PROTECTIVE DEVICES IN BASILARCHIA. 263 



close to but scarcely touching' the eaten edge of the leaf; and as fast as the 

 leaf is eaten, it removes this packet (contiiuially added to until it becomes 

 about as big as a small pea), farther and fsirther down the midrib away 

 from its perch, always keeping it near the eaten edge. It should be noted 

 that it is so loosely attached, the bits of leaf at all possible angles, that it 

 is moved by the least breath. Meanwhile the caterpillar has been growing 

 larger and more conspicuous and thus in greater peril from its enemies. 

 There are two possible services that this odd packet may render. A 

 spider wandering over the leaf and observing its motion may seize it and 

 thinking it has a prize hurry away with it and leave its architect unharmed. 

 This seems to me rather a strained suggestion, for a wandering spider 

 would probably proceed to investigate it on the spot. Another expla- 

 nation seems more probable. It should be remembered that the leaves 

 preferred l)y these creatures as food are mostly such as are easily shaken 

 by the wind, and as the caterpillar moves with the leaf and with all the 

 surrounding leaves (in a continual fluttering in the case of the trembling 

 aspen, and to a less degree in the other food-plants) this of itself is a 

 protection to it, as it would more readily escape observation as an object 

 distinct from the leaves, all being in motion together ; but on the more 

 stable leaves, like the willow and especially the Rosaceae and the oaks, the 

 motion in a feeble wind would not be sufficient to be serviceable, and here 

 at least the packet comes into play. An object in motion among others at 

 rest is a most noticeable thing, a fact well recognized among animals, as a 

 host of them show when they fear being seen. This packet attached by 

 loose silken threads moves, as stated, with a breath of wind and so would 

 distract attention from its architect near by, who has taken pains to place 

 it at the farthest remove from his perch, while still (to avoid undesirable 

 steps) on his daily track. If this be really its object, it is surely one of 

 the oddest devices in nature. 



The species of Basilarcliia all pass the winter while in the caterpillar 

 state and but partly grown. The caterpillar has moulted at least once 

 (devouring its cast-off clothing, by the way, doidjtless that it may not 

 attract attention) and has to prepare against the inclement season. This 

 it does in a very shrcAvd way, which is all the more remarkable because no 

 trace or semblance of it is seen in caterpillars of the broods that attain 

 their entire growth in the same season. When the proper time approaches, 

 warned thereto possibly by the dryness of its food, or by the cooler nights, 

 the caterpillar constructs a little nest, sometimes from the still unfinished 

 leaf on which it was born, sometimes from one which it prepares specially 

 at greater pains ; this is done l)y eating away or biting oft' the unnecessary 

 parts, and leaving on either side of the base of the leaf little flaps just 

 large enough, when drawn together, bottom side up and meeting above, to 

 form a cylinder into which it can squeeze ; a projecting shelf is also left 



