once to lay the 

 foundation of the next generation. And with the gall- 

 making saw-dies the same is true. Mr. Cameron says that 

 he has often bred the Hy of the willow-gall above described, 

 but has never met with a male. Mr. F. Smith, however, 

 was more fortunate ; but still in his case there was only a 

 single male to several hundreds of females. But when we 

 come to the gall-gnats we find examples of a still more 

 remarkable method of ensuring the perpetuation and multi- 

 plication of the race. The reproductive power is carried 

 two stages back in the history of the insect, and we find 

 not merely the perfect insect, but even the larra gifted 

 with the power of producing young, for which reason the 

 process is called "pedogenesis. ' Fig. 10 shows the larva 



Fig-. 10. — Larva of Cecidomyid containing five voinig lar\iu. 

 (After Pagenstecher.) Mueli maguifieil. 



of a cecidomyid which contains within itself five other 

 larvae. These are produced where they are seen, and they 

 subsist upon the tissues of the parent larva, ultimately 

 absorbing all the contents of the skin, within which they 

 lie free. Rupturing the skin, they escape and proceed with 

 their development. Sometimes several generations of 

 larvic are produced in this way before we revert again to 

 the full-grown Hies. Of course this process involves the 

 destruction of the parent larva, which has no more chance 

 of completing its own development than if it had harboured 

 a set of ichneumon maggots. But notwithstanding this, 

 there is a net gain in the number of individuals ultimately 

 existing, so that the species is in the long run benefited, 

 though by the sacrifice of some of its members. 



Our last instance of dipterous galls is from the brilliant 

 and banded-winged TrypHidce. The best known of this 

 group is, no doubt, the common thistle gall. The stems of 



Its name 



Pio. 11. — "Wing of 

 Urophora carthti^ Gall- 

 luaking fly of thistle. 

 Magnified seven dia- 

 meters. 



one of our commonest thistles {Cnicus arvetisis) are often 

 seen to be swollen near their tips into a large, green, oval, 

 wen-like body. This is the gall of the very handsome 

 dark-bodied fly whose wing is shown in Fig. 11. 

 is Uroplioni cardtii. The gall is 

 sometimes partly concealed by the 

 leaves of the thistle, the growth 

 of which it does not interfere with. 

 On cutting open the gall we find a 

 solid mass of tissue, in which are 

 excavated a number of pear-shaped 

 chambers, each containing a single 

 white maggot. There may be as 

 many as eight of these cells in 

 a single gall, or there may be no more than one ; usually, 

 however, there are several. The fly is very common, and 

 is pretty sure to be found in plenty wherever the thistle 

 grows. Another species belonging to the same genus 

 attacks the flower-heads of the common " knapweed " or 

 " hard-head," an extremely abundant weed with thistle- 

 shaped but not prickly flowers. In this case, however, the 

 gall is invisible till the flower-head is pulled to pieces, as 

 it occupies the position of the ovaries of the florets, and is 

 therefore concealed by the scaly cup, or involucre, which 

 surrounds them. But though invisible they can be felt 

 from the outside. If the dead flower-head be squeezed 

 between finger and thumb, it will yield to the pressure and 

 more or less flatten out when no gall is present ; but 

 when the gall is there, it will be felt as a hard lump which 

 will prevent collapse. 



{To be cuntinued.) 



THE LIGHT-CHANGES OF Y CYGNI. 



By Miss A. M. Clerke, Authoress of '' The System <// the 



Stars" and "A Popular History of Astronomy duriny the 



Nineteenth Century," dc, d-c. 



ACQUAINTANCE has so far been made with nine 

 variable stars of the Algol type. They are essen- 

 tially characterized by normal stedfastness of 

 light, interrupted at intervals of a few days or 

 hours by brief phases of darkening. These 

 sudden dips to a lower magnitude represent stellar eclipses ; 

 for the certainty on the point, attained spectroscopically in 

 the case of Algol, may safely be extended to its sister stars. 

 They are all, then, very rapid binaries, the orbits of which 

 lie edgewise towards the earth. Plainly, however, this 

 latter condition is extrinsic to the systems themselves ; 

 our position with regard to them determines its realization. 

 If it remained unfulfilled, the loss, so to speak, would be 

 ours. The mutual relationships of the stars would be just 

 the same ; only we should remain ignorant of them unless 

 the spectroscope came to our assistance. This it could 

 only do if the combined brightness of the revolving bodies 

 sufficed to bring them well within range of our present 

 instruments, and if, in addition, a large proportion of their 

 orbital velocities were directed, at quadratures, in the line 

 of sight. 



Two such spectroscopic binaries are particularly well 

 kno^n, and each exemplifies a variety of the class. Thus, 

 Spica (a Virginis) is composed of a brilliant sun, and a 

 relatively obscure, though powerfully attractive body ; 

 /3 Aurigie of two nearly equal suns. We should accordingly 

 expect to find the same varieties represented among 

 eclipsing binaries. And the companion of Algol seems, 

 in fact, to be completely dark ; for if it gave any sensible 

 light, a minor obscuration sliould be recorded without fail 

 just half-way from each principal eclipse to the next. 



