THE EVOLUTION OF THE CATERPILLAR 625 



least activity, it is bound to be a more potent influence than 

 any method which entails activity. In the Geometers, or 

 Loopers, by far the larger number of individuals, if not of 

 species, are green in colour. Pure green is not the most effec- 

 tive disguise, however, even though the particular shade 

 harmonises exactly with that of the leaf, for every leaf is more 

 or less marked with streaks of lighter colour, caused by the 

 natural venation. It is quite in accord with this, therefore, 

 that the majority of caterpillars have stripes, either running 

 lengthwise of their bodies, or transversely across them. Most 

 often these streaks are whitish or yellowish, but others are of 

 darker shades of green, black, or brown. 



Perhaps no greater degree of perfection in this direction 

 has been reached than in the case of the common Winter Moth, 

 a Geometer found in profusion on almost every kind of British 

 fruit and forest tree. In spite of the fact that this insect 

 forms probably the chief food of numerous small birds, it 

 maintains its position as the most abundant of tree-feeding 

 caterpillars, and its protective coloration is, subject to modifi- 

 cation in opposite directions, of lighter or darker colour and 

 marking, exactly that which theoretically affords the best 

 disguise. 



While emancipated from the necessity of making rolls, the 

 Geometers do, in many cases, retain the power of silk spinning 

 during their larval period and make use of this material as a 

 safety line in the same way as their leaf-rolling cousins. 



There is in this group the beginning of a further de- 

 velopment of the exterior appearance which, so far as can be 

 understood at present, is solely connected with protective 

 resemblance. This is the appearance of humps, warts, and 

 similar protuberances in irregular positions on the body. The 

 connection between these humps and protective resemblance 

 does, it must be confessed, seem rather remote in some cases, 

 but the well-marked instances where it is known to serve that 

 purpose, lend colour to the general theory that this is the sole 

 object served by them. These protuberances may be present 

 on one or more segments of the body, but in the same species 

 they are always in one place, sometimes being located on the 

 back and sometimes on the sides. The general idea concern- 

 ing them is that anything which tends to break the regularity 

 of outline tends towards concealment, by making the object 

 blend more naturally into its surroundings. The strongest 

 evidence of this is found among certain Geometers which have 

 the habit, when resting, of stretching out their bodies stiffly, 

 attaching themselves by the anal feet, in which position they 

 resemble small twigs and in some cases may easily be over- 

 looked on this account. Indeed, in the case of the Thorn 



