280 ILLINOIS ACADEMY OF SCIENCE 



SEXUAL DIMORPHISM IN THE ACANTHO- 

 CEPHALA^ 



H. J. Van Cleave, University op Illinois 



Frequently, animals differing widely in appearance 

 belong to the same species. Polymorphism, as this con- 

 dition is called, occurs in various groups throughout the 

 animal kingdom and is especially prominent in insect 

 states and in the colonies of some of the coelenterates, es- 

 pecially among the Siphonophora. Under these condi- 

 tions individuals representing each of the different forms 

 carry on restricted, specialized functions for the colony 

 or state. The causes of such diversity of form within the 

 members of the same species are not understood thor- 

 oughly. There are those who contend that polymorphism 

 has arisen as a result of a division of labor among the 

 individuals while others adhere just as strongly to the 

 view that because the individuals are different they are 

 thereby fitted for only one kind of special work and con- 

 sequently each does the work for which bodily structure 

 fits it. The type of bodily difference accompanying re- 

 stricted function most frequently encountered in the ani- 

 mal kingdom is that associated with the differentiation 

 of the sexes. 



Sexual dimorphism, as somatic differences between 

 males and females is termed, is of frequent occurrence 

 throughout the animal series. However, most of the 

 published accounts dealing with this phenomenon have 

 been concerned with the most conspicuous instances such 

 as those in which male and female differ so profoundly 

 in superficial characters that they might well be taken as 

 representatives of entirely distinct species. Technically, 

 any somatic difference, however slight, which enables one 

 to differentiate males from females without an examina- 

 tion of the gonads, may be considered as sexual dimor- 

 phism. In many species certain restricted organs or 

 parts of the body, not directly associated with the repro- 

 ductive process, display distinctive differences in the two 



^ Contributions from the Zoological Laboratory of the University 

 of Illinois, No. 162. 



