216 



KNOWLEDGE 



[November 1, 1898. 



and involuntary which take place in the life of an organism. 

 The experiments did not lend much support to the hypo- 

 thesis of protective coloration, but pointed strongly 

 towards a different reason for the presence or absence of 

 the darli brown colour. 



By first enclosing single caterpillars which were about 

 to spin in pockets made of various materials, some dark, 

 others light, and, secondly, enclosing families of cater- 

 pillars in white and black muslin bags, abundantly supplied 

 within with crumpled pieces of white and brown paper 

 respectively, so as to make the surroundings as uniformly 

 light or dark as possible, Mr. W. Bateson has shown that 

 the colour of the cocoon is independent of that of the 

 material on which it is built, for dark cocoons were attached 

 both to dark and light substances indifferently, and vice 

 rersii. But it was observed that the more natural were 

 the surroundings of the caterpillar, and the less it was 

 interfered with at the close of its larval career, the greater 

 was the tendency to form a dark cocoon, the light ones 

 being made specially by those which had been removed 

 from their food-plant. Continued association with the 

 food-plant up to the time of transformation thus seemed 

 necessary for the construction of cocoons of full colour. It 

 was further observed that the caterpillars had the power of 

 ejecting from the mouth a green liquid, which, apparently 

 by a process of oxidation, became brown on exposure to the 

 air ; and that in all probability a similar fluid was voided 

 from the hinder extremity of the alimentary canal. There 

 were many indications, which we have not space here to 

 detail, that it was this fluid, derived from the partially 

 digested food, and ejected from the alimentary canal, which 

 stained the cocoons and gave them their brown colour, and 

 that when light cocoons were made, the caterpillars had 

 probably already voided this fluid before their isolation, 

 and therefore possessed no further store to stain the cocoon 

 as soon as it should be made ; or, by being removed too 

 early from their food, failed to make the fluid, and were 

 therefore unable to give the cocoon the usual layer of 

 colouring. Thus the colour of the cocoon would seem to 

 be dependent, not upon the predominant tint of the sur- 

 roundings, but upon the condition of the caterpillar at the 

 time it is made. 



Of course it still remains an open question why this 

 hquid should be poured out upon the cocoon at all, and in 

 what way it may conduce to the well-being of the insect 

 that its abode during its time of quiescence should be thus 

 varnished. The colouring substance, though soluble in 

 water at first, becomes incapable of being washed out when 

 once the cocoon is stained, thus proving itself a fast colour. 

 It is an obvious suggestion that this dyeing of the cocoon 

 may have something to do with rendering it impervious to 

 the rains of winter ; but it is hardly safe to hazard such a 

 conjecture, since other equally plausible suppositions might 

 no doubt be made, which yet experiment might show to be 

 untenable ; it is better therefore to wait for the results of 

 further experiments before coming to any definite con- 

 clusion. It should be remarked that there seems no 

 particular need of further protection to the chrysalis than 

 that which it already possesses in being enclosed within 

 the very tough skin of the cocoon. Any insectivorous 

 animals which might otherwise feel disposed to prey upon 

 it would apparently be sufficiently kept at bay by this very 

 unmanageable material, so that special efforts for the 

 concealment of the besieged' after the acquisition of so 

 excellent a breastwork would seem to be only wasted 

 energy. 



The egger moths are so called because of the very 

 compact, egg-like cocoons they make. The two species 

 best known in this country are the oak-egger, a fine large 



Fig. 2. — Cocoon 

 of Oak Egger 

 Moth. 



yellowish -brown moth which appears in summer time, and 

 the small egger, a much smaller species of a reddish-brown 

 colour tinged with ashy grey, which makes its de'ljiit in 

 mid-winter. The cocoons of both of these are very 

 similar — parallel sided, and equally rounded at each end 

 (Fig. 2). The silk is close and compact, and forms a 

 parchment-like shell, thin but hard, and 

 extremely smooth within ; in each case it 

 is an exceedingly close-fitting cell, which 

 is not more than half the length of the 

 caterpillar that makes it ; hence it follows 

 that, as the artificer is accommodated 

 within during the whole of the time of 

 construction, it finds itself very cramped for 

 room as it turns about and carries its thread 

 from side to side. The small egger, in 

 some unaccountable way, always leaves a 

 minute perforation in one side of its cocoon, 

 an arrangement which its larger relative 

 does not imitate. From this hard and 

 compact shell the moth makes its escape 

 by pushing oft" from one end an irregular piece, which looks 

 as if it had been bitten round. When we remember, how- 

 ever, that the moth is entirely destitute of biting jaws of any 

 kind, the real method of escape is seen to be a great puzzle, 

 for the separation of this portion of the cocoon clearly implies 

 the severance round its edges of the crossing and recrossing 

 threads of which the cocoon is composed, and it is diificult 

 to understand how that should be done without a biting or 

 cutting instrument. Careful watching of the emerging 

 moth is here of little use, for the cocoon is opaque, and the 

 attack is made from within, and when we catch the first 

 glimpse of tlie head of the newly-fledged insect issuing 

 from the cocoon the deed is already done, and the escaping 

 prisoner has kept its secret as effectually as Messrs. 

 Maskelyne and Cooke in their escape from within their 

 closely-corded box. 



The problem has, however, recently been somewhat 

 satisfactorily attacked in the case of a different insect, and 

 one imprisoned, it might be imagined, more entirely 

 beyond hope of escape than either of the above. The 

 moth in question is the puss moth [I>icranurn riinila), of 

 whose extraordinary caterpillar we gave our readers a 

 description some time ago (Knowledge, February, 1893). 

 This creature selects a crevice in the bark of a tree, and 

 excavating it into an oval hollow, roofs itself over on a 

 level with the surrounding bark, with a layer of mixed 

 silk and chewed bark, which closely imitates the colour 

 and roughness of the true bark, so that often nothing less 

 than the closest and most minute scrutiny avails for the 

 discovery of the cocoon. This roof of mingled silk and 

 sawdust does not reveal distinct threads, as has been the 

 case with the other cocoons, but seems to be composed of 

 one continuous mass ; it becomes so hard and unyielding 

 that the blade of a knife, unless very firmly pressed, slides 

 ofi' and makes no impression on it. This intensely hard roof 

 completely covers in the dumpy brown chrysalis into which 

 the brilliant caterpillar changes, and from this at the end 

 of the following spring issues the large fluffy soft-bodied 

 moth, with its whitish wings prettily pencilled with grey 

 streaks, in a manner which recalls the markings of a 

 tabby cat. In this case, as before, a portion of the hard 

 shell is burst oft' just above the anterior end of the chry- 

 salis, making a hole just large enough for the moth to 

 squeeze itself out. Thus the problem is repeated, and 

 with an added intensity : how can a soft-bodied creature, 

 with no jaws or claws, work its wa,y through a partition 

 almost as hard as if it had been made of metal '? 



For the solution of the problem tlie analytical chemist 



