322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



again, by some unknown physiological regulation, life soon ceases 

 after its purposes are accomplished. 



Early in May, usually about the time the buds on the apple trees 

 are opening and sending out the new leaves, the cankerworm eggs 

 hatch and send out a destroying army of young caterpillars. Ac- 

 cording to Doctor Hunter, the time of hatching depends on the tem- 

 perature. Eggs kept in the laboratory with a temperature of 72°, 

 he says, will hatch in 10 days; eggs under outdoor conditions with a 

 mean temperature of 48° hatch in 30 days. Each caterpillar, 

 or cankerworm, leaves the egg through a round hole in the blunt 

 end, and, as it emerges, assumes a length of about one-eighteenth of 

 an inch, fully twice that of the egg from which it comes cut. The 

 empty shell remains as a delicate, transparent capsule glistening 

 with its iridescent colors. The young caterpillars are snak^^-lookin:: 

 little things, despite their large heads and flaring hind legs (fig. 6) ; 

 but their curious manner of walking by humping up the back as 



the rear end of the body is brought 

 forward, and then straightening 

 out again to repeat the humping, 

 at once catches the eye, and we 

 recognize in the tiny creatures 

 familiar acquaintances — measuring 

 worms/ 



Some of the young worms are 

 blackish, others are brown or green. 

 Fig. 6.— Newly hatched spring canker- ^^^ ^ost of them are marked by 



worms (much enlarged) ,, . . , , 



a narrow yellow stripe along each 

 side of the body. Their special feature, however, which gives them 

 the snaky look just mentioned, is the lack of most of those legs 

 on the abdominal segments that are ordinarily characteristic of 

 caterpillars. The form and structure of a typical caterpillar was de- 

 scribed in the Smithsonian Report for 1922 (p. 353, fig. 17, A), 

 where it was shown that the caterpillar body consists of 13 seg- 

 ments, the first 3, or thoracic segments, carrying each a pair 

 of jointed legs, while of the remaining 10 or abdominal segments, 

 the third to the sixth, inclusive, and the tenth have each a pair of 

 soft, thick, unjointed " prolegs.^^ The spring cankerworms have pro- 

 legs only on the sixth and tenth segments. Hence their looping 

 method of walking. 



The full-grown spring cankerworms (fig. 7) are from three- 

 fourths of an inch to a full inch in length. In general they are of 

 a dark color, but they vary from pale olive brown or yellowish to 

 mottled brown and black, with one, two, or three broken yellowish 

 lines more or less distinct along the sides. On the back of the eighth 



