VARIATION IN HEAD LENGTH OF SPERMATOZOA 511 



7 is unimodal with the mode at 11.7 m- This indicates either a 

 single group or two so close together as to give a unimodal result 

 (see Zeleny and Faust '15 a, p. 193). 



There are no pubhshed spermatogenesis data for this species. 

 Descriptions for different species of beetles show in all cases, 

 except one, two groups as regards chromatin content. Leaving 

 out the possibilities of random sampling and of selective elimina- 

 tion within the two groups the present species seems to be either 

 without distinction among its spermatozoa or else has two groups 

 differing but slightly from each other. 



7. Berosus striatus, a coleopteran. The material was obtained 

 in early April and the spermatozoa were all ripe and active. 

 The frequency distribution of the five hundred measurements 

 is given in figure 8. The curve is distinctly bimodal with approxi- 

 mate equality in the two groups. The modes are at 16.1 ix and 

 17.2 IX with a ratio of 1.00 : 1.07. 



There are no cytological data for this species but as was stated 

 in discussing the last form the descriptions for all but one species 

 of beetles give two kinds of chromosome groups among the 

 spermatids. 



DISCUSSION 



The present data as a whole substantiate the view that di- 

 morphism in size of spermatozoa is of common occurrence in 

 those groups of animals in which two chromosomal classes of 

 spermatids are of common occurrence. Of the seven species 

 described five are hemipterans and two coleopterans. In both 

 of these orders a great majority of the species so far studied show 

 a quantitative difference in chromosomal content among the 

 spermatids though in each case there are some species which 

 show no difference or only a slight one. 



Among the five hemipterans three, Corizus lateralus, Euschis- 

 tus variolarius and Cosmopepla carnifex, show two well marked 

 size groups. The other two species are not so clear. One, 

 Leptocoris trivittatus, is probably dimorphic though the curve 

 is irregular. The other, Reduviolus ferus, is doubtful because 

 the upper mode is very low and may be a chance projection in 



