ECDYSIAL CLEAVAGE LINES OF ACARIFORM MITES (ARACHNIDA, ACARI)

Among mites the ancestral ecdysial cleavage line, or line of dehiscenc e (delta), is inferred by outgroup comparison to be prodehiscent: U-sh aped, passing around the front of the mite just above the insertions o f the appendages, such that the mite ecloses anteriorly. From preserve d and living individuals and exuviae, we found prodehiscence (or its s light variations) to be widespread in Acariformes. It appears to be pe rvasive in endeostigmatic mites, eupodine Prostigmata, and basal taxa in the Oribatida (Enarthronota, Palaeosomata); it is dominant in eleut herengone Prostigmata and is present in at least one anystine family ( Caeculidae). Three general modes of dehiscence are considered to be de rived within acariform mites. (1) Merodehiscence is a transverse split ting of the dorsal cuticle at or near the juncture of proterosoma and hysterosoma; it evolved separately in three groups of Prostigmata (Tet ranychidae, an undefined subgroup within Cheyletidae, and active insta rs of Parasitengona) and in a genus of Astigmata (Histiogaster). (2) T ransdehiscence is a transverse splitting of the dorsal hysterosomal cu ticle anterior to the opisthosomal glands; it occurs in middle-derivat ive oribatid mites (the paraphyletic Desmonomata), and new observation s show it to be widespread in Astigmata, lending support to the hypoth esis that the latter group evolved from within Desmonomata. (3) Circum dehiscence is a circumferential splitting of the hysterosomal cuticle that may be incomplete anteriorly; it has long been known to character ize the monophyletic oribatid taxon Brachypylina, but it is convergent with a similar dehiscence in an unrelated family, Lohmanniidae. Trans dehiscent and circumdehiscent mites eclose posteriorly.Astigmata exhib it the greatest variety of modes of dehiscence, including the three de rived modes and a probable reversal to prodehiscence in Algophagidae. Furthermore, heteromorphic deutonymphs (hypopi) may ecdyse differently from other immature instars of the same species.