50 ANIMAL AGGREGATIONS 



Szymanski (19 13) further studied the formation of feeding aggre- 

 gations of Arye caterpillars. Groups of these young caterpillars 

 gather on their species of food plant and arrange themselves on the 

 leaves so that they cling with their thoracic legs on the upper surface 

 while the posterior end hangs down curled around the edge of the 

 leaf. They arrange themselves so, side by side along the margin of 

 the leaf. When the larva at the tip has eaten to the main vein, it 

 may do one of three things: (i) turn around and go to the base of the 

 leaf and begin feeding there; (2) leave the leaf entirely; or (3) cross 

 over to the opposite side and begin feeding there. The older cater- 

 pillars tend to lose this regular arrangement and behavior. 



By the usual type of analytical experiments the Arye caterpillars 

 are shown to be positive to light, negative to gravity, and positive 

 to certain touch stimuli. The method of locomotion consists in the 

 extension of the anterior end and the drawing-up of the posterior. 

 The posterior end shows a definite motor reflex upon stimulation. 

 Thus, if touched at the posterior end, the posterior half of the body 

 is raised. A similar reaction is given if the substratum is gently 

 shaken. If one side is touched, the same response may occur, to- 

 gether with a bending-away of the touched part. 



If one tests out the method of colony formation, one finds that 

 when the larvae are placed at the base of the food plant they will 

 crawl up on it, since they are positive to light and negative to grav- 

 ity. When the first leaf petiole is encountered, they will turn aside 

 onto that because it is narrower than the main stem, and for the 

 same reason they will move along the edge of the leaf. On the leaf 

 they move to the side most strongly illuminated, or to the side far- 

 ther from the ground, as the case may be. 



The larvae crawl here and there over the leaf, passing over each 

 other; or they may touch the larvae ahead and cause them to move 

 forward. Finally one begins to eat, and gradually all settle to eating. 

 The positions taken may be accidental, for wide spaces may occur 

 between larvae, while others are closely crowded. The piece of leaf 

 between two larvae becomes eaten away, so that eventually the 

 head of the second larva touches the posterior end of the first. This 

 causes the latter to raise its posterior end, as in the test experiments 



