BEHAVIOR OF THE ANT-LION. 



28 7 



and along the ventral surface of each there runs a prominent 

 tube through which the juices of the victim are sucked. This 

 tube is composed of two parts. Along the whole of the ventral 

 surface of each mandible [Fig. 13-4] there is a deep groove with 

 incurved edges. Another mouth part [Fig. 13.8], probably the 

 maxilla, fits so tightly into this groove of the mandible that, even 

 when viewed with a 2/3-inch objective, the two seem to form a 

 single structure. With that power, on the underside of each 

 mandible one sees two ridges. These mark the junctions of the 

 two pieces; but, unless you had previously dissected a mandible, 

 you would not suspect that there were two pieces and that they 

 were not rigidly united. Turn the insect on its ventral surface, 



FIG. 6. Shed chrysalis skin of ant-lion. 



carefully disarticulate the mandible, and, with a pair of dissecting 

 needles, gently push it forward. Thus the other mouth part will 

 be gradually drawn out of the mandible and left attached to the 

 ventral part of the front of the head. 



The ant-lion preys upon living invertebrates. How does it 

 distinguish the living from the not-living? There may be several 

 factors which help it solve this problem. The following experi- 

 ments show that one attribute by means of which the ant-lion 

 differentiates between desirable and undesirable prey is the 

 exhibition of restlessness: 



Experiment i. I fastened a bit of straw to the end of a silk thread. 



