Ecdysial cleavage lines of acariform mites (Arachnida, Acari)

Abstract

Among mites the ancestral ecdysial cleavage line, or line of dehiscence (a), is inferred by outgroup comparison to be prodehiscent: U‐shaped, passing around the front of the mite just above the insertions of the appendages, such that the mite ecloses anteriorly. From preserved and living individuals and exuviae, we found prodehiscence (or its slight variations) to be widespread in Acariformes. It appears to be pervasive in endeostigmatic mites, eupodine Prostigmata, and basal taxa in the Oribatida (Enarthronota, Palaeosomata); it is dominant in cleutherengone Prostigmata and is present in at least one anystine family (Caeculidae).Three general modes of dehiscence are considered to be derived within acariform mites. (1) Merodehiscence is a transverse splitting of the dorsal cuticle at or near the juncture of proterosoma and hysterosoma; it evolved separately in thrcc groups of Prostigmata (Tetranychidae, an undefined subgroup within Cheylctidae, and active instars of Parasitengona) and in a genus of Astigmata (Histiogaster). (2) Trarnsdehiscence is a transverse splitting of the dorsal hysterosomal cuticle anterior to the opisthosomal glands; it occurs in middle‐derivative oribatid mites (the paraphyletic Desmonomata), and new observations show it to be widespread in Astigmata, lending support to the hypothesis that the latter group evolved from within Desmonomata. (3) Circumdehiscence is a circumferential splitting of the hysterosomal cuticle that may be incomplete anteriorly; it has long been known to characterize the monophyletic oribatid taxon Brachypylina, but it is convergent with a similar dehiscence in an unrelated family, Lohmanniidae. Transdehiscent and circumdehiscent mites eclose posteriorly.Astigmata exhibit the greatest variety of modes of dehiscence, including the three derived modes and a probable reversal to prodehiscence in Algophagidae. Furthermore, heteromorphic deutonymphs (hypopi) may ecdyse differently from other immature instars of the same species.