Orientation 



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of the branches where no doubt the juiciest leaves are found. The 

 caterpillars later may return to the nest by following the silk threads 

 they have left behind them on their upward journey, but they will 

 not ordinarily move further downward over fresh surfaces. The two 

 wild cherry trees shown in Fig. 6.15 have been completely defoliated 



Fig. 6.15. Wild cherry trees in southern Rhode Island defoliated by tent cater- 

 pillars (Malacosoma), whose nests are seen in the forks of the branches. Neigh- 

 boring oak trees were not attacked. 



by tent caterpillars. Following the consumption of all the leaves in 

 their tree, many caterpillars starve, or die of disease (Craighead, 

 1950) but others crawl or fall to the ground and eventually find their 

 way to another tree. For some reason, as yet unknown, this species 

 does not ordinarily eat oak leaves, and in the area photographed the 

 oak trees only a meter or so away were not attacked. As the time for 

 pupation approaches, negative geotaxis becomes strong and the ani- 

 mals tend to climb up any vertical object encountered. In a neighbor- 

 ing area a series of fence posts was found, on the top of each of which 

 was a seething mass of tent caterpillars that had gathered there as a 

 result of the reactions just described. Since the animals were slaves to 

 their geotaxis, they remained on the fence posts, and, finding nothing 

 else to eat, they proceeded to devour each other. 



When an animal moves vertically under normal illumination from 

 above, it is often difficult to ascertain whether the animal is reacting 

 primarily to gravity or to light; that is, if it moves upward, it may be 



