148 THE STORY-BOOK OF SCIENCE 



dition is decided on, a caterpillar leaves the nest 

 and pauses at a certain distance to give the others 

 time to arrange themselves in rank and file and form 

 a battalion. This first caterpillar has to start the 

 march. Following it, others place themselves, not 

 one after another, like the processionaries of the 

 pine, but in rows of two, three, four, and more. The 

 troop, completed, begins to move in obedience to the 

 evolutions of its file-leader, which always marches 

 alone at the head of the legion, while the other cater- 

 pillars advance several abreast, dressing their ranks 

 in perfect order. The first ranks of the army corps 

 are always arranged in wedge formation, because 

 of the gradual increase in the number of caterpillars 

 composing it; the remainder are more or less ex- 

 panded in different places. There are sometimes 

 rows of from fifteen to twenty caterpillars march- 

 ing in step, like well-trained soldiers, so that the 

 head of one is never beyond the head of another. 

 Of course the troop carpets its road with silk as it 

 marches, so as to find its way back to its nest. 



1 'The processionaries, especially those of the oak, 

 retire to their nests to slough their skins, and these 

 nests finally become filled with a fine dust of broken 

 hairs. When you touch these nests, the dust of 

 the hairs sticks to your hands and face, and causes 

 an inflammation that lasts several days if the skin is 

 delicate. One has only to stand at the foot of an 

 oak where the processionaries have established them- 

 selves, to receive the irritating dust blown by the 

 wind, and to feel a smart itching. " 



"What a pity the processionaries have those 



