326 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



them belong to a family of moths known as the Geometridse, so 

 named from the measuring habits of their caterpillars. The reduc- 

 tion of the abdominal legs of the caterpillars is an accompaniment 

 of the looping style of progression, but does not occur to the same 

 degree in all species. In one group of the family the caterpillars 

 retain the full number of prolegs, which is five pairs, but the first 

 three pairs are reduced in size (fig. 11), the first pair being very 

 small, the others successively larger to the fourth pair, which is of 

 ordinary proportions. These caterpillars probably represent an 



Fig. 11. — A geometric! caterpillar (Brephos infans) having the full number of abdominal 

 prolegs, but with the first three pairs reduced in size ( X 2% ) 



early stage in the evolution toward suppression of the first three 

 pairs of abdominal legs. The fall cankerworm, with a rudimentary 

 third pair of prolegs (fig. 8), is in a stage near the other end of the 

 series, while the spring cankerworm (fig. 7) and others that retain 

 only two pairs represent what appears to be the degree of leglessness 

 most practical for the looping gait. All measuring worms, however, 

 do not belong to the Geometridse, the familiar cabbage looper being 

 a member of quite a different family, the Noctuidse. 



When the cankerAvorms are full grown, about the 1st of June, they 

 cease their feeding and let themselves drop from the trees at the ends 

 of threads run out from their spinnerets. On reaching the ground 



they burrow into the earth and bury 

 themselves at a depth of from 2 to 5 

 inches beneath the surface. Here the 

 spring cankerworms, after from four 

 days to a week, change to pupse (fig. 12). 

 The fall cankerworms, however, after 

 their burial is accomplished, inclose 

 themselves in small oval cases or cocoons 

 made of earth particles firmly bound together with threads of silk, 

 and then rest in the cocoons for about 30 days before changing to 

 pupa). The pupae of the spring cankerworms remain in the ground 

 through the summer, fall, and winter, transforming early the follow- 

 ing spring into the moths which come out at this season and whose 

 history has been given. Most of the pupse of the fall cankerworm 

 undergo their final transformation late in the fall of the year of 

 their interment. 



The nature of the pupa of a caterpillar and its metamorphosis 

 to the moth was described in connection with the tent caterpillar 

 in the Smithsonian Report for 1922 (p. 253, fig. 17.) It was 



Fig. 12. Pupa of spring can- 

 kerworm in the ground 



(X 21/,) 



