86 



KNOWLEDGE 



[May 1, 1898. 



"baceous plants sucb as buttercups, marked with curious 

 zigzag lines of most fantastic shapes, very fine at the com- 

 mencement and increasing in breadth till they abruptly 

 terminate in a club-shaped end (Fig. 4) in the centre of 

 which perhaps a little dark spot and swelling may be seen 

 marking the position of the caterpillar itself. Sometimes, 

 instead of a zigzag line, there is merely a discoloured 

 irregular blotch ; sometimes, again, the line follows the out- 

 line of the leaf, coasting, as it were, along the various 

 indentations on its margin. But whatever be the precise 

 character of the markings, they are always evidence of 

 the presence, either at the time or shortly before, of a 

 leaf-mining insect of some sort or other. Of course it 

 is only very minute species that can subsist in this way ; 

 but almost all the smallest of the Tinese, amounting 

 to hundreds of species in Britain alone, are habitually 

 leaf-miners. It must not, however, be supposed that 

 all such mines in leaves are caused by the caterpillars 

 of Lepidoptera ; other kinds of insects have similar 

 habits, especially some of the smaller flies, a well- 

 known mine in primrose leaves, for example, being caused 

 in this way. 



Each mine is the register of the entire larval history of 

 one single insect ; its intricacies and windings represent 

 the episodes of the tiny career, and its extent gives an 

 accurate measure of the total amount of food consumed by 

 the insect from its birth till its pupahood, and in fact, one 

 might probably say, till its death, for as a chrysalis it 

 takes no food, and, as a rule, no doubt the same is true of 

 its brief adult life. In each of these mines, then, we have 

 a record, indelible as long as the leaf on which it is traced 

 endures, of the exact amount of work done in the way of 

 the destruction of vegetable produce by a single insect during 

 the whole of its career ; and yet, so minute are they, that 

 sometimes half a score of them will be found on a single 

 leaf of no more than moderate dimensions, the great mass 

 of it even then remaining untouched, so that it only seems 

 as if the leaf were adorned here and there with an elegant 

 tracery of filigree work. The little moths whose larvae ai-e 

 the excavators of these mines are oftentimes some of the 

 gems of the insect world, and were they only larger, would 

 attract universal attention by their splendour. Snowy 

 white or jet black wings, with bars, streaks, or spots of 

 burnished gold or silver, purple or orange, adorn many of 

 them, but they are often too minute for their beauty to be 

 fully recognized and appreciated without the help of a 

 lens. 



The second method of home construction is that of fold- 

 ing the edges of a leaf upon one another so as to make an 

 enclosure, or else rolling it up into a coil. There is a large 

 group of small moths that, from the frequency of this habit 

 amongst them are called Tortrices, i.e., " twisters " ; they 

 are also known as " leaf-rollers." There is a general 

 similarity of style and shape about them that makes it 

 usually an easy matter to distinguish them from members 

 of other groups. When resting with wings closed, they 

 generally appear as distinctly triangular little moths, with 

 almost straight edges to the fore-wings after a great bend 



at the shoulder ; but some- 

 times the outline is more like 

 that of a bell (Fig. 5). Many 

 of them are excessively 

 abundant, not only in woods 

 and hedges, but also in 

 gardens. One of the common- 

 _ ^ ,. , , est and best known is the 



n r^l r 1 "n " lovely little emerald green 

 (Tortrice.1), in position moth {Tortn.r niulana), -which 

 of rc5t. is a very familiar object in 



woods in June. It infests a number of trees, but 

 especially the oak and hornbeam, and from its associa- 

 tion with the former has sprung its popular name of 

 "green oak moth"' (Fig. 6). The caterpillars appear 

 in May, while the oak leaves are young and tender. 

 The operation of leaf-rolling is a truly remarkable process, 

 when we consider the size and help- 

 lessness of the insect. Taking advan- 

 tage of some natural curvature in the 

 leaf, the young caterpillar plants itself 

 at the spot, and bending its head from 

 side to side, runs a few short threads 

 from its spinneret tightly across from 

 the extreme point that can be reached 

 on the one side to a corresponding 

 distance on the other — then placing 

 itself at the centre of the little band 

 so constructed, and thereby slightly 

 pressing it down and therefore bending 

 the leaf still more, it runs another 

 set of threads in a slightly different 

 direction, but crossing the others at 

 the centre. So it proceeds till it has 

 made a strong band, which fixes the 

 edge of the leaf in the somewhat 

 curved position into which it has been 

 bent. By similar operations at inter- 

 vals along the same line, a long roll 

 is produced across the leaf ; other 

 little bands of silk, added to the outer 

 surface of this in the same way, cause 

 it to be rolled over still further, thus 

 commencing a second turn of the coil, 

 and so on till ultimately as much as 

 half the leaf may become rolled up, 

 rolling is not always the same, even with the same species ; 

 sometimes it is longitudinal to the leaf (Fig. 7), sometimes 

 transverse, or again diagonally placed. It seems not im- 

 probable that the bands of thread, after 

 they are run across, contract somewhat 

 in length so as to increase the curvature 

 of the leaf, by pulling the edges together ; 

 otherwise it is difficult to understand 

 whence sufficient power could be obtained 

 by so small and weak a caterpillar, work- 

 ing with a gum which is in a semi-fluid 

 condition as it issues from the spinneret, 

 to overcome the rigidity of so stiff a leaf 

 as that of the oak tree, and coil it up as 

 deftly as human fingers could do it. It 

 is said that if the caterpillar, in the 

 course of coiling the leaf, comes across 

 a vein which is too tough and stubborn 

 to bend as required, it weakens its 

 resistance by biting it partly through, 

 thus manifesting a marvellous degree of 

 intelligence. 



The coil so made is open at each end, 

 and the caterpillar then places itself 

 inside and remains there unless dis- 

 lodged, feeding upon those surfaces of 

 the leaf which line the coil, carefully 

 avoiding, however, causing any damage 

 to the last turn of the coil, which would 

 of course bring about the opening up 

 and destruction of its snug little domicile. 

 When it has devoured so much of its 

 walls as is available and safe, it gives 

 another turn or two to the coil, thus bringing what 



Fio. 6. — Caterpillar of 

 Green Oak Motli, 

 hanging from coiled 

 oak leaf. 



The direction of 



Fro. 7.— Lilae leaf 

 rolled by Cater- 

 jiillar. a. Silk 

 fastenings. 



