3.3 Specialized morphology to adapt an obligate parasitic life
The skin of mammalian species is composed of epidermis, dermis and hypodermis (Figure 3a ). As a permanent parasitic mite species, scabies mites burrow into the epidermis of the skin, and resident at the interface of stratum lucidum and stratum granulosum (Figure3b ). Belonging to a member of chelicerate, the adult scabies mite body plan comprises the anterior prosoma bearing the chelicerae, pedipalps, and the four pairs of walking legs, and the posterior opisthosoma (Figure 3c ). After long time of adaptation, the four walking legs of scabies mites has become extremely stubby to adapted to its parasitic environment in the epidermis, while for other mites such as surface living and free-living mites, the length of walking legs remains long (Figure 3c ). Interestingly, when we observed the morphology of species in class Acari, we found that for species in the superorder Acariformes, the anterior and posterior two pairs of legs were distantly distributed and the proportion of the abdominal segments were reduced, while for species in the superorder Parasitiformes and the velvet spider, all the legs were very close to each other and the abdominal segment is relatively longer (exceptMetaseiulus occidentalis , which seems a transactional species) (Figure 3c; Figure 3d, left panel ). Therefore, we analyzed the structurally and functionally conserved Hox genes of these related species, including lab , pb , Hox3/Zen , Dfd ,Scr , ftz , Antp , Ubx , Abd-A , andAbd-B , hoping to find potential clues to explain this phenomenon. The results showed that S. scabiei has eight of ten canonical Hox genes that are present in the arthropod ancestor, except Zen andAbd-A , and no gene duplication events (Table S14 ). The loss of Zen and Abd-A seems common and unique for species in superorder Acariformes (Figure 3d, central and right panel ).