122 



KNOWLEDGE 



[July 1, 1892. 



The Leaf-Butterilv 



(Afttr AVallacu.) * 



and without actually seeing the insect settle upon the spot 

 it is absolutely impossible to find it. To increase the 

 delusion, no two individuals of these insects are precisely 

 alike on the under surface ; while many of them have little 

 black patches, or dots, exactly resembling the dark fungous 

 growths so often found on decaying leaves. Fortunately 

 for the reader who desires to verify this extraordinary 

 instance of inanimate mimicry, a case is now exhibited in 

 the central hall of the Natural History Museum with 

 several of these insects attached to a bough with faded 

 leaves, and it is curious to watch the visitors to this case 

 and see how often they fail to distinguish all the butter- 

 flies from the leaves which they imitate. 



The nest best instance of inanimate mimicry among 

 Insects occurs in the so-called stick- and leaf-insects, which 

 are allied to our grass-hoppers and cockroaches. The 

 stick-msects, of which some are found in Southern 

 Europe, have long, slender bodies and limbs, of a dark 

 colour, and so exactly resemble dry sticks that it is 

 almost impossible to distinguish between the one and the 

 other. To increase the resemblance, these insects when 

 at rest have the habit of placing their legs uusymmetrically. 

 On the other hand, the leaf-insects, or "walking leaves," 

 of India, both in colour and form, so exactly simulate 

 green leaves that they may be passed dozens of times 

 without attracting attention. All the legs of these curious 

 creatures are furnished with irregular flat expansions 

 looking precisely like bitten leaves ; whUe the head and 

 fore part of the body forms a kind of stalk expanding 

 behind into a broad and flattened abdomen, covered by 



• We are indebted to Messrs. Macraillan & Co. for this figure. 



the horny wings which are veined and netted so as to 

 form an almost exact imitation of a leaf. 



We might cite many other instances of inanimate 

 mimicry among insects, but we must pass on to show that 

 this phenomenon is by no means confined to this group of , 

 animals. Perhaps we should scarcely expect to find this 

 kind of mimicry in such a comparatively highly organized 

 a creature as a fish ; yet there is a group of fishes, familiar 

 to those who have kept aquaria, under the name of sea- 

 horses, in which it is exhibited in its full perfection. The 

 ordinary sea-horse attaches itself to a sea-weed or some 

 other object by curling its tail tightly round it ; and all these 

 fishes have the habit of anchoring themselves by their tails 

 in some way or another. In all of them the hard, horny 

 body is furnished with a number of prominent ridges and 

 spines ; but in one, a peculiar group from the AustraUan seas, 

 these spines attain an enormous development, many of 

 them being prolonged into irregular filaments or streamers 

 of skin, which are especially developed throughout the 

 long and slender tail. As these streamers float in the 

 water they so exactly resemble both in colour and shape 

 the particular kind of sea-weed to which these fishes are 

 in the habit of attaching themselves, that the whole 

 creature seems but part and parcel of the fucus ; so that 

 when on the sea-bottom it must be impossible for any 

 carnivorous rover to distinguish between the animal and 

 the vegetable. 



One more instance of this kind of mimicry, and we must 

 close this portion of our subject. This example is taken 

 from the mammalian, or highest class of animals, and, 

 although not such a perfect imitation of form as those we 

 have already mentioned, is very remarkable as occurring 

 so high up in the animal kingdom. Most of our readers 

 are probably acquainted, at least by name, with those 

 lowly South American mammals known as sloths. These 

 animals are inhabitants of the great forest regions of that 

 continent, and are of a sombre greyish colour, very like 

 that of the gnarled and lichen-clad boughs, from beneath 

 which they are wont to hang back-downwards. Not only, 

 however, is their general colour like that of a Uchen- 

 covered branch, but their coarse grey hairs actually develop 

 a growth of lichens upon themselves to complete the 

 resemblance to their surroundings. It is, indeed, clear 

 that the long grey coat of the sloths has been produced 

 for the sole purpose of this protective mimicry, for when 

 this is removed there is found beneath an under coat of 

 softer fur marked by yellow and black stripes, which may 

 be pretty confidently regarded as the original coloration of 

 these animals. 



We have now to consider animate, or true mimicry, in 

 which one animal imitates the form, and generally the 

 habits, of another in order to participate in the immunity 

 from foes enjoyed by the latter, owing either to the possession 

 of some formidable weapon, or to its unpalatable nature as 

 food. In all cases of this kind of mimicry it is essential 

 that the mimicked animal should be numerically far more 

 abundant than the mimicker, as otherwise predatory 

 creatures would soon learn that the inocuous and palatable 

 animal was more likely to be captured than the harmful 

 one. Undoubted cases of true mimicry are most common 

 among insects, and it is to these alone that our 

 observations will be confined. We may also observe 

 that mimicking insects, as a rule, mimic other insects, 

 although it has been considered that some large cater- 

 pillars mimic snakes, and certain moths certainly imitate 

 birds. 



We will first refer to some excellent and well-marked in- 

 stances of mimicry which occur among the insects of our own 

 country. Most of us are probably familiar with those large 



