Mat 1, 1893.] 



KNOWLEDGE. 



86 



collections of stars bear somewhat about the same pro- 

 portion one to the other, it is all but certain that the 

 members of those two collections travel through space with 

 approximately equal average velocities. This test has been 

 applied by Prof. Kapteyn to several groups of Sirian and 

 solar stars, variously selected and discussed, with the result 

 of eliciting no systematic difference between their paral- 

 lactic and their real displacements. That is to say, the 

 small differences that emerged were alternately in opposite 

 senses. Thus, he derived a mean angular value of 0-196" 

 for the sun's yearly translation (q) as viewed at right angles 

 from 94 first-type stars common to Stumpe's list of proper 

 motions and the Drapi-r Cataloijue ; while 325 similarly 

 selected second-type stars gave, forthe same amount of mean 

 total proper motion (/u), a valueof (y=0-16G". Taken singly, 

 and interpreted strictly, this result would of course imply 

 that the solar stars were, on the whole, more distant, con- 

 sequently swifter-moving bodies, in the proportion of 196 to 

 166, than the Sirian stars compared with them. And a 

 similar investigation of 189 Auwers-Bradley stars, with 

 proper motions between 0-08" and 009', brought out a 

 difference of the same character. But a more extensive and 

 more searching inquiry suggested an opposite inference. 

 Here the more stringent method was adopted of comparing 

 the perspective element of proper motion, not with the total 

 movement, but with the exclusively original element 

 directed across the line drawn from each star to the sun's 

 apex ; and the stars considered were 230 solar and 338 

 Sirian, contained both in Auwers's and in Pickering's cata- 

 logues. The upshot was to make the Sirian stars this 

 time appear further off and quicker, in the ratio of 37 to 

 81. It seems, then, difficult to contest, in the present state 

 of knowledge. Prof. Kapteyn's conclusion that stellar rates 

 of travel are independent of spectral distinctions. 



CATERPILLARS' DWELLINGS.-IL 



By E. A. Butler. 

 (Continued from page 63.) 



THE solitary caterpillar, as might be anticipated 

 from the nature of the case, greatly excels the 

 gregarious one in ingenuity. The two great 

 problems of its life are, as usual, the safety of its 

 person amidst the dangers with which the outer 

 world teems, and the assurance of a sufficiency of daily 

 food. Having, in order to secure these ends, to rely 

 entirely upon its own individual exertions, unaided either 

 by parent or companion, and being moreover, so far as 

 British species are concerned, a small and naturally 

 defenceless creature, it endeavours to combine the solution 

 of the two problems by a single device, either constructing 

 its abode in the immediate neighbourhood of its food, or 

 in a truly marvellous way making what is in reality an 

 edible dwelling. In either case it becomes a little hermit, 

 cut off from the world at large, and merely, like Prospero, 

 "master of a full poor cell." In carrying out these 

 designs, one or other of three methods is usually adopted, 

 and these we may roughly distinguish as the method of 

 the " tunnel," the method of the " enclosure " or " coil," 

 and the method of the " portable case." Not that these 

 constitute alternative plans, either of which may be adopted 

 by each species as inclination directs or circumstances 

 demand. On the contrary, each species has its own 

 peculiar method of procedure, which it invariably adopts, 

 thus exhibiting that uniformity of inherited habit to which 

 the name instinct is applied. 



The tunnel makers construct a silken tube or rim in the 

 midst of their food, within which the proprietor can rest 

 secure from attack, and be at the same time within easy 

 reach of supplies. Amongst the heads of umbelliferous 

 flowers, we sometimes see silken tunnels of this kind, open 

 at both ends, and running between the stalks of the 

 different components of the umbel, other threads acting as 

 cables for mooring the structure, and drawing the flower 

 head together. Several of the smaller species of moths, 

 especially amongst the Tinea?, adopt some such plan as 

 this. Considering the immense number of insects of all 

 orders that visit the flowers of the Umbelliferne, one can 

 see that these web-and-timnel-forming caterpillars are 

 much freer from disturbance than they would be if they 

 fed in the open. There may be scores of little creatures of 

 one kind and another crawling about overhead, ransacking 

 the expanded head of flowers, but the caterpillar in its 

 silken home, though close beneath them, is safe from 

 molestation, enjoying a sort of privacy in public. Similarly, 

 the caterpillars of the meal and tabby moths form silken 

 galleries amongst the flour and chaff on which they feed ; 

 and so also do some of the clothes moths, the silk in this 

 case being largely mixed with particles of the woollen 

 fabric which they are doing their best to ruin. 



Other excellent illustrations of gallery-making are to be 

 found in the honey-comb moths, the chief of which 

 {GiiUeria rerella) is SO different in the male and female that 

 Linne made two species out of it. The moths enter bee- 

 hives in the evening, when the bees are resting, and lay 

 their eggs on the combs. The caterpillar is a pale whitish 

 creature, which feeds on the wax of which the combs are 

 made, and as such depredations would of course be 

 resented by the bees, some thoroughly reliable sting-proof 

 shelter becomes necessary if the marauders are to carry out 

 their enterprises successfully. Hence there is, in this case 

 again, a very good reason for the construction of the silken 

 tube, and the caterpillar, apparently conscious of the risk 

 it would run if exposed, takes good care to spin its tube 

 as far as it travels, and never to thrust out more than just 

 the end of its body, that part being defended by a hard, 

 thick skin, while the rest is soft and unprotected. 



In this connection we may briefly notice those cater- 

 pillars that excavate minute 

 tunnels or mines between the 

 upper and under skins of 

 leaves. Here the timnel consists 

 simply of the narrow, linear, 

 zigzag space left by the insect 

 as it devours the soft cellular 

 parts of the leaf that lie between 

 the two surfaces. The green- 

 ness of the leaf is not due to 

 any colour in its outer sldn, 

 which is almost colourless and 

 transparent, as may be proved 

 by tearing a piece of it off. 

 The green part is contained 

 solely in the cellular tissue that 

 constitutes the central layers 

 of the leaf, and therefore when 

 these are removed, a clear, 

 transparent, or whitish track is 

 left, which, however, usually 

 shows a dark streak down its 

 centre, consisting of the Une 

 of excrement left by the eater- 

 pillar as it progresses along its 

 life-long course. Thus we 

 often see the leaves of different trees, shrubs, and her- 



FlQ. 4. — Portion of Nut leaf, 

 showing (aj mine of a small 

 Caterpillar (Nej^ticula micro- 

 iheriella). 



