228 



KNOWLEDGE. 



[Decembek 1, 1892. 



anrl diguified a figure. Mr. Poulton indeed would reduce it 

 to a mere question of mechanics. He points out that the 

 attitude is only occasionally seen in the first two stages of 

 the caterpillar's life, and that after that period it becomes 

 much commoner, being most marked in the fourth stage, 

 (.(■., the last but one. At this time the caterpillar usually 

 rests upon the mid-rib of a leaf on its under surface, and 

 later on, on upright stems, with its head towards the top of 

 the stem. In either case, as soon as the front part of the 

 body is released from contact with its support, it would 

 naturally, by reason of gravitation, tend to fall backward, 

 and the curved position would therefore result as a com- 

 promise between this tendency and the strain put upon its 

 muscles in the attempt to neutralize the effect of gravity by 

 becoming stiff and rigid. But though this expilanation 

 would no doubt account for much, it would not furnish any 

 reason for the lifting of the body in the first instance, nor 

 explain why the insect is not satisfied with doing as so 

 many other caterpillars do, and indeed as it did itself when 

 younger, namely, resting stretched out at full length on its 

 support, and holding on by all sixteen legs. Amongst 

 British insects we find some sixteen species of hawk moths, 

 but they do not all assume the sphinx-like attitude, which 

 is not marked in those that live in trees, while most of 

 those that feed on low plants do not exhibit it. 



When a sixteen-legged caterpillar, resting comfortably 

 at full length, wants to begin to walk, it first advances the 

 last pair of claspers, raising them and setting them down 

 again nearer the last pair but one. This can easily be 

 done, for it will be remembered that the two preceding 

 segments have no legs, so that there is always a larger 

 space between the last two pairs than between the others. 

 Then, while it holds tight with this last pair, the others 

 are successively raised from behind forwards, and planted 

 firmly in the same order a step in advance, the legs of the 

 opposite sides being, of course, moved together, and not 

 alternately, as the perfect insect would do ; at the same 

 time a sort of wave of muscular movement travels along 

 the body. The wave at last reaches the thoracic segments 

 and the front part is thus moved on. The operation is 

 repeated again and again with considerable rapidity, and 

 thus the caterpillar manages to get over a good deal of 

 ground in a short time, the actual pace depending, of 

 course, to some extent upon the species. 



Some caterpillars, such as those of the rare but beautiful 

 Kentish glory (Endromis versicolor), and the moths called 

 " prominents " {Xotodontiil(r), do not make much use of the 

 last pair of claspers, and habitually rest with them lifted 

 up and not adhering to any support. In other cases, they 

 are not only unused but actually absent, and in their place 

 we find a pair of stiff appendages forming a kind of tail, 

 which is of no use for prehensile purposes. This is notably 

 the case in the larva of the puss moth (Dicnviura rinida). 

 This extraordinary insect, which is scarcely less remark- I 

 able in form than the "lobster" itself, is a common species i 

 and may be found in the summer feeding on the leaves of 

 willow and poplar trees. It throws the front part of its body 

 into a somewhat sphinx-like attitude, and at the same time 

 elevates the last three segments, which taper towards the , 

 tail and terminate in a pair of stiff tubular sheaths, prickly 

 on the outside. From within each of these there can be shot 

 forth rapidly a long, flexible, whip-like organ (Fig. G) of a 

 beautiful pink colour, cylindrical and hollow, but closed at 

 the extremity. It is moved out and in much in the same 

 way as the tentacles of a snail, that is, b}' a process of 

 turning inside out and the reverse. The mechanism by 

 which this is effected is twofold ; it is extended by having 

 blood forced into it, and as long as it remains full it 

 retains its form and length, being at the same time capable 



Fig. 6.— Whip and 

 sheath from tail 

 of Caterpillar of 

 Puss Moth, luag- 

 nified 4 diameters 

 (after Poulton). 



of being flourished about lilie the lash of a whip ; it is 

 drawn back again by means of muscles 

 which run up its tubular cavity, and 

 are attached at its apex. The tip is 

 first drawn in, and then, following it, 

 more and more of the column, till the 

 whole is withdrawn into the sheath. 

 The action can be easily understood if 

 one imagines the finger of a glove to be 

 fully extended, and a thread to be passed 

 up it inside and fastened to the top ; 

 on pulling gently at the thread, the tip 

 descends into the hollow of the finger, 

 and thus the whole finger, if the sides 

 remain sufficiently firm, is gradually 

 drawai back in a reversed position into 

 itself and towards the palm, that 

 surface which was at first an outward 

 convexity now becoming an inw'ard con- 

 cavity. Of course, if means of exit were 

 not provided for the blood contained 

 in the whip-like filament, the muscular 

 contraction would fail to withdraw it, 

 as may be shown by tying a silken 

 thread round its base when it is erected, 

 so that the blood cannot return into the 

 body, and then it will remain extended 

 as long as the ligature is present. 

 Mr. Poulton has made very careful microscopical 

 sections of this little organ, and has shown that it is most 

 exquisitely constructed. The pink colour is due to a 

 layer of thickish cells of spindle-shaped outline placed 

 beneath the colourless outer layers of the skin. A little 

 beneath this again lie bands of muscular fibres in the form 

 of a hollow cylinder, and within this is a band of nerve 

 fibres and a number of nerve cells, all the interspaces 

 being filled with blood. The large comparative size of 

 the nervous portion indicates the great sensitiveness of the 

 organ, and accounts for the wonderful rapidity with which 

 it can be moved. The whole organ in fact is extremely 

 delicate, and slight damage done to it, especially at the 

 point where it joins the tubidar sheath, would entirely 

 destroy the power of shooting it forth ; this apparently is a 

 misfortune that usually happens sooner or later, for the 

 adult insect generally loses control over its filaments, and 

 they may then be found lying helplessly within the 

 sheaths. When, however, they are in good working order, 

 the insect, as soon as it is touched, dashes its head round 

 in the direction of the irritation and immediately shoots 

 forth the tail filaments and flourishes them about ; so that 

 there can be little doubt that their purpose is to scare 

 away ichneumon flies that may chance to alight upon its 

 back. How great a need there is for this is evidenced by 

 the fact that, notwithstanding this means of defence, a 

 considerable proportion of the caterpillars perish through 

 the attacks of a large reddish-yellow ichneumon ; without 

 this and other defences which they have, no doubt the 

 mortality would be much greater. Like the puss moth, the 

 "lobster" caterpillar also possesses no terminal claspers, 

 but a pair of long rod-hke appendages instead ; but they 

 are not hollow, and therefore possess no filaments. They 

 help to heighten the effect of the "suggested monster" 

 when the tail is turned forward and brought near .the 

 head, by becoming separated and thus simulating a pair of 

 antennas ; but when the caterpillar is lying extended they 

 are brought close together, and then simulate the stalk of 

 the withered beech leaf which the caterpillar suggests. 

 Other caterpillars belonging to the same group, those of 

 the "hook-tips," so called from the hooked fore-wing of 



