Classification of screwworms (Diptera: Calliphoridae) by larval spine morphology

The morphology of screwworm, Cochliomyia hominivorax (Coquerel), larval spines was affected by larval substrate and geographic effects on variation. Spine morphology of individual larvae was used to determine criteria for discrimination analysis classification. Classification criteria for types of wounds and host species were determined for larvae from central and southern Mexico. Larvae from marginal populations (United States, Caribbean Islands, Costa Rica) were classified by these criteria. Classifications were 27% accurate for wound type and 20% correct for animal species. When single egg masses were split and larvae were reared on laboratory diet and animal substrates, analysis of spine types showed significant rearing environment and sibship (egg mass) effects. Cluster analysis of geographic groups showed that larvae from central populations were closely clustered, whereas marginal groups were morphologically dissimilar from each other and from the central populations. These results indicate that larval spine morphology has limited value in taxonomic or systematic studies of screwworms and that mating compatibility was not related to spine similarity among populations. The significant differences between wound-reared and artificial media-reared larvae indicate that differences in spine morphology could be used to verify that samples from the field were indeed taken from animal wounds.