6 



CATERPILLAR 



ings or ecdyses ; begins to develop some of the adult 

 structures, and falls into a quiescent pupa, chry- 

 salis, or aurelia stage. The pupa is usually ( except 

 in butterflies) insheathed in a silken cocoon, may 

 be fixed or free, suspended by one thread or more 

 to leaf or branch, or hidden away underground. 

 Among butterflies, the uninsheathed pupa may be 

 fastened head downwards by a single silken rope 

 fastened to the tail end (pupae suspenses), or head 

 upwards with an extra suspensor round the body 

 ( suscinctce ). The intimate structural changes asso- 

 ciated with the remarkable alteration in habit of 

 life have not yet been fully followed. One of the 

 7iiost important features is the appearance on the 

 caterpillar of what are known as 'imaginal discs,' 

 which appear to arise from the skin, and give 

 origin to the limbs and wings of the adult 

 insect. In the quiescent pupa stage very im- 

 portant changes go on, amounting to more or less 

 of a remaking or the entire body ; but it is not 

 possible within the present limits to describe the 

 changes undergone by the digestive, nervous, and 

 other systems, or the very marked transformation 

 of the mouth appendages. The cocoon in which 

 the pupa becomes the perfect insect may be alto- 

 gether absent, or very slight, or strikingly compact 

 and protective. Some firm cocoons open very 

 neatly from the inside by valvular lips, and in other 

 cases the moth is known to soften the walls of its 

 prison by means of some secretion. The gilded 

 colour of some pupae ( chrysalids indeed ), so mineral- 

 like in appearance, not improbably arose, it has 

 been suggested, in hot dry countries, and had a 

 protective value among dry rocks. It has been 

 shown experimentally to arise as a direct con- 

 sequence of bright surroundings. The cocoon 

 occasionally consists solely, or almost solely, of 

 the hairs of the larva ; in some cases leaves, 

 wood, earth, &c. are used in construction ; in 

 most moths it is spun. The work of spinning the 

 usual cocoon is carried on almost ceaselessly, some- 

 times for four or five days, and Trouvelot calculates 

 that the larva of Polyphemus in distributing its 

 silk must have moved its head to and fro about 

 254,000 times. 



A very primitive insect type is represented by 

 a widely distributed genus Peripatus (q.v. ), which 

 remains permanently at a sort of caterpillar level, 

 and serves to connect jointed-footed animals or 

 arthropods with worm-like forms. The caterpillar 

 may be interpreted as in part a recapitulation 

 of this historical stage in the evolution of insects. 



Fig. 2. Peripatus : 



Survival of ancestral insects (from Moseley). 



The caterpillar thus represents the prolongation of 

 an ancestral and embryonic stage, while many of its 

 characters have arisen as secondary adaptations to 

 its peculiar mode of life. Between each moult there 

 is a period of quiescence, and this becomes greatly 

 prolonged in that all-important moult in which the 

 mouth organs of the larva are modified into those of 

 the adult. Free life at a period so momentous would 

 evidently be disadvantageous even if it were pos- 

 sible. The pupa, furthermore, may come to have 

 a secondary importance other than that of simply 

 being an exhausted quiescence at the final moult- 

 ing ; it may come to oe of vital moment as a pro- 

 tective phase, by means of which the insect sur- 

 vives the cold of winter or the drought of the 

 dry season. In one case (Cossus) the preparatory 

 stages may be prolonged for three years. From 



another point of view the life-history may become 

 more intelligible viz. from the side of its physi- 

 ology. In the embryonic development the young 

 form is built up at the expense of its endow- 

 ment of food-capital. The first chapter is one 

 of passivity and living on past gains. When 

 these are exhausted, and the embryonic processes 

 completed, the larva emerges, hungry, voracious, 

 active. In its ravages and moultings it exhibits 

 alternate minor rhythms of activity and passivity. 

 Finally having laid up a store of food-capital in the 

 recesses of its 'fatty oody,' it falls asleep into the 

 more emphasised quiescence of the pupa stage. 

 During this phase of fasting and passivity, and 

 of life sustained by past gains, momentous clianges, 

 associated with gradual loss of weight, take place, 

 and the final debut is made by the appearance of 

 the active, frugal, sexually-mature, comparatively 

 short-lived adult. It is not yet possible to ration- 

 alise the details of the life-history, but in the 

 alternations of activity and passivity common to 

 all living organisms, and here more marked than 

 in any other case, the solution must be sought. 



Protection and Colour. Caterpillars are evi- 

 dently enough tempting juicy morsels to birds and 

 other insect-eating animals ; their slow movements 

 render them liable to ready capture, and, as 

 Wallace has pointed out, their soft-walled tense 

 structure is 'extremely dangerous, for a slight 

 wound entails great loss of blood, while a moderate 

 injury must prove fatal.' It is therefore not sur- 

 prising to find that caterpillars, in common with 

 the larvae of other insects, have found out, or have 

 become the subjects of, various devices for evading 

 their enemies. The more conspicuous forms almost 

 always possess some unpleasant attribute in taste 

 or smell, either in the tissues generally or in special 

 glands. Weismann notes how a curious lashing 

 about of .the tail may preserve one form, and how 

 the juices of another attract a protective body- 

 guard of ants. Others are &o uncanny in the dis- 

 position of their hair-tufts and colour, or in the 

 ' terrifying atti- 

 tudes ' which they 

 assume, that their 

 cautious though 

 hungry foes leave 

 them alone. But 

 distaste and re- 

 pulsion may on 

 stress of hunger 

 be overcome, and 

 only a relative- 

 ly small number 

 of larvoe trust 

 to this mode of 

 defence. Thus 

 others are in 

 their colour and 

 markings so like the plants on which they feed, 

 or the ground on which they crawl, that they 

 avoid detection, and this protective resemblance 

 is often not merely general and superficial, but 

 detailed and exact. Thus some Avlien fixed as 

 pupae to the stems of plants, are almost indistin- 

 guishable from knobs or stunted twigs. In the 

 twig-like attitude the supporting thread is some- 

 times dispensed with. Others are like little splin- 

 ters of wood, or the curled margins of withered 

 leaves. The hairs and fleshy tubercles may prevent 

 the casting of a sharp shadow. Nor is the ' mimicry ' 

 confined to resembling the parts of plants, but a 

 palatable insect may probably save itself by approxi- 

 mating in colour to one that is distasteful. Mr 

 Bates observed a large caterpillar deceptively like 

 a small venomous snake. Protective resemblance 

 may be further abetted by unpalatable taste or 

 unpleasant smell. Some forms hide during the 



Fig. 3. Terrifying attitude of larva 

 of Dicranura vinula ( after Poul- 

 ton). 



