CHAPTER XVI 



UNIT-CHARACTERS OF INSECTS 



The so-called " silkworm " is the larva of an Asiatic moth 

 which feeds principally on the leaves of the mulberry tree. 

 The " worms " when full grown spin a silken cocoon (which 

 furnishes the silk of commerce) within which they complete 

 their metamorphosis into the moth stage. As moths they 

 mate and the females lay eggs. In some races there is only 

 one generation a year, the eggs laid one summer hatching the 

 next spring. These are said to be univoltine, having one 

 flight or mating period annually. In other races there are 

 two or more broods a year depending on temperature condi- 

 tions. These are said to be bivoltine or multivoltine. In 

 crosses between univoltine and bivoltine races the eggs laid 

 have the character of the mother's race, being purely ma- 

 ternal structures. Thus, eggs laid by a univoltine mother 

 refuse to hatch before the following season, whatever the 

 racial character of the male that fertilized the eggs. And 

 eggs laid by a bivoltine mother are regularly bivoltine regard- 

 less of the father's racial character. But the females which 

 hatch from cross-bred eggs are really heterozygous as regards 

 voltinism. Their eggs show the dominant {univoltine) char- 

 acter but their daughters, the F2 females, are some univol- 

 tine, others bivoltine, in the ratio, 3:1. 



Races of silkmoths differ by numerous characters, many 

 of which are Mendelian. Toyama has enumerated more 

 than a dozen such Mendelizing characters found in the larva 

 alone. Some races differ in the number of larval moults, 

 which may be either three or four. Tri-moulting is dominant 

 over tetra-moulting in crosses. The blood of the larva may 

 or may not be yellow colored, yellow blood being dominant. 

 Yellow-blooded larvae spin yellow cocoons so that there is a 

 correlation between blood-color of the larva and the cocoon- 



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