Haploid individuals, however, are usually non-viable, and parthenogenetic offspring usually have the diploid chromosome number. Individuals are hermaphroditic and reproduction can be either sexual or parthenogenetic. Marbled crayfish are triploid animals, which may be the main reason for their parthenogenetic reproduction. Like all squamate obligate parthenogenetic lineages, parthenogenetic Teiids are hybrids. Besides differences between sexual and parthenogenetic morphs, winged and wingless morphs exist. Meiosis II involves the separation of sister chromatids in both sexual and parthenogenetic species. Many others classify the endomitotic variants as asexual and consider the resulting embryos parthenogenetic. Sister chromosome pairing maintains heteroozygosity in parthenogenetic lizards. DNA genotyping demonstrated that individual zebra sharks can switch from sexual to parthenogenetic reproduction. Although parthenogenetic species do occur, most harvestmen reproduce sexually. Most insect species reproduce sexually, though some species are facultatively parthenogenetic. The water flea "Daphnia pulex" alternates between sexual and parthenogenetic reproduction. Primarily known from lab studies of parthenogenetic "Aspidoscelis neomexicanus", simulated mating behavior can increase fertility. A few species of parthenogenetic centipedes are known. A few species are known to be parthenogenetic, reproducing without the need for males.