254 CATERPILLARS AND THEIR MOTHS 



and several other persons, have found them high on 

 the trees. Examination of over three hundred cocoons 

 proved to us that as many males as females, in pro- 

 portion, came from baggy cocoons, and as many females 

 as males from slender cocoons, and that both baggy 

 and slender ones were found low down and high up. 

 Another writer calls the baggy cocoons " the lowland 

 form " and the slender ones " the hillside form," but 

 we have found both forms on one tree, and even on 

 one twig. 



The only way in which we have ever been able to 

 tell male and female pupte, without opening the 

 cocoons, has been by weight. As far as we have 

 tested them in this way, the heavier cocoons held 

 female pupae and the lighter ones male pupae, but we 

 have not yet reached a point of certainty which would 

 let us dare to say that this was a sure test. We have 

 always found the males less numerous than the females. 



The cocoons found vary in color, even before they 

 have " weathered," and we once had a cecropia larva 

 spin a cocoon which was silvery white at first, then of 

 an exquisite pale green. It has faded somewhat in 

 the course of years, but is still green, the only one we 

 have ever seen or heard of, though the silkworms 

 often spin green cocoons in Italy. 



The egg-period varies, being sometimes three weeks. 

 The length of larval life also varies from four weeks 

 to eight or nine. 



Box-elder is a favorite food-plant, but we have had 

 best success with apple, wild-cherry, and willow, and 

 the largest cecropia we ever saw was found on a willow 

 in a swamp. 



