INSECTS INJURIOUS TO THE SWEET POTATO 433 



conceals the insect. Upon this fork is heaped the excrement and 

 cast skins of the larva, and w en covered by this " umbrella " it is 

 with great difficulty that the larva is distinguished from a bit of 

 mud or a bird-dropping. The manner in which this fork increases 

 with the size of the larva is rather interesting. At each molt, the 

 fseci-fork of the last stage is held upon the new fseci-fork, and in 

 this way those of the different stages are telescoped, the one inside 

 the other, and the stage of growth of the larva may be readily 

 determined by the number of cast skins held on the fork. From 

 the likeness of this burden to a pack, the larvae are often known 

 as " peddlers." In order to more firmly bind the excrement and 

 cast skins to the fork, the larvae fasten them together by a fine 

 network of silken threads, which are attached to the spines at the 

 sides of the body. When fully grown the larva fastens itself to a 

 leaf, its skin splits open along the back, and from it comes the 

 pupa, which is held to the leaf by its caudal fork, which is securely 

 encased in the fseci-fork of the larval skin. About a week later 

 the adult beetle emerges, eats for a few days and then disappears 

 from the sweet-potato patch until the following spring, doubtless 

 feeding on morning glory until it enters hibernation. 



The Two-Striped Sweet-potato Beetle * 



This is usually the most common of the tortoise beetles attack- 

 ing sweet potatoes. The beetle is pale or brownish-yellow, 



FIG. 310. The two-striped sweet-potato beetle (Cassida bivittata Say): I, 

 larvae on leaf; 2, larva; 3, pupa; 4, beetle all enlarged. (After Riley.) 



striped with black as shown in Fig. 310, and the larva is yellowish- 

 white, with a longitudinal band along the back, on either side of 

 * Cassida bivittatce Say. Family Chrysomelidce. 



