Multivariate morphometric discrimination of nymphal and adult forms of the blacklegged tick (Acari: Ixodidae), a principal vector of the agent of Lyme disease in eastern North America

A morphological study of postlarval stages of the blacklegged tick, Ixodes scapularis Say, was conducted to examine congruence between northern (formerly I. dammini Spielman, Clifford, Piesman & Corwin) and southern morphotypes. The character set totaled 17 for females, 25 for males, and 28 for nymphs. Populations from 6 geographic areas, F1 progeny from reciprocal crosses between I. scapularis from Massachusetts and Georgia, and I. pacificus Cooley & Kohls from California were measured. Characters, except cornua length in nymphs, were positively correlated with PC1, indicating it was a general-size eigenvector. Characters used previously by others to distinguish northern and southern forms had a highly positive allometric relationship to general size. In canonical variate analysis (CVA) of nymphs, canonical score 1 (CAN1) discriminated I. pacificus from all other groups, canonical score 2 (CAN2) discriminated the remaining groups in a pattern that correlated with latitude, and canonical score 3 (CAN3) separated the western (Minnesota and Missouri) from the eastern groups. Size-free CVA indicated that north-south discrimination was size dependent, but that variation between Missouri and all other groups was not size related. Mahalanobis distances between groups within stages were significant with the exception of the 2 groups of female progeny of reciprocal crosses (Massachusetts x Georgia). Analysis of variance and Student-Newman-Keuls tests revealed that each geographic population differed from all other groups in at least 1 nymphal character. Nymphs from northern areas (Minnesota, Massachusetts, Maryland) differed from those from southern areas (Missouri, North Carolina, Georgia) in characters associated with the basis capitulum (longer intercornua and interauricular distances, basis capituli, and hypostome internal files, larger auriculae, but shorter cornua). Western populations (Minnesota, Missouri) differed from eastern populations in idiosomal characters (broader scutum, larger coxae III and IV). Frequency polygons of characters with the greatest differences indicated that data are continuous and geographic variation is overlapping. Thus, the data support the previous contention of conspecificity of I. scapularis and I. dammini. I. scapularis appears to be a polytypic species with a widespread geographic distribution exhibiting north-south and east-west morphological clines in eastern North America.