NO. 35G8 MELANOTUS — QUATE AND THOMPSON 17 



author have examined types of similis, and there is no doubt that that 

 species is the same as the one known a,sjissilis. 



Melanotus similis exhibits a good deal of variation throughout its 

 range. Variations in the ratio of the thoracic wddth to length, in the 

 size of the pronotal punctiu-es, in the shape of the distal enlargement 

 of the male paramere, and the spines of the female biu-sa have been 

 observed. Some of the variation appears in a regular pattern with 

 an east-west cline and some is oriented in a north-south pattern in an 

 irregular manner. The two patterns are noncorrelated and inde- 

 pendent of each other (fig. 2). 



The eastern populations of M. similis have a relatively wdder 

 pronotum which generally narrows to the west and finally the narrow- 

 est pronotum is found in the Southwest. There is an approximate 

 east-west clinal variation, but this is irregular through the Midwest. 

 Table 1 shows the ratios of the pronotal length/width. There is no 

 significant difference between adjacent populations, but, as shown in 

 the chart, there are significant differences between nonadjacent popu- 

 lations. All measurements were made of males with dissected 

 genitalia. Females are not included because samples of uniform 

 sizes were not available in dissected specimens; however, less ex- 

 tensive analysis of females indicates that a similar pattern also exists 

 in that sex. 



The male genitalia show variation roughly correlated with the 

 pronotal width/length ratio. No satisfactory means of expressing 

 the difference qualitatively was found, but the differences in various 

 parts of the range are illustrated in figures 5a-e. Generally, the 

 distal enlargement of the paramere is longer and the subapical in- 

 dentation is more abrupt in eastern populations, but there is less 

 constancy than in the pronotal variation and only a weak clinal 

 trend is noted. From a taxonomic viewpoint, the differences are not 

 great enough to cause difficulty with identification, and the male 

 genitalia are the most reliable criteria for recognition of the species. 



The female genitalia display marked differences in the number of 

 spines on the bursa. Females can be sorted roughly into three 

 classes on the basis of the bursal spines. In the northern and western 

 part of its range, the three types are found ^vith a little dominance of 

 the "normal" type, which has few spines; in the southeastern United 

 States there is almost exclusively the "spiny" type, which has a 

 large number of spines (fig. 2). There are intermediates to the two 

 types which unite them in a gradient series, and no formal taxonomic 

 discrimination is warranted. 



Ratio of width/length of pronotum of Melanotus similis (analysis 

 based on 20 males from each area, total of 140 specimens) : 



221-532—67 2 



