228 



KNOWLEDGE. 



[December 1, 1898. 



same, and will usually serve to distinguish a Braconid 

 from an Ichneumonid pretty readily. 



Both of these groups of parasites make cocoons, many 

 of which closely resemble those of the Lepidoptera, except 

 that they are, most of them, smaller. But as the larvsB are 

 naked maggots instead of hairy caterpillars, of course the 

 cocoons are always composed purely of silk, and contain no 

 hairs. As might be expected from the shape of the perfect 

 insects, the cocoons are of a long, narrow form, equally 

 rounded at each end ; they are of all shades of colour, 

 from the purest white to the deepest black. A large 

 reddish-yellow Ichneumonid, Paniscus testacfus by name, 

 which is very destructive to the puss moth, makes a thick, 

 rough, dull blackish-brown cocoon. Except by those who 

 rear the hosts, these cocoons are not likely to be seen, as 

 they are to be found only inside the cocoon of the moth ; 

 for the parasite does not reach maturity till after its host 

 has immured itself. Another ichneumon, strikingly like 

 the last, and easily mistaken for it, though belonging to a 

 different genus {Opliion), makes a totally different cocoon ; 

 it is much like those of the egger moths, only narrower — 

 oval, yellowish-brown, and covered with a network of 

 black threads. A species of Braconid {/wle testaceator), 

 exceedingly like this again, though much less common, 

 provides for itself a most elegant envelope of a satiny white 

 appearance. Thus we have three species which superficially 

 bear a close resemblance to one another, but which con- 

 struct totally different cocoons. Some of the smaller kinds 

 make very pretty cocoons ; a little Ichneumonid parasitic 

 on the weevil (Ih/peni rariiibilis), above described, makes a 

 minute reddish-brown one with a neat white band round 

 the middle (Fig. 9), and many others are similarly adorned. 

 Many of these cocoons may be found 

 amongst the herbage in woods, hedges, 

 &c., though their small size of course 

 necessitates a careful search ; after the 

 disclosure of the flies, they appear to last 

 for some months, and may be found 

 lying about amongst dead leaves and 

 other rubbish. 



Amongst Braconids, there is a large 

 number of small species which are great 

 scourges to the caterpillars of moths and 

 butterflies; they live gregariously in their 

 hosts, and when full fed they eat their 

 way out, and construct their neat little 

 white, buff', or yellow cocoons in clusters 

 round and over the shrivelled remains of 

 their victim. The caterpillar of the common white butterfly 

 (Pieris rapw) may often be seen on walls, treated in this 

 way, its crumpled and distorted carcase being almost 

 hidden under a cluster of little oval silken spindles. One 

 curious genus makes glossy brown cocoons, from one end 

 of which a loug, stout, and much-twisted anchoring thread 

 proceeds, to the extent of some three or four times the 

 length of the cocoon (Fig. 9) ; the method of its formation 

 is not easy to understand. Some gregarious Braconids 

 unite their cocoons together, deeply imbedding them m a 

 common mass of silk which entirely conceals their outlines, 

 and makes the whole look more like a single cocoon of a 

 moderate-sized moth than a cluster of small ones. The 

 possibility of such an association, of course, depends upon 

 their all reaching the pupating stage at the same time ; 

 but there is no difficulty about this, for the eggs are all 

 deposited in the host's body on the same occasion, and as 

 the maggots are always completely surrounded by their 

 food, the juices and tissues of their host, they all have 

 equal chances of development, and therefore reach 

 maturity together. The flies also emerge in company. 



Flo. 9. — Uaiuloil 

 and tailed Co- 

 coonsof Iclmeu- 

 nion Flics. 



Sfttnct Notes. 



Plate II. in the October number of Knowledge was 

 made from a photograph of the photosphere taken by 

 Dr. .J. .Jansseu on the 10th of June, 1890. The sun's 

 disc would have a diameter of 90 centimetres, or rather 

 more than 35 inches on the scale of the plate. 



De Morgan, in an interesting article in the Philosojihical 

 Magazine for April, 18-18, on Thos. Wright " of Durham," 

 attempts to show that Wright, whose theories of the Milky 

 Way and Saturn's ring have recently attracted fresh 

 attention, passed the greater part of his life in London, 

 and not in the north of England. Amongst other proofs 

 that Wright lived in London, De Morgan points out that 

 he illustrated the dimensions of the solar system by 

 comparisons vrith distances in the metropoUs. Mr. R. A. 

 Gregory has recently made a much more extended com- 

 parison of the same kind. He says : "If the dome of St. 

 Paul's Cathedral is taken to represent the sun, Mercury is 

 proportionately represented by a skittle-ball at Chancery 

 Lane ; Venus, by a globe rather less than one foot in 

 diameter revolving at the distance of Charing Cross from 

 the Cathedral dome ; the earth, by a one-foot globe 

 situated at Buckingham Palace ; and the moon, a cricket 

 ball circling round her at a distance of thirty feet. The 

 radius of the orbit of Mars is represented by the distance 

 from St. Paul's t) South Kensington Museum, the planet 

 itself being about the size of a small football. The minor 

 planets are exemplified by small shot revolving at the 

 distance of Hammersmith ; Jupiter, by a globe eleven feet 

 in diameter just beyond Richmond ; Saturn, a nine-foot 

 globe at Staines ; Uranus, a four-foot globe at Reading ; 

 and Neptune, by a globe four and a half feet in diameter 

 at Oxford. On this scale the nearest star has a distance 

 of five hundred thousand miles. 



Prof. Michelson's " Interference Refractometer," de- 

 scribed in a recent number of Astronomy und Astro- Physics, 

 is likely to prove of great use to spectrosoopists. Among 

 some twenty lines in the solar spectrum examined by it 

 (though all appear to be simple lines when examined by 

 the eye), the great majority proved to be highly complex. 

 Thus, the red hydrogen line is a double, whose components 

 have the intensity ratio seven to ten, and whose distance 

 is about one-fiftieth of the interval between the sodium 

 lines. Each component of the yellow sodium lines is a 

 double, whose components have also the intensity ratio seven 

 to ten, and whose distance apart is about one-hundredth 

 of the distance between the principal components. 

 Thallium gives a double line, whose components are in the 

 ratio one to two, at a distance of about one-fiftieth of that 

 of the sodium lines ; while each component has a small 

 companion, whose intensity is about one-fifth of that of 

 the principal lines, at a distance of about one three- 

 hundredth of that of the sodium lines. The green 

 mercury line is made up of a group of five or six lines, 

 the strongest of which is itself double (or perhaps triple), 

 the distance of the components being less than one five- 

 hundredth part of that between the sodium lines. These 

 distances, small as they are, says Prof. Michelson, can be 

 measured within about one-twentieth part, so that by this 

 means it is possible to detect a change of wave-length 

 corresponding to the ten-thousandth part of that between 

 the two sodium lines. 



