ARTHROPODA. 283 



The males are often smaller, more active, and more highly 

 colored than the females (see "sexual dimorphism," 145). 

 Sometimes the members of a single sex are dimorphic, as in 

 the workers and queens among the bees. This is correlated 

 with individual division of labor in the social insects. 



The sexual organs are usually paired, and in the female 

 consist of the ovaries (which maybe subdivided into ovarioles) , 

 oviducts, receptacula seminis in which spermatozoa are stored 

 at copulation, accessory glands, sometimes external copulating 

 and egg-depositing organs. The male has testes; vasa de- 

 ferentia, which may have special enlargements for the storing 

 of spermatozoa and the formation of sperm masses; and ex- 

 ternal copulatory organs. See figures of the sexual organs 

 of the honey-bee or other representative insect in the reference 

 texts. Compare them with those of the snail. 



321. Parthenogenesis. In several insect types the eggs 

 have the power of developing without being fertilized by the 

 male element. Its occurrence is determined primarily by the 

 absence of males, but even when males are present the female 

 may often deposit unfertilized eggs. She is influenced to do 

 this possibly by the special conditions of temperature, nutri- 

 tion, and the like, to which she is subject. 



The individuals resulting from parthenogenesis may differ 

 very materially from those produced by the normal sexual 

 method. In the case of the bee, the males or drones come from 

 unfertilized eggs, and the workers and queens from fertilized. 

 The cause of the differences between workers and queens is 

 apparently one of nutrition purely. Biologically, partheno- 

 genesis is to be considered as a modified or abbreviated form 

 of sexual reproduction, in which the stimulus to cleavage 

 comes from some source other than the male cell. 



322. Development. After fertilization the nucleus divides 

 as described for other forms, but usually, on account of the 

 abundant yolk which the eggs contain, complete segmentation 

 of the cell is not effected. After a series of divisions some of 



